My father has a Walter Woods amp that has been broken for many years.  It stopped working because of one of two things (he can't remember what happened it's been so long):  

1) running the amp with too low of a load on the speakers - it's an 8 ohm amp that might have been run at a lower ohmage

2) mismatched fuse - when I got it from him the fuse was missing.  He remembered having trouble finding the correct fuse for it (2.5 amp 125 volt slo-blow), and may have used the wrong one (not so smart)

Does anyone have any recommendations on how they would troubleshoot the amp?

Thanks very much,
travis

is it S/S or Tube ?,
JD

It's solid state.  I was wrong about the impedance - it's a 4 ohm amp.  I found a link or two that show/tell a little about the amp.

http://www.doublebassguide.com/4_amps.html (scroll down to the walter woods section)

Here's an excerpt (link is at bottom) that seems pertinent:

Walter Woods Mi 100 - 2 channels (No Website) "This MI100 is from 1987 and has 2 channels. The user manual explains that the equalisation frequencies (bass, medium and treble) are slightly different for both channels. These corrections are efficient (+/-16dB). The front panel, red brushed aluminium , is tilted for easier knob access. The inside view shows us the typical architecture of these amps. The pre-amp circuit board is vertically mounted. This solution is possible by the use of specific jacks and potentiometers. This choice optimises space but makes it less accessible for difficult repairs. You must dismantle in order to solder. For this pre-amp, Walter decided to use a couple of transistors to substitute audio integrated circuits. Was it to obtain a better noise characteristic? I think not. Moreover, this pre-amp has bad distortion (between 3 and 10%!). Later, Walter will give up this very trendy concept for the following models. A central barrier improved the shield of the pre-amp and the rigidity of the box. The power amp, with bipolar transistors equipped, is very classical. It delivers 75W/8 Ohms and 105W/4 Ohms. The power supply deserves consideration. This is the key of the concept. It delivers around 100W but its size is no bigger than a handful of cherries. On the schematic, there is a mysterious rectangle named " switching power supply ". In fact, this is an auto-oscillator working around 100khZ. Compared to power supplies with fixed frexency, this solution is less cumbersome because it reduces the quantity of components, but may cause fallibility problems. Purists will note two points: the frequency of the oscillator varies according to the load and the transformer heats up because it works in the magnetic saturation"

Does this make sense to you guys?  

http://www.jazzguitarfaq.com/FAQ00012.htm

Thanks again for the help.

I went out and got a fuse for it, and hooked it to an 8 ohm speaker.  When I turned it on, the light came on like it wanted to work, but there was no sound at all from the speaker.  No pop when I turned it on, no static, nothing.

Warning: long!
number one: the impedance won't be much of a problem (unless it's a gross mismatch - read short)

number two: stick in the correct fuse, and make sure you have some spares. Make absolutely sure it's set to the appropriate voltage setting for your local mains supply. Turn it on and see what happens. If the amp works, vow never to use the wrong fuse again and be happy. If it is dead, move on to the next step. If any flashes, bangs, whisps of smoke etc. occur, note them and immediately switch the amp off. Report back here.

number three: Have a look.
Identify the point the AC comes into the amp (not plugged into mains!!). This will also be where the switching PS is. There might be some input filtering (usually in the form of a metal box that's part of the AC connector and a varistor - a big round disk connected across the AC lines. Ignore these for now)
The entire PS circuit should consist of an oscillator (probably a collection of transistors, resistors and caps. small signal stuff), a power rectifier (probably 4 1N400x) with a bit of electrolytic filtering, one or more power (MOS)FETs (cooling!!) that modulate the rectified AC (and are controlled by the oscillator) , a small transformer and a not so heavy rectifier with small caps. Assorted small coils, opamps can also be in there but the basic build-up is rectifier-filter-modulation-transformer-rectifier-filter.
Phew.

Intermezzo:
What a switching PS does is rectify AC mains coming in, then modulating it at a much higher frequency.  Galvanic separation and voltage conversion can now be accomlished with a much smaller transformer: the core size is determined by the frequency it needs to work with (larger for lower frequencies) so a 100kHz transformer can be much smaller than a 50Hz one,  able of handling the same power. After this stage, you also need a much smaller cap to filter out noise (high frequencies, remember?)
I don't quite get the saturation bit: saturating the core indeed makes for lots of heat, and other nasties. Pretty white smoke even, when you take it too far. Perhaps this is possible because of the higher frequencies.
Another benifit of switching PS is that these things can monitor and regulate themselves. These things moslty work with square waves (hmm...saturating, huh?),  so think of a square wave. "low" is 0V or off, "high" is max voltage or on.
Now, get this. If you change the ratio of ON versus OFF (so you change the pulse width, but not frequency), you can see there's a different amount of charge pumped into the filter cap after that , right?
This means you can (within reason) control the output voltage by changing the ON pulse width. You get more voltage with longer ON pulse width. The same thing goes for increasing frequency while keeping pulse width constant.
So, if you make the oscillator react to output voltage, it regulates itself to
always put out the same voltage, no matter what you feed it. This is how those modern light-weight cellphone adapters work, that take both 110 and 230V without setting switches or something.
Still here? Respect.

Okay, end of intermezzo, step...
number four:
review: the amp is completely dead (no lights, no hiss, no NOTHING), the fuse is still ok and there are no obvious burned components or loose wires and such. <edit> I just realised the amp might use several PS's, with only the poweramp running off the switching one. Check. Measure if the power transistors are getting power(30V or more), and check if the preamp is getting power(5-30V). if everything IS getting power the problem is not in the supply section and you've read all this for nothing
If the poweramp is the only thing not getting power, we're still on track.
<end edit>

Now, plug the AC cord in with the amp opened up and BE CAREFUL!
switch the amp on again. Just to be sure shield your eyes at this time. Always shield your eyes when switching on a suspect device when the case is open. Stuff occasionally flies. At speed.
Still nothing?
Check that there's actually AC coming INTO the amp. This is best done at the large, first rectifier bridge. That ok?
Check that the rectifier is churning out rectified voltage by *carefully* measuring across the larger caps just after the rectifier. That also ok?
That means it's either the transformer (which I doubt - although, that saturation blurb...), the power MOSFETs, or the PS oscillation won't start up.
See if you can find, in the Switching Power Supply, one or two large resistors. I'm talking 1W at least, probably a little more. These should be the only ones like that around the supply. Take out your iron. Unplug the amp if you haven't already.
lift the resistor(s) at one end so you can measure them. It's very likely these are a LOT higher resistance than they should be (a lot of Megs to infinite) - there's your culprit. Replace them with the correct value, with a powerrating AT LEAST the same as whats in there.
This is the most common problem with switching PS's, and causes the oscillator/MOSFET combo to simply not work.
Otherwise it might be the MOSFETs, transformer, or....

I hope this helps...

Thanks so much for all the help!  Next weekend I'm gonna tackle the Woods and see what I can figure out.

(1) Look for obvious failures. In SS amps, you often find small burn marks or burned out components by just looking. If this has happened, repair the part and in some cases the charred PCB material first. Charred PCB material is conductive, and must be cut or sanded away.
(2) In most SS amp failures, the outputs die. Before you ever power the amp, test the output transistors and find out if they're dead. Bipolar (NPN and PNP) transistors tend to fail shorted, MOSFET outputs tend to fail open. Check the driver transistors too, as SS amp failures often have a chain of destruction that goes back into the power amp drivers, sometimes even the voltage gain stages.
(3) Make up a soft start adapter. This is a jury-rigged AC power cord that puts a 120Vac light bulb socket in *series* with the AC power line. Light bulbs auto-limit AC line current in inverse proportion to their wattage. For instance, a 25W bulb will let about 200ma of AC line current through. If you put a whole amplifier in series with it, then the bulb limits AC current to less than 200ma even if the amp is dead shorted. This lets you use an assortment of bulb wattages to let more current through to the amp as you fix things, and also lets you measure voltages, etc. without popping fuses like popcorn.

The standard disclaimer goes here: if you don't already know how to wire and work on AC power line circuits, don't go messing with them, even to the extent of opening up that amp. If you don't already know how, get someone who does know how to do this safely to help you. It's not worth dying for.

(4) Once the outputs, drivers and other devices are known good, do a limited power on test with the series light bulb. Start with a low wattage bulb, 25 to 50W. Use no load (that is, an open circuit) for this test, as SS amps don't care if they run a high resistance load.  The light bulb brightness is an indicator of current drain. A good amp will have an initial pulse of bright, then settle down to a dim glow. Constant full brightness indicates a short somewhere.

(5) The first test is for DC on the speaker outputs. If there is DC on the outputs, something is major dead, usually an output, but possibly any device in the power amp. Once you establish no shorts, no DC on the outputs, measure the power supply voltages. These will be balance bipolar ( plus and minus ) in modern SS amps. Usually they will be about +/-30 to +/-70Vdc. No power supply = no operation.
Check at the power supply caps first, then at the output transistors.

That'll get you started.

RG put in the obvious firts steps I missed in my switching-supply-enthusiasm (experience keeps one's head screwed on nice and firm), but:
RG wrote:

(3) Make up a soft start adapter. This is a jury-rigged AC power cord that puts a 120Vac light bulb socket in *series* with the AC power line. Light bulbs auto-limit AC line current in inverse proportion to their wattage. For instance, a 25W bulb will let about 200ma of AC line current through. If you put a whole amplifier in series with it, then the bulb limits AC current to less than 200ma even if the amp is dead shorted. This lets you use an assortment of bulb wattages to let more current through to the amp as you fix things, and also lets you measure voltages, etc. without popping fuses like popcorn.

This is a very good tool in most cases, but it is my inderstanding you should NEVER use something like this, or a variac (even worse), on switching supplies. The circuitry wants to force a certain output no matter what comes in, and may go rather haywire if you feed it something lower than what it is designed for.
RG?

Sorry - you're correct - I skipped reading that part.

The high frequency power supply puts this in a different perspective. The power supply is more likely than the rest of the amp to be failing. That being the case, a novice should think very carefully about whether they will survive the repair process.

I designed switching power supplies professionally for a while. I survived some of my dumb mistakes, but part of that was luck, part of it was a fairly complete "cocoon" of protective equipment and experienced advisors. The risk of dying while working on a high voltage switching power supply is even higher than that for a conventional AC power supply. The AC line rectification capacitors alone store a fatal charge even with the power supply off unless specifically designed to drain it away.

An indicator light is a good sign, if it's an LED. That would say that the power supply is functioning. I'd amend my advice to leave out the AC series adapter, and to start with measuring the power supply voltages. If you have +/-30 to +/-50Vdc out of the power supply, fine, go ahead and work on debugging the amp.

If the power supply is not providing good power, I'd advise a non-experienced person to junk the power supply and get a good one from somewhere, or get an experienced tech to work on the power supply. The chances of dying are just too high to start mucking about inside one of these for a novice.

Ok. I guess I'll get qualified help.  My experience is mostly tinkering with guitar pedals, and getting fried doesn't seem like much fun.  I feel bad for you guys typing out all this help and then me wimping out, but it would appear that your advice may have saved my life.

Thanks again guys,
travis

Hi,

Quote from: tcobrettiI feel bad for you guys typing out all this help and then me wimping out, but it would appear that your advice may have saved my life.

Don’t feel bad, this is a great tutorial on things to look for when fixing broken amps. This is timely for me as I have 3 stereo amps to repair and 2 possible tubed guitar amp projects this summer.

A BK Stereo 140 where one channel is severely distorted. All the voltages seem ok and powers supply seems ok.  It would be nice to get my good stereo working again.

An Accuphase power amp where my friend put in one of the channel circuit cards in backwards which caused that channel to fail. He told me it had a bit of fireworks inside when he turned it on. He claims the one channel sounds fine. He gave me the amp about 4 years ago. It probably hasn’t been plugged in for 6 years.

A Peavey CS400 power amp that the output transistors have all been removed. Someone started to take it apart to repair and then gave up. So it is missing a bunch of small spacer hardware and screws. So I’m not exactly sure how the whole heat sink assembly, that has the output transistors on it, is to be hooked-up.

I rescued out of the garbage a power amp section from an old RCA Victrolia console and its speakers. This looks like it could have been one of the top models in the late 40’s or early fifties. One of the capacitors has a 1947 date on it. So I was thinking of working on a low power (5 watt?) guitar amp. I have no idea when this was last plugged in.

I also picked up an old tubed Sansui stereo receiver. Not sure of the power but it looks like it would be around 20 watts per channel, very nice looking transformers.  I have no idea when this was last plugged in.

I think I will start by building the soft start (light bulb) adaptor since none of the above amps are using a switching power supply.

Regards,
Will

No amount of typing is too much to save a life.

These sorts of question are not too uncommon so now we have a great thread with great info that I'm sure will get used over and over by others.

Take care,
-Peter

PS: I've been knocked silly (OK, silly-er ) by the charge sitting in the primary rectifier stage of a switcher without bleeder resistors, and that was a couple of DAYS after the supply was plugged in last. In 120V land, those caps sit at about 170 volts and they hold enough power to do a couple of serious spot welds. :shock:

Hey Peter, RG, how many times somebody Meggar a Diode and then hand it to ya, talkin owch !  :lol:  not to funny at the time if it happens to be me !  ac and dc are very lethal and seeing somebody hit by high voltage is something you don't wanna see or smell.
you have to have High respect and use all safety precautions that go with the type of voltage you are dealing with. all it takes is 1 sec. for a mistake and that 1 sec. could be your last.
JD

This doesn't specifically apply to amplifiers, but is in keeping with capacitors holding charge.  About twenty years ago, a tech was repairing one of the computers where I worked.  We're talking a DECSystem 2065, not a PC.  Anyway, he must not have been looking because he managed to get one of the capacitors in the power supply to discharge, which sent him flying several feet.  Amazingly, he was unhurt -- but he was "all shook up".

Yeah, I've been there too. Still have the scar.
Will: I have experience with several CS400's and 800's (old style, not the newer ones - no LED bars, logo embedded in the front. These are actually better IMHO than the newer stuff)
If the cooling tunnel is gone (well,  the cooling-fin assembly) you might have a problem getting it all to fit. The temerature sensor hooks op to that as well.
Next time we service one of these, I might take some pictures for you (although it might be a while - they're in ampracks built into the building, more or less)