slightly OT: Repairing a Peavey PA-200 power mixer...

[Marek]

Hello!

First of all, sorry for this 'crossposting', but you guys are the only community I kinda know and trust. And, the issue here is sort of related to what's normally going on around here, anyway...

I've posted a few photos of the power amp I got today, which I am supposed to repair:

http://pa-200.morokvasic.com/

Now, I have a few questions:

1. Can I switch on the amp to test it, or should I first try to find out what exactly is missing, and replace it, and only than switch it on?

My father thinks that, since few components are missing ( 3 resistors and a power transistor) they (or the lack thereof) could pull further components into death, the moment the amp is switched on... Unfortunately my dad can help only over the phone, and that's kinda complicated... What do YOU think should be done in this particular case?

2. Does anybody have the schematic for this thing? (Peavey PA-200)

I contacted Peavey, and a very nice  service dept. guy  said that the service manual is on its way (email), but... e-e. Nothing happens...

3. What is the function of the huge black transformer on the left hand side
of the photo? And the huge metal capacitor (3500 MFD)? Is the small transformer on the right hand side the 'normal' 220V -> 15/18/36V or so transformer??

4. The 0.68 Ohm 10 Watt resistors are connecting to the emitters of the Power transistors. The green wires are connectiing the two middle taps of the small transformer to the collectors. That leaves the red wires to carry the signal to the base of the power tranny. Did I get it right?

5. I'm open for whatever suggestion you might have (except for 'take it to the qualified service person' :-)  I want to try and do it myself, that's the point of this...).

Thanks in advance,
Marek

[niftydog]

first impressions - you're pretty screwed unless you can get a hold of the exact schematic diagram. It could be fixed without it, but it'll take a fair bit of skill and calculation.

Can I switch on the amp to test it

I don't see why not, but from the looks of it that won't tell you anything that you don't already know. There's a chance that your Dad is right, but without actually knowing what was pulled out and it's relation to the schematic it's almost impossible to say.

guy said that the service manual is on its way (email), but... e-e. Nothing happens...

hassle them again, it'll be worth it if you get the schematic.

What is the function of the huge black transformer...

all that is power supply.

I don't know enough about amps to speculate about the other transformer, maybe a transformer coupled power amp... does that even make sence?!

Hell, I'm a video tech, ok!!!  :oops:

Someone here is sure to chime in on some other details for you.

What is that cylindric black thing between the two power transistors???

thermostat.

Transistor in a TO-220 (?) housing is a "EP430".

doesn't gel, what's the other marking on that one? C7631 or something like that?

oh, and that RCA "transistor" sounds wrong... it say 7915 which implys it's a voltage regulator, but without knowing the fourth one it's hard to say.

[Tubebass]

One of my bandmates has one of those....he used to use it as his guitar amp! I can borrow it and check part values etc. if you need the info. I can also take digital pix of the board and send them to you if your E-mail inbox is up for some fairly large files.
    Let me know.
                                       Dave

[Marek]

Hey, guys!! Thanks for the info!.

niftydog, you're right, there is another line printed on the 'EP430' thing and it says:  'A7831'.  

The 4th power tranny is most probably the same type as the rest, cos' the wiring is absolutely the same (they are wired in parallel, two and two).

That's what you get when you take it to a very cheap repair shop. It really gets 'fixed'... :?

Dave, I sure could use some photos of the:

1. four transistors at the back-plate of the housing. ( one just needs to unscrew those plastic protective caps. 1 screw per tranny only)

2. The top side of the power PCB: I need to see the resistor colors of those four 1/2W resistors which are located on the right side of the huge
10W resistors.

3. As many photos of the PCB as you can make.... :-)

*** WARNING! ACHTUNG! ATTENTION! PAZNJA! VNIMANIE! ***

I THINK THAT THE HUGE METALL CAPACITOR ON THE LEFT HAND SIDE OF THE BACK PLATE CARRIES 'serious' VOLTAGE!!

"DONT TOUCH! OTHERWISE - OUCH!"

(I always knew I should have started writing songs and making money doing it... this example would make a good late 80's rap hit :-) )

OK, that's it for the moment. Thanks again.

Greetings,
Marek

[Marek]

Forgot to write my email address:

marcomenator@gmail.com

--------------------------------------------------

And...

If anyone here has a schematic for this thing (Peavey PA-200 power mixer with the PA-120 Century Series power amp), please email it to me!

Thanks,

Marek

[Marek]

Friends,

the saga continues...

I got the PA-200 schemos today. Peavey Service ruleZ!  :-)

http://pa-200.morokvasic.com/PA-200_schematic.pdf

Well, here's what I found out so far (photos included):

http://pa-200.morokvasic.com/pa-200-2.html

niftydog, you were right, the small transformer is actually "driver transformer", it drives the 4 power transistors.

The thing (small transformer) seems to be dead. It shows 77,4 Ohm on the primary, 6,4 on secundary(1), and nothing(open) on secundary(2). Maybe my measuring is wrong. I havent taken it out of the circuit...

Further, the Transistor supstitutions are as follows:

"953" = 2N3391A
"RCA 62792" = 2N3055 ( Toshiba et all. )
"EP430C" = TIP 29C

-------------------------------------------------------------------------------

R.G., Mark Hammer, Jack Orman... all you gurus, how would You start repairing this thing?

--------------------------------------------------------------------------------

Greetings,
Marek

[Marek]

Hello again!

Now I have bought all the parts, exchanged every component in the circuit which comes after the driver transformer (except for the ceramic caps, I figured that they didnt die ), and everything looks fine... Only,
I haven't plugged it in yet, I've tried testing with a 9V battery first (the logic was that 9V battery cannot destroy anything in the circuit, and it will simulate power supply, only smaller :-) )

Well, the V+ test point is OK - 8,56V.

Now, the test point where V+/2 is expected gives way less than V/2.. something around 0,35V (!).

Any suggestions?

Greetings,
Marek

[niftydog]

you should test that transformer out of circuit. I know it's probably a pain, but it's the only way to be sure.

can you give me some direction to those test points on the schematics? I can't see 'em for looking!

[Marek]

niftydog, thanks for the reply.

About the 'test points':
where DC enters the circuit, it should read according to the schematic + 74 V. That's my "V+" test point.

At the collectors of the lower two (2N3055) power transistors it should read exactly the half of the "V+" = 37V DC (according to my logic & by looking at the schematic). That's my "V/2" test point.

About the V/2:
The resistance should be absolutely the same from the Ground rail and from the V+ rail. It's 1K + 15 Ohm from both sides ( although on the PCB I'm getting 1050 Ohm from Ground to V/2 and from V+ to V/2 I'm getting only 720 Ohm I bought mega new, mega cool, mega chic 1K 5W 5% resistors and soldered them in the board in place of the old ones!!

--------------------------------------------------------------------------

Ha! I plugged it in the 220V!! And nothing happens... :-(

So, first I have tested the Power Transformer, took the secondary winding contacts from the PCB, and measured: ~ 56V (slightly more). Cool! ...not... When I plugged the AC contacts back into the PCB, I wasn't getting any(!?) voltage anymore. (I measured at the rectifier diodes)

So i took the (secondary winding ) AC contacts out of the pcb again and connected a 100W light bulb across them. Cool, it lights up quite brightly.

Now I reconnected the AC secondary windings to the board again. Nix... No voltage no nothing. And none of the parts are getting warm or hot?!

If the power transformer is dead, how come the light bulb lights up so brightly? If it's not dead - I'm getting ~56V AC from the secondary winding after all - then why am I not getting any voltage from it when connected to the PCB??

I tried the contacts with the 'beepser' (continuity tester wich beeps:-) and everything is fine, that is - the cables/connectors are OK.

Any idea? Thanks so much for your help. I can't wait to hear from you again. :-)

Greetings,
Marek

[niftydog]

you need to be a bit more careful with your DC checks. (that is; measuring components such as resistors)

Those resistance readings you're getting in the biasing circuitry might well be correct; You cannot reliably measure components in circuit with a multimeter.

If you're going to the trouble to replace those resistors, that's well and good, but you should follow up and test the old ones out of circuit to see if they are actually cactus, and the new ones in circuit to see if you're getting the same readings as before.

(Incidentally, note that part of the circuit is associated with that untested coupling transformer! The fact that the 1st secondary winding doesn't react at all sounds bad to me.)

Have you measured from the DC rail to ground? Could be a short there somewhere.

Does this capacitor look fried?

Looks like it's been pretty hot.

I really think you should suss out that transformer properly, you could be wasting your time and money when all the while it's the bung transformer to blame.

some other tips to help eliminate causes; you can disconnect the pre amp from the supply by pulling the 2k 5W resistor at the top of page two. You can also disconnect the primary of that (suss) coupling transformer by pulling the 700ohm 5W just below it. (it actually says 700 5k, but that seems like a typo.)

[Marek]

niftydog, thanks again for your help. I agree that this troubleshooting is getting nowhere unless I take out the driver transformer and suss it, as you say :-)

If you're going to the trouble to replace those resistors, that's well and good, but you should follow up and test the old ones out of circuit to see if they are actually cactus, and the new ones in circuit to see if you're getting the same readings as before.

I've taken those thick resistors out & checked them... Originally, those 1K 5W limiting resistors were: 810 Ohm (between V+ and OR) and 734 Ohm ( between V/2 and BLU ). While being soldered in the PCB they were: 435 Ohm and 735 Ohm.

My new resistors read: 735 Ohm between V+ and OR and 975 Ohm between V/2 and BLU

The transformer gives:  992 Ohm between BLU and YEL and 5,6 Ohm between GR and OR (measured in the circuit).

Now my next step is taking out the transformer, and measuring it. I'll get back to you with the results. Altho' I'm hoping so much that this driver transformer is not dead... Where would I get a similar one to replace it? :-(

But, not having any AC voltage on the PCB, while there are ~ 56 V AC across the secondary when measured "in the air"  (not connected to the pcb) wonders me. Neither the Amp Fuse nor the Circuit breaker in the house installation are reacting...

Well, that's it for now. The moment I have some new info, I'll get back to you.

Thanks A LOT for helping so far. If you ever come to Berlin, consider yourself invited for a drink...

Greetings,
Marek

[niftydog]

The transformer gives: 992 Ohm between BLU and YEL

again, to be sure you need to take it out of circuit... BUT I would normally expect a much lower reading because the coil resistance is usually very low. At least you would expect something more like what you're getting between GR and OR.

not having any AC voltage on the PCB, while there are ~ 56 V AC across the secondary

That doesn't surprise me... because the AC is rectified and filtered imediately after the secondary. There might be DC, but if there's AC then you have a problem!

Where would I get a similar one to replace it?

beats me. You could try the peavey techs again and ask if they still stock spares or maybe they can give you specs with which to get a custom transformer wound elsewhere.

[Marek]

Quote:
not having any AC voltage on the PCB, while there are ~ 56 V AC across the secondary

That doesn't surprise me... because the AC is rectified and filtered imediately after the secondary. There might be DC, but if there's AC then you have a problem!

Heh, I know that rectifier diodes rectify the signal, and produce DC out of AC voltage, but I was measuring AC at those spots on the rect. brige which should in any case show AC voltage.

Consequently, I don't have any DC across the big filter-capacitor either... It's a very wierd situation. I thought maybe the oxidaton on the contacts could be producing a huge transition resistance?? And therefore no AC on the PCB tracks. And no DC altogether :-(

Thanks for the input.

'Stay in touch' :-)

Greetings from Berlin,
Marek

[niftydog]

ah, misunderstood you.

Have you measured the resistance from the DC rail to ground? There may be a short circuit there which would short out the DC.