Hello everybody
Normally I just lurk during term time due to not having any time to actually make any effects pedals, but as of this morning I have a problem that needs quick fixing and I don't know anyone else to ask. My lovely Technics Stereo Amplifier (part of my hi-fi) died this morning. I've had it for 33 years and it's been used almost everyday with no problems until today when it no longer turns on. All the audio in my house plays through this amp, so the house has been horribly quiet today... None of the LEDs light up and no sound comes out of it. I did notice yesterday when I turned it on that it took about 30 seconds to power up (normally it takes about 3 seconds), but didn't think any more of it.
I've checked the fuse in the plug and that has continuity. I tried plugging it into other power sockets (no change). I've opened up the amplifier and found a fuse next to the power input. I checked that (it has a metal filament in it) and that also has continuity. My next plan is to clean the 33 years of dust there is inside it and look for obvious burned resistors or capacitors (there is so much dust that I can't tell if any are dead), but before I do that, I do recall reading somewhere about how amplifiers (even when unplugged and turned off) somehow store lots of electrical charge which makes touching the insides of them rather dangerous? Or is that just guitar amplifiers? Is there anything I should be careful to avoid touching or doing while cleaning the dust out? I've got a can of compressed air to clean it with or should I just wipe everything with a dry or damp cloth? Any tips to avoid me doing something stupid or painful would be gratefully received!
Thanks!
Been a few years and my memory is fading but I had a Technics amp which had failed. Took me ages to realise it had an overheat protection circuit in it and the cooling fan for the power transistors had failed. Caused silence but not sure about the display.
As I recall, the pcb was conveniently silk screened to indicate pre amp, power amp etc sections but nothing about this overheat circuit
It's the big caps that are dangerous...although tube amps usually have higher voltages inside than transistors. Still a good idea to discharge them before you go poking around. People suggest 100k 1w resistor, an alligator clip to ground, an insulated probe.
The other rule is "one hand in your pocket."
This amp is +/-43V (or +/-30V) on the big caps. Unless you have a heart defect, brushing this will tingle like hell but will not kill you.
Check the switches you don't know about. This amp has a 4-8-16 ohm switch which gives the different power rail voltages. If it has been bumped into the in-between point, or grossly tarnished inside, no go. Work it back and forth and leave it all the way in the right setting. (If in doubt, lean toward 4 Ohms.)
You may also have a 100V-115V-210V-220V-240V switch. The in-between points are no-go.
It does seem to have a cut-out in the power transformer, also various fuses on the line side of the transformer.
You want a Volt-Meter. The big caps should show 43V (or 30V) each.
QuoteNone of the LEDs light up and no sound comes out of it. I did notice yesterday when I turned it on that it took about 30 seconds to power up (normally it takes about 3 seconds), but didn't think any more of it.
I've checked the fuse in the plug and that has continuity. I tried plugging it into other power sockets (no change). I've opened up the amplifier and found a fuse next to the power input. I checked that (it has a metal filament in it) and that also has continuity. My next plan is to clean the 33 years of dust there is inside it and look for obvious burned resistors or capacitors (there is so much dust that I can't tell if any are dead), but before I do that, I do recall reading somewhere about how amplifiers (even when unplugged and turned off) somehow store lots of electrical charge which makes touching the insides of them rather dangerous? Or is that just guitar amplifiers? Is there anything I should be careful to avoid touching or doing while cleaning the dust out? I've got a can of compressed air to clean it with or should I just wipe everything with a dry or damp cloth? Any tips to avoid me doing something stupid or painful would be gratefully received!
Check the AC voltages at the output of the transformer. Then the DC voltages after the rectifier and on the various supply rails.
https://www.audioservicemanuals.com/t/technics/technics-su/150519-technics-su-600-schematics
First of all thanks ever so much everyone! I've cleaned the interior with canned air and wiped over it with a cloth so that I can see inside and nothing looks obviously burned or dead alas (at least to me anyway)
Quote from: stallik on March 05, 2022, 07:00:42 PM
Been a few years and my memory is fading but I had a Technics amp which had failed. Took me ages to realise it had an overheat protection circuit in it and the cooling fan for the power transistors had failed. Caused silence but not sure about the display.
As I recall, the pcb was conveniently silk screened to indicate pre amp, power amp etc sections but nothing about this overheat circuit
Sadly this doesn't seem to have a fan anywhere in it... the PCB isn't obviously labelled into sections as you mentioned, but it does have labels on most parts (nothing about power though)
Quote from: PRR on March 06, 2022, 12:59:12 AM
This amp is +/-43V (or +/-30V) on the big caps. Unless you have a heart defect, brushing this will tingle like hell but will not kill you.
Check the switches you don't know about. This amp has a 4-8-16 ohm switch which gives the different power rail voltages. If it has been bumped into the in-between point, or grossly tarnished inside, no go. Work it back and forth and leave it all the way in the right setting. (If in doubt, lean toward 4 Ohms.)
You may also have a 100V-115V-210V-220V-240V switch. The in-between points are no-go.
It does seem to have a cut-out in the power transformer, also various fuses on the line side of the transformer.
You want a Volt-Meter. The big caps should show 43V (or 30V) each.
Thanks, I will start measuring things and see what results I get. And I'll look at the switches you mentioned - there is a switch on the back for the ohms rating for speakers, so I'll look at that. I think to measure the capacitors I will need to take a lot of the amp to bits because the solder points for them are between the PCB and the chassis base.
Quote from: idy on March 05, 2022, 10:15:32 PM
It's the big caps that are dangerous...although tube amps usually have higher voltages inside than transistors. Still a good idea to discharge them before you go poking around. People suggest 100k 1w resistor, an alligator clip to ground, an insulated probe.
The other rule is "one hand in your pocket."
I can't discharge a capacitor without removing it from the PCB can I? Or can I? All the videos I watched about this seem to involve a capacitor that is already not attached to anything... I do have insulated probes and the DMM says it is rated to 1000V DC and 750V AC.
Quote from: Rob Strand on March 06, 2022, 04:50:17 AM
Check the AC voltages at the output of the transformer. Then the DC voltages after the rectifier and on the various supply rails.
https://www.audioservicemanuals.com/t/technics/technics-su/150519-technics-su-600-schematics
OK will try that, I think the outputs are the 6 terminals at the top of the transformer (they all connect to ribbon cable that feeds into the PCB). What is the rectifier?
Thanks for the link to the schematic; I've photoshopped the images together in case it's of use to anyone (and increased the contrast to make it easier to read)
(https://i.imgur.com/vTVx95X.png)
Hello everyone
Some updates, although I suspect the answer is the transformer is dead :(
(https://i.imgur.com/TBdmq8U.png)
This is the power section, measured with the speaker switch set to 4ohms in case that matters. Everything seems fine until I measure the output of the transformer, which is 0.01V AC on all the terminals... so power is going into the transformer, but nothing is coming out...
(https://i.imgur.com/G00JRXB.png)
This is the input of the transformer, I can't get a probe to the furthest input, but the first two are at 31V AC - does that sound like a normal value?
I've tried looking for any info about the transformer online (SLT5M479-W1), but the only mention of it is in a service manual for a similar technics amplifier to this one, so I assume that means it's not available anywhere :(
Do I now have to resign the poor amp to the dustbin? I guess I can't complain - 33 years of service is pretty impressive.
QuoteThis is the input of the transformer, I can't get a probe to the furthest input, but the first two are at 31V AC - does that sound like a normal value?
No. You should see mains voltage getting to the transformer input terminals.
It might not be be the transformer (yet). It might be something simple like a solder joint on the mains side or
perhaps even a faulty mains switch, or a bad connection around the power selector.
Plenty to check.
What you need to do is establish that *each* mains input line is connecting all the way through to the transformer.
*** Completely remove the power and disconnect the unit from the mains. ***
Using the ohms tester on 200 ohms trace:
1) Transformer check
Referring to the schematic, the three wires correspond to the transformer primary winding
and the transformer primary winding through the thermal fuse (inside the transformer).
The direct transformer connection is available on transformer but there is no wire soldered to it.
Measure the resistance between each of the three mains terminals of the transformer.
You expect to see a low DC resistance into the transformer: perhaps roughly 5 to 10 ohm for a 120V winding ; I'm assuming that's what you have. The resistance across the thermal fuse should be less than 1 ohm. When you are on the outer terminals the thermal fuse resistance you add to transformer resistance and your resistance measurement should reflect that.
One possible fault is the thermal fuse has opened. That means you will get an open circuit across it but still see
the transformer DC resistance on the unused terminal. In this case the transformer itself is OK.
Another fault is the transformer has opened and you will only see a short across the thermal fuse.
[If you have a voltage selector and a stack of transformer taps then identifying what is what won't
be straight forward. It's important the thermal fuse is checked. But if you measure across two
mains voltage taps it is normal to measure a near short circuit, this is where the small decimal
points can guide you about what is going on.]
If you see something wrong in this area stop there.
2) Wiring Check
- With the unit disconnected from the mains, enable the power switch.
- Using the ohms tester on 200 ohms trace *each* mains inlet wire through to the transformer input terminals.
- It's best to use the ohms scale and follow the wires by eye. You want to see 0.5 ohms or less for the wires.
Be aware if you put the meter on the wrong wire you will get false ohms reading (5 to 10 ohms) through
the transformer, which is the wrong way through the circuit. You want to follow the wires with < 0.5ohms
all the way from the mains inlet to the transformer on *each* lead.
Thanks Rob for all your suggestions. I unplugged the power and then unscrewed a lot of the amplifier and was able to get the transformer out, although it is still attached to the main PCB through solder (not the PCB that the power socket is attached to). I can't set my DMM to a 200ohm trace (I don't have a setting for this) but I was able to stick the black probe on one of the three input terminals and the red probe on one of the other input terminals and regardless of which I put where I got the same result: 16.7 ohms (see below). If I put the DMM probes to the six output terminals I get between 1.6 - 2.7 ohms resistance depending on which probe is on which terminal.
(https://i.imgur.com/bPAoWKW.jpg)
Regarding the thermal fuse - I can't figure out how to get inside the transformer - there are no screws and the metal plates it is made of appear to have been squashed / slotted together using some sort of hydraulic press... I have no idea if I can get into the housing - I assume the thermal fuse is inside this? Also the input terminals have thick transparent orange gunk all over them - it is solid and not sticky but I wonder if that should be there? It looks like something melted...
I will try your wiring check tomorrow; it's late here and I have to be up early for work :(
Moid,
I have a Technics SA GX130. I had only one issue in more than 30 years -> the power switch which is non-mechanical. No power at all. I had to take it to Service, few $$$.
The Tape Deck TR515 needs repair too, stuck mechanism, new rubber belts and motors mantainance.
SL PG300 CD player tray motor sometimes gets stuck. A gently touch and problem solved! :icon_lol:
mac
Quoteprobes to the six output terminals I get between 1.6 - 2.7 ohms resistance depending on which probe is on which terminal.
Not unreasonable. If you short your meter terminals together what ohm reading do you get 0.8 ohm or 1.0 ohm, 1.4 ohm?
The thing is to get the true ohm reading you should subtract off the shorted ohm reading. What that does is give you a more accurate reading and let you judge shorts or low ohms measurements better.
Anyway at this point the secondary looks OK.
QuoteRegarding the thermal fuse - I can't figure out how to get inside the transformer - there are no screws and the metal plates it is made of appear to have been squashed / slotted together using some sort of hydraulic press... I have no idea if I can get into the housing - I assume the thermal fuse is inside this? Also the input terminals have thick transparent orange gunk all over them - it is solid and not sticky but I wonder if that should be there? It looks like something melted..
The fact you get 16.7 ohms on two terminal makes me think the transformer and the thermal fuse is OK. For the three terminals, you got 16.7 ohm between the "common as drawn" and the other two. Do you get a very low resistance between the other two? I suspect the thermal fuse is OK and measuring across those two terminals is measuring across the thermal fuse.
That being the case I'd start tracing the continuity of the mains inlet through the fuse and switch and then to the transformer. I'm really expecting some sort of bad solder joint along the way and that's the problem!
On one of the variants of the schematic the mains goes through two coils (possibly that's a single core). The soldering could be bad in that area. You can even get cases where the coil wires break.
Quote from: Rob Strand on March 06, 2022, 05:10:55 PM
2) Wiring Check
- With the unit disconnected from the mains, enable the power switch.
- Using the ohms tester on 200 ohms trace *each* mains inlet wire through to the transformer input terminals.
- It's best to use the ohms scale and follow the wires by eye. You want to see 0.5 ohms or less for the wires.
Be aware if you put the meter on the wrong wire you will get false ohms reading (5 to 10 ohms) through
the transformer, which is the wrong way through the circuit. You want to follow the wires with < 0.5ohms
all the way from the mains inlet to the transformer on *each* lead.
Hi Rob
First, sorry about the slow reply, my job is horrible at the moment. I finally got a few minutes this evening to look at the amplifier.
Thanks very much for your suggestion - I did that and the resistance stayed at 0.4- 0.6 ohms throughout the circuit with the exception of a few places where it drops to 0 as long as I measure the lines that run from the lower of the two metal prongs in the power socket input. This was notably the two outer prongs of the power connection from the power board PCB to the input of the transformer. The inner prong was 0.5 ohms, the other two were 0 ohms. I tested this by holding one of my DMM probes to the metal input tip of the power socket and the other to the part of the circuit I was testing. I tried both metal tips in the power socket because they both have their own path across the power PCB, and was careful to make sure I didn't accidentally hit the wrong part of the circuit! If I measure from the top metal prong in the power jack I get 0.3 ohms everywhere on that line.
So I guess that part of the circuit is working? I removed the power PCB and transformer from the chassis of the amp so I could get better access to them and discoverd the power PCB has a capacitor hidden underneath it that I couldn't see before. It's a 103 (10nF) and I measured it without any power in the circuit and got a reading of 11.5nF - so close enough to be OK I think? The schematic shows this capacitor as being 10nF.
You asked about shorting my DMM probes together? I get 0.6ohms when I do that.
Quote from: Rob Strand on March 08, 2022, 12:20:09 AM
The fact you get 16.7 ohms on two terminal makes me think the transformer and the thermal fuse is OK. For the three terminals, you got 16.7 ohm between the "common as drawn" and the other two. Do you get a very low resistance between the other two? I suspect the thermal fuse is OK and measuring across those two terminals is measuring across the thermal fuse.
Sorry I'm not sure what you mean by 'common as drawn'? Do you mean put one probe on the centre prong and another probe on one of the two outer prongs? Presumably while it's plugged into the mains? Or something else?
Quote from: Rob Strand on March 08, 2022, 12:20:09 AM
That being the case I'd start tracing the continuity of the mains inlet through the fuse and switch and then to the transformer. I'm really expecting some sort of bad solder joint along the way and that's the problem!
On one of the variants of the schematic the mains goes through two coils (possibly that's a single core). The soldering could be bad in that area. You can even get cases where the coil wires break.
There are two copper coils on the main PCB, but nowhere near the power PCB... I will try to take some photos in daylight.
Quote from: mac on March 07, 2022, 06:35:54 AM
Moid,
I have a Technics SA GX130. I had only one issue in more than 30 years -> the power switch which is non-mechanical. No power at all. I had to take it to Service, few $$$.
The Tape Deck TR515 needs repair too, stuck mechanism, new rubber belts and motors mantainance.
SL PG300 CD player tray motor sometimes gets stuck. A gently touch and problem solved! :icon_lol:
mac
Hi Mac, yes if I can't fix this myself I will have to look for a repair shop... I made the mistake of looking at what a new Technics amp costs these days and almost had a heart attack! £2000! I had no idea they were so boutique! I guess 15 year old me had good taste in hifi - that amp cost me £100 in the late 80s!
QuoteI made the mistake of looking at what a new Technics amp costs these days and almost had a heart attack! £2000! I
:o :o
For only £1,999.99 I sell you mine, free shipping! :icon_lol:
mac
Quote from: moid on March 13, 2022, 07:24:47 PM
Thanks very much for your suggestion - I did that and the resistance stayed at 0.4- 0.6 ohms throughout the circuit with the exception of a few places where it drops to 0 as long as I measure the lines that run from the lower of the two metal prongs in the power socket input. This was notably the two outer prongs of the power connection from the power board PCB to the input of the transformer. The inner prong was 0.5 ohms, the other two were 0 ohms. I tested this by holding one of my DMM probes to the metal input tip of the power socket and the other to the part of the circuit I was testing. I tried both metal tips in the power socket because they both have their own path across the power PCB, and was careful to make sure I didn't accidentally hit the wrong part of the circuit! If I measure from the top metal prong in the power jack I get 0.3 ohms everywhere on that line.
So I guess that part of the circuit is working?
Well, from that test you would think so. However, if the both mains wires connect all the way through from the mains plug through to the transformer then you should be seeing mains voltage at the transformer in normal operation. Your mains voltage measurements in Reply #6 don't show this. So I'm not sure what's not matching up here.
I'm also not exactly sure which version of the wiring you have. Three mains terminals on the transformer implies the lower version on the schematic. (That's assuming three transformer wires in Reply #6 are in fact the main input. Your 16.7 ohm measurements in Reply #6 look in the right ball-park.)
Your mains voltage is 110V but the simpler mains wiring in the lower part of the schematic shows only 220V and 240V. For this version there is no mains *outlet* on the back of the unit and no mains voltage selector switch (see upper mains wiring).
It would help if you can confirm exactly what version you have.
(https://i.postimg.cc/ph3M7KmP/Technics-SU-600-Schematics-Mains-Wiring.png) (https://postimg.cc/ph3M7KmP)
Quote
I removed the power PCB and transformer from the chassis of the amp so I could get better access to them and discoverd the power PCB has a capacitor hidden underneath it that I couldn't see before. It's a 103 (10nF) and I measured it without any power in the circuit and got a reading of 11.5nF - so close enough to be OK I think? The schematic shows this capacitor as being 10nF.
Quote
You asked about shorting my DMM probes together? I get 0.6ohms when I do that.
Yes. What that means is the 1.6 ohm and 2.7 ohm secondary measurements in Reply #6 are actually 1.0 ohm and 2.1 ohm. As a rough judgment that looks OK for secondaries.
Quote
Quote from: Rob Strand on March 08, 2022, 12:20:09 AM
The fact you get 16.7 ohms on two terminal makes me think the transformer and the thermal fuse is OK. For the three terminals, you got 16.7 ohm between the "common as drawn" and the other two. Do you get a very low resistance between the other two? I suspect the thermal fuse is OK and measuring across those two terminals is measuring across the thermal fuse.
Sorry I'm not sure what you mean by 'common as drawn'? Do you mean put one probe on the centre prong and another probe on one of the two outer prongs? Presumably while it's plugged into the mains? Or something else?
So in reply #8 (upper/lower as per your pic) you have two resistance measurements on the transformer:
- Upper to center terminal 16.7 ohm
- Lower to center terminal 16.7 ohm.
If your mains wiring follows the lower version of the wiring in the schematic then the transformer has three terminals. two are across the windings and two are across the thermal fuse. Based on that I would guess measuring from the Upper to the Lower you should see close to 0 ohm. As a sanity check you should confirm this is true. If you follow the mains wiring you should see PCB tracks going to only two of the three transformer terminals. The unconnected terminal should be where the thermal fuse connects to the winding - as per lower version of main wiring in the schematic.
Quote
There are two copper coils on the main PCB, but nowhere near the power PCB... I will try to take some photos in daylight.
It's possible your board doesn't match the mains wiring on the schematic. If you see two coils far from the mains wiring then I suspect those are around the power amplifier IC (L401 and L402). Those aren't the coils shown on the lower version of the mains wiring.
This is an interesting fault. I do have a few observations to note.
1. Moid is in the UK which generally uses 220 - 240VAC so the amp would most likely use that voltage.
2. The 220V transformer's primary only uses 3 connections whereas the multiple input voltage transformer uses 7. The pictures that moid has uploaded shows only 3 pins.
3. The measured voltages are odd. Where were the test probes placed?
I suggest that a continuity / resistance test be done on the power lead itself. I've had those fail in some very odd ways before. Also, resolder the power switch as there appears to be a fractured solder joint there.
Edit: Also test the resistance of the power switch when it is on. They do fail and not always in an obvious manner. Don't forget to resolder the switch.
Both Rob and Jim have mentioned in their comments below some interesting things about mains appliance plugs used in the UK that I had forgotten about. But it did remind me of another little fun issue with older equipment with cylindrical glass fuses. Replace the fuses even if they test OK. That includes any in the amplifier itself. After many years of use the fuse element develops minor fractures which don't cause any problems until you pass current through it. A multimeter doesn't pass enough current so the fuse seems fine but the fuse won't work when you try to power up something like a transformer. The fuse goes open circuit but when you take the fuse out and test it with a multimeter it now tests fine. This is a rare fault but it does happen. Now some people will say BULL#@IT!!. Feel free to do so, but it happened to me twice last year. The fuses were between 12 to 17 years old and not in a heavy current situation.
Quote1. Moid is in the UK which generally uses 220 - 240VAC so the amp would most likely use that voltage.
So he is! That puts a spanner in the works for the 97V measurement he made earlier on!
That reading could be entirely spurious.
Maybe it's the mains plug? Don't UK mains plugs have fuses? Worth checking.
If it dates from the times when UK electricals were supplied without any mains plug and before they started dressing the wire ends with bootlace crimp ferrules - you had pre-stripped solder tinned wires to screw into your plugs terminals. This was always a reliability problem since the solder was lead-based and lead flows under pressure so contact gets looser and dirtier over time.
Quote from: mac on March 14, 2022, 12:13:07 PM
QuoteI made the mistake of looking at what a new Technics amp costs these days and almost had a heart attack! £2000! I
:o :o
For only £1,999.99 I sell you mine, free shipping! :icon_lol:
mac
I'm glad to see that mate's rates are still observed on DIYStompboxes :) I'll give your kind and generous offer some thought! Also sorry to everyone for disappearining, work has been awful. I now have a whole week of holiday (yay!) so will start replying to everyone's helpful suggestions.
I'm reading through all the suggestions (thanks very much everyone!). I apologise for not explaining that I live in the UK so my local electricity runs at 220-240V. I'm so used to thinking in 9VDC for stomp boxes that I completely forgot that everyone else in the world runs their electricity at very different amounts!
I will try changing the fuse in the plug itself as Slowpoke suggested, and I did find a fuse near the power input to the amplifier which is a glass one with a wire inside - my multimeter says it is fine, but I'm heeding Slowpoke's advice and wil try to get a replacement... which might be rather hard. The use has 7800mA 250V written on it, and I can't find those for sale anymore. Is it OK to use 8 Amp fuses instead? I can get some on Amazon quickly which are the fast blow glass fuse type:
https://www.amazon.co.uk/Pack-F8AL-Fast-Blow-Glass-Fuses/dp/B01M5CHTUE/ref=sr_1_7?crid=17486DM7CVV1X&keywords=8%2Bamp%2Bfuse&qid=1649685138&sprefix=8%2Bamp%2Bfuse%2Caps%2C99&sr=8-7&th=1 (https://www.amazon.co.uk/Pack-F8AL-Fast-Blow-Glass-Fuses/dp/B01M5CHTUE/ref=sr_1_7?crid=17486DM7CVV1X&keywords=8%2Bamp%2Bfuse&qid=1649685138&sprefix=8%2Bamp%2Bfuse%2Caps%2C99&sr=8-7&th=1)
I was unsure about swapping for a higher amp fuse because I'm sure someone once told me if I did that and the components in the circuit were rated for less than the fuse, those components could fry before the fuse would trip? I might not have remembered that correctly, but it sounds logical.
It's T800mA. T for time delay or slow blow.
Thanks anotherjim! you're totally right, I looked at the fuse in direct sunlight and what I thought was a 7 is actually a T! Just ordered some from ebay, hopefully they'll get here before the weekend. I'll be so happy if it's just a busted fuse :)
There's some good news and some bad news! First - the amplifier is fixed :) Yay! Rejoice! Dance around madly because I can play records and listen to the audio on my computer correctly! The bad news - I have totally wasted everyone's time :-[ Sorry everyone, especially Rob. I took Slowpoke's advice and changed the fuse in the mains plug (I did previously test this and my DMM said it was fine), but I swapped in a new 13amp fuse and lo and behold the amplifier turned on! Which would've been totally awesome if I hadn't had been resting my thumb on the power section PCB at the time, with it right on the two prongs of the mains input... jump? yes, almost hit the ceiling! I tell you what, 240volts is quite a wake up; works faster than caffeine that's for sure. Also the noise I made scared the crap out of my wife... which is not the best thing to do when you're playing with electricity.
I guess with the duff fuse in the plug I was getting 97V out, but not the 240 the transformer needed for the amplifier to work? The mains plug shows 245 Volts now! And Anotherjim was right - this amplifier was supplied without a plug when I bought it, I remember my dad showing me how to wire the plug - the plug itself is so old that it says made in Britain on it!
So... all's well that end's well?
Thanks so much everyone, I really appreciate your input and help as usual. I'll get back to breaking stompboxes soon!
QuoteThere's some good news and some bad news! First - the amplifier is fixed :) Yay! Rejoice! Dance around madly because I can play records and listen to the audio on my computer correctly! The bad news - I have totally wasted everyone's time
QuoteSo... all's well that end's well?
Thanks so much everyone, I really appreciate your input and help as usual. I'll get back to breaking stompboxes soon!
Great news indeed. No waste of time at all, that's life when you are debugging. The good thing is you didn't need to find any hard to get/expensive replacement parts.
QuoteWhich would've been totally awesome if I hadn't had been resting my thumb on the power section PCB at the time, with it right on the two prongs of the mains input... jump? yes, almost hit the ceiling! I tell you what, 240volts is quite a wake up;
Not good. It's a risk when you work with this stuff. I've developed a healthy sense paranoia over the years.
On safety, it amazes me that so many power transformers put the primary winding tags on the top side where it will be so easy to accidentally touch (our 240v) while the (usually) safer, low-voltage secondary tags hide out of the way down below. It wouldn't be so bad if they at least supply a safety cover (some do, but it's an optional extra that nobody stocks).
So, is there an electrical reason to place the primary of an EI transformer above the secondary?
Quote from: anotherjim on April 12, 2022, 04:23:12 AM
On safety, it amazes me that so many power transformers put the primary winding tags on the top side where it will be so easy to accidentally touch (our 240v) while the (usually) safer, low-voltage secondary tags hide out of the way down below. It wouldn't be so bad if they at least supply a safety cover (some do, but it's an optional extra that nobody stocks).
So, is there an electrical reason to place the primary of an EI transformer above the secondary?
I've seen the primary both on the top and on the bottom of transformers with either terminals or with wires. As far as transformers inside of equipment go, the majority have the primary on the bottom - the primary connections usually have wires. There's also over wound types, and side connections (rotated transformers).
I've used transformers from this series in the past (originally from the UK). I was never fond of those exposed terminals. I used to cover the "spokes" out the top and the wire terminal with heatshrink. Recently they have started to offer plastic shrouds for the connections which you buy separately.
(https://res.cloudinary.com/rsc/image/upload/b_rgb:FFFFFF,c_pad,dpr_1.0,f_auto,q_auto,w_700/c_pad,w_700/F0503978-02)
Here's soldered primary at the bottom (very common in AU for 15VA's and 30VA's from 70's to 90's),
[Click to embiggen]
(https://i.postimg.cc/1VKvKLDX/tx-pic-pt6672.jpg) (https://postimg.cc/1VKvKLDX)
(https://res.cloudinary.com/rsc/image/upload/b_rgb:FFFFFF,c_pad,dpr_1.0,f_auto,q_auto,w_700/c_pad,w_700/F0503978-02)
doesn't the australian standard say this needs a split bobbin? does it have a split bobbin obscured?
I have a question.
QuoteI took Slowpoke's advice and changed the fuse in the mains plug (I did previously test this and my DMM said it was fine), but I swapped in a new 13amp fuse and lo and behold the amplifier turned on!
13 Amp fuse? is that the standard plugtop-thing fuse the english comedians are always cracking funny about? and it was dead, you say? so what/where was the 800mA slo-blo one located? how does the 13A go before the 800mA one? or is something I miss?
Quote from: anotherjim on April 12, 2022, 04:23:12 AM
On safety, it amazes me that so many power transformers put the primary winding tags on the top side where it will be so easy to accidentally touch (our 240v) while the (usually) safer, low-voltage secondary tags hide out of the way down below. It wouldn't be so bad if they at least supply a safety cover (some do, but it's an optional extra that nobody stocks).
Kind of related to what you said - there was a plastic cover (loose) over the power PCB area that would prevent anyone who opened the amplifier up from geting shock if they had been stupid enough to touch that part while it was on... but I of course took the cover off so I could get to the power PCB to test it :)
Quote from: duck_arse on April 12, 2022, 10:52:50 AM
I have a question.
QuoteI took Slowpoke's advice and changed the fuse in the mains plug (I did previously test this and my DMM said it was fine), but I swapped in a new 13amp fuse and lo and behold the amplifier turned on!
13 Amp fuse? is that the standard plugtop-thing fuse the english comedians are always cracking funny about? and it was dead, you say? so what/where was the 800mA slo-blo one located? how does the 13A go before the 800mA one? or is something I miss?
The very same fuse mr duck! The fun thing was that with the DMM on that fuse I got continuity and it seemed fine - but switch it for a new fuse and the plug (and amplifier) suddenly worked! The 13 amp fuse was in the plug body itself - look below
(https://i.imgur.com/ZM96SWw.jpg)
So that plug was in the wall socket. The cable coming out of the plug went into the power input and power PCB of the amplifier. The 800mA fuse was part of the power PCB inside the amplifier.
Quotedoesn't the australian standard say this needs a split bobbin? does it have a split bobbin obscured?
On the actual units you can see the design uses a split bobbin. You can see the different wire thicknesses through the vent slots in the bobbin.
I don't have a copy of the latest Australian standard but I suspect it doesn't dictate a split bobbin; for example Toroid power transformers are always over-wound. Typically the standards dictate creepage and clearance distance together with breakdown specs.
RS components have been selling this style of transformer since before 1990 (IIRC back then people referred to them as Radio Spares). Over time the designs get revised to comply with changes to standards. The common transformers in au (Ferguson/A&R Transformer/DickSmith/Jaycar/Altronics/Arista) have been revised over time for the same reason. The incorporation of thermal fuses and recommended fuses in the transformer data is a result of changes to standards. I rememeber back in the 70's when Ferguson were one of the few manufacturers to claim they conformed to standards!
Quote13 Amp fuse? is that the standard plugtop-thing fuse the english comedians are always cracking funny about? and it was dead, you say? so what/where was the 800mA slo-blo one located? how does the 13A go before the 800mA one? or is something I miss?
I was going to check out the fuse characteristic but I don't think it's that.
[For completeness: I^2t rating of 13A BS1362 fuse is 870 A^2 sec whereas 800mA slow blow is 3.37 A^2 sec.
That indicates under pulsed loads the 800mA slow blow would be expected to fatigue first.]
Fuse are complicated because they can blow at currents less than the rating. They fatigue over time when exposed to spikes etc. Slow fuses are affected less than fast fuses but with the big rating difference of 13A vs 800mA even that might not account for the failure.
I think the simple answer is the 13A fuse probably suffered minor damage at some point which made it fatigue earlier. Fuses in the mains plug can suffer mechanical shocks if the plug is dropped. Fuses in the amp are unlikely to suffer mechanical shocks as often and the fact there are many springy mechanical mountings between the enclosure and the internal fuse holder will soften any blows. Over 20+years the minor weakness shows its head.
It's all speculation based on a sample of one!
Eek! :icon_eek:
Quote from: moid on April 12, 2022, 04:39:59 PM
(https://i.imgur.com/ZM96SWw.jpg)
For my peace of mind (and yours, I hope!), please rewire that plug so that the cable grip holds onto the black outer insulation, and the live and neutral are trimmed to an appropriate length.
UK plug fuses don't have a good mechanical seal of the end caps for the fuse wire so fatigue/corrosion happens over time.
Anything made now will have a plug like this ready fitted & fused...
(https://i.postimg.cc/NjTgZB4x/uk-plug.jpg)
The fuse wire is simply soldered or welded at the centre of the end caps. A partial/intermittent fault is possible that may pass a DMM continuity test, but any serious current flow causes a voltage drop.
The first thing I do with dead equipment is measure resistance between the AC plug live & neutral pins (cable plugged into the equipment but not AC supply of course) and switch it on. The load of whatever is inside (transformer primary usually) should show a relatively low resistance on the DMM.
QuoteK plug fuses don't have a good mechanical seal of the end caps for the fuse wire so fatigue/corrosion happens over time.
Anything made now will have a plug like this ready fitted & fused...
After going over the thread, that has to be it.
I only remembered Moid checking the fuse in the amp back in reply #18,
QuoteI will try changing the fuse in the plug itself as Slowpoke suggested, and I did find a fuse near the power input to the amplifier which is a glass one with a wire inside - my multimeter says it is fine,
But he had actually tested the mains fuse way back in his opening post,
Quote
I've checked the fuse in the plug and that has continuity. I tried plugging it into other power sockets (no change). I've opened up the amplifier and found a fuse next to the power input. I checked that (it has a metal filament in it) and that also has continuity.
The worst fuses for corrosion are the bullet-nose fuses used in cars made in continental Europe. Back when I had a Volkswagen (never again!), functions would cease to work then magically reappear. I finally realized that you had to spin the fuses one turn while still in their holders about every six months or there would be more trouble. Maybe if you put the original fuse back in, it would work properly since the act of removing or spinning a fuse is to grind off the corrosion.
Volkswagens need regular exorcisms to remain driveable. Even Lucas electrics are better.
Quote from: bluebunny on April 13, 2022, 03:39:23 AM
Eek! :icon_eek:
Quote from: moid on April 12, 2022, 04:39:59 PM
(https://i.imgur.com/ZM96SWw.jpg)
For my peace of mind (and yours, I hope!), please rewire that plug so that the cable grip holds onto the black outer insulation, and the live and neutral are trimmed to an appropriate length.
Guess what everybody, I'm back! Like a bad penny, I always turn up. I have to resurrect this thread because the damn amplifier has died again :(
First of all I would like to reassure bluebunny (and any other anxious observers who were kindly concerned that I was about to electrocute myself again) that the power plug is now repaired to a hopefully higher standard (see below)
(https://i.imgur.com/q7Y9p04.jpg)
(https://i.imgur.com/HSNnpfw.jpg)
Onto the actual problem. Typically this has happened because I've finally got August off as annual leave so was looking through some layouts wondering what to build this Summer and went to turn the amp on to listen to some music at the same time and the amplifier no longer turns on (has worked perfectly since I changed the fuse in the plug). Yes, it's becoming obvious that the universe hates me, or at least there is some random spiritual entity that I have offended that is scared of me having the time to assemble a fuzz pedal and so throws these distractions at me to make sure I can't enjoy life! I assumed the fuse was knackered again, so put another one in the plug... no joy. I tried two other 13 amp fuses just being paranoid but the amp still doesn't turn on. I stuck my DMM probes into the two inputs of the plug (the end that connects to the amplifier - not actually connected to the amplifier at that moment) and got a reading of about 242V AC, which seems right for the UK, so I think I can assume the fuse is OK and power is fine.
Next I plugged the power cable into the amplifier and started taking readings of the power section. Here's where I'd like to check my method of doing this with you, because I got different results depending on where I put the black probe of the DMM, and I am worried I'm measuring things wrong. If I place black and the red probes on the two soldered prongs (one probe to each prong) of the PCB where the power comes into the amp I will get around 242V AC. However to measure the current elsewhere in the circuit should I be connecting the black probe to the ground point on the back of the amplifier (see below)
(https://i.imgur.com/0Z9TOfb.jpg)
and then using the red probe to touch parts of the circuit?
Or should the black probe remain on the top prong of the power input?
(https://i.imgur.com/tclTaPi.jpg)
And use the red probe to touch the rest of the circuit?
I suspect this is correct (black probe used as above) but before I start measuring the rest of the circuit and doing it wrong, I'd love someone else's opinion please :)
I'm being very careful not to touch any PCB areas with my fingers this time! I've removed the transformer so that I could take readings from where the power circuit connects with the transformer - if my power readings are correct and the way I'm measuring are fine I will progress onto measuring the transformer. At the moment the socket that connects to the transformer shows the following readings (first number refers to the socket closest to the camera / me) when the power button is on on the amp
01: 1.8V AC
02: 242V AC
03: 0.471V AC
Any help / opinions / commiserations would be gratefully received! I hope all your Summer's are going better than mine!
Black probe close to power in, not that little ground post on the panel next to phono. Should not be different, but the post is for grounding a turntable. Maybe it is chassis ground.
Careful about words; "current" you measure (in amperes) by putting the meter in series and allowing power to flow through it, dangerous at high voltage. Voltages are what you are measuring, and what you should be measuring.
Because you don't have a grounding wire on the mains plug, the AC voltage ought to be measured between L & N at the transformer's primary input. However, Neutral is grounded somewhere back in the supply feed but it doesn't prove the plug neutral is connecting if you measure the live from the chassis ground.
*Brits should probably say "earth" for the plug/socket safety pin and not "ground" but the rest of the world seems to understand ground better.
Quote from: anotherjim on August 05, 2022, 04:32:59 PM...Brits should probably say "earth" for the plug/socket safety pin and not "ground" ....
"PE" will fool all the Americans.
If you have some sort of AC voltage on the PT
secondary, stop fooling with the 240VAC side! It works, obviously.
On the
secondary I normally stick Black in any handy screw-hole, call it common. In any domestic Hi-Fi, chassis IS common. The phono screw WILL also work, but don't grip a probe too good, unless you smoosh it out of shape.
Take the Red probe first to the BIG power caps. Expect zero on a common wire, plus 43V one side, negative 43V the other side.
And beFORE you connect any speakers, be sure you see near-ZERO on the Speaker terminals!
Thanks chaps for the fast replies! I can't do anything to test things until Wednesday (off to see my mother who lives a very long way from us). I was hoping to do some testing today, but one of our kitchen drawers decided it would jam instead (it was the one with all the knives in it of course) so lost most of the day to dismantling / drilling holes until I could free the stuck object - a knife had got stuck inside a drawer compartment and then also jammed behind part of the carcass of the cupboard, what a day!
Quote from: idy on August 05, 2022, 01:17:47 PM
Black probe close to power in, not that little ground post on the panel next to phono. Should not be different, but the post is for grounding a turntable. Maybe it is chassis ground.
Careful about words; "current" you measure (in amperes) by putting the meter in series and allowing power to flow through it, dangerous at high voltage. Voltages are what you are measuring, and what you should be measuring.
Brilliant, thanks very much Idy, I'm glad I was right (I was assuming it would be the same as an effects pedal, but when I noticed that post it did make me question myself). Sorry about the language confusion, my electrical knowledge is not very good.
Quote from: anotherjim on August 05, 2022, 04:32:59 PM
Because you don't have a grounding wire on the mains plug, the AC voltage ought to be measured between L & N at the transformer's primary input. However, Neutral is grounded somewhere back in the supply feed but it doesn't prove the plug neutral is connecting if you measure the live from the chassis ground.
*Brits should probably say "earth" for the plug/socket safety pin and not "ground" but the rest of the world seems to understand ground better.
Aaahhhh so 'earth' is a British thing? I have always wondered if the two terms meant the same or were different. I'm not sure which of (two of) the primary inputs are the Neutral one (I've worked out the central one is live!). So shall I try a reading with red probe on the centre input and then hold black to each of the other inputs to see what AC rating they show? Thanks anotherjim
Quote from: PRR on August 05, 2022, 06:18:53 PM
If you have some sort of AC voltage on the PT secondary, stop fooling with the 240VAC side! It works, obviously.
On the secondary I normally stick Black in any handy screw-hole, call it common. In any domestic Hi-Fi, chassis IS common. The phono screw WILL also work, but don't grip a probe too good, unless you smoosh it out of shape.
Thanks PRR - when you say PT secondary you do mean the seven prongs that voltage comes out of the transformer? I assume I should see a lower voltage reading on those prongs? I will test them all and get back to you on Wednesday.
The other reason I queried where to place the black probe was because I assumed that if it touched any bare metal on the chassis, that would be fine to establish ground; but when I tried this I got really low voltage readings on the power input prong (it went from 25 - 32 V AC!) which seemed really odd to me when I was sure it would be about 240V AC at that point. Maybe the metal has some sort of coating on it that would give a strange reading?
> I queried where to place the black probe was because I assumed that if it touched any bare metal on the chassis, that would be fine to establish ground; but when I tried this I got really low voltage readings on the power input prong (it went from 25 - 32 V AC!) which seemed really odd to me
You do not have the Earth/Ground pin connected. There is no connection from chassis back to 240V power source. (That's fairly common in small hi-fi.)
QuoteBecause you don't have a grounding wire on the mains plug, the AC voltage ought to be measured between L & N at the transformer's primary input. However, Neutral is grounded somewhere back in the supply feed but it doesn't prove the plug neutral is connecting if you measure the live from the chassis ground.
Yes, it's not a reliable mains measurement connecting the DMM to the chassis.
Quote
*Brits should probably say "earth" for the plug/socket safety pin and not "ground" but the rest of the world seems to understand ground better.
In au we also use "Earth" for mains earth. Ground is more for circuits.
If you look at the pic of the back of the unit you can see a symbol with two encircled squares. That means "double insulated" and typically the mains earth isn't connected on such equipment.
(https://i.imgur.com/0Z9TOfb.jpg)
For those not into HiFi the "GND" terminal on the back of the unit is normally used to connect a ground lead from the turntable (ie. Phono Input). That normally connects to the chassis ground at the amp.
If you want a safely grounded chassis, get a 13A plug and some green/yellow wire and terminate one end to the lug earth pin and take out the fuse. At the other end of the wire fit a croc' clip. However, also keep one hand away from the chassis when probing around high voltage parts. Electricity discharged to earth via your hands is likely to interrupt your heart!
Any voltage you measured using a floating (disconnected) chassis for the meter return can be stray/induced. The meter has a high impedance so will pick these voltages up even though they are feeble and can't do any work.
The power transformer primary only works of the voltage it's given, so that's where you want to measure - both probes across the primary feed. Only then will you know the actual supply voltage to the primary.
When something that did work stops working, don't ignore stupid simple causes such as the dog chewing through the speaker wires etc...
Don't forget the unit has its own fuse, the one right near the mains inlet PCB. (pull the mains plug when looking at this one.)
... and maybe that fuse isn't connecting to the fuse socket.
You can also check the switch contacts are closing. (mains plug pulled out if using ohms/conductivity.)
If you can connect the neutral correctly you should be able to get 240V all the way upto where the transformer connects to the the mains inlet PCB. If not backtrack until you find where it stops.
Some good news / closure to those who were wondering what happened to the amplifier (OK probably nobody, but I thought I'd leave the fix here in case anyone else needs it in future). I tried testing the amp and measuring the voltages on the transformer, then tried to get underneath the PCB to measure the big power capacitors (which would require a complete dismantlement of the chassis) and spoke to my brother in law (who is an electrician) who, after I described what I was proposing doing, strongly suggested that I either check my life insurance policy or maybe get someone else to look at it (not him of course, I mean why would he help...). Anyway my wife suggested that perhaps it was worth someone else taking a look at it (because the life insurance policy expired when we paid off our mortgage a few year's ago...) so I found a shop an hour's drive from me and took it in and they fixed it in an hour!
The fault (there were two) was a dry solder joint on the power jack connection to the PCB (which meant that if you tilted the amplifier slightly at certain angles power would go through, and not at other angles!) and another similar problem with the two brackets/ grips (no idea what these are called) that hold the time delay fuse on the inside power PCB - those were also loose, so he adjusted them and re soldered the connections to the PCB. So nothing had actually died; just the solder had given out! And the repair bill was £36, which was a bargain. So all's well that end's well!
If you're in the UK and within driving distance of Bedford and need an amp repairing, I can totally recommend Mick Newman at MC Electronics.
QuoteSome good news / closure
Good news indeed, was bound to be something like that.
Funny that you got two doses of trouble.
Double trouble seems like my style! I doubt I would have ever found those problems myself, I guess that's why it is totally worth paying someone with years and years of experience (and the added bonus of not killing myself of course!)
QuoteI guess that's why it is totally worth paying someone with years and years of experience (and the added bonus of not killing myself of course!)
Very true. Unfortunately a lot of debug problems which come up on the forum can be fixed easily but there's no one you can take it to - so you end up with very long threads solving very simple problems.