Repairing a vintage solid-state guitar amplifier

The power amp appears to be direct coupled so I'd guess a few blown up transistors back down the line too.

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    Dave Plowman        dave@davenoise.co.uk           London SW
                  To e-mail, change noise into sound.

use a variac (variable AC source) to slowly bring the voltage up. wire a light bulb in series with it. (about 100W for this case i guess) if the lamp starts to glow brightly when you bring up the voltage with no audio signal then something is still wrong.

Some amplifiers of this vintage were DC couple all the way back to the pre-amp'. The only sure way to repair them was to replace every transistor on board and very carefully check all resisistors and caps' before firing up.

You may also find such devices as pots to balance the output transistors bias which if not properly set will again send out smoke signals.

The things really were a pain in the arse with some amps' blowing up even if the loudspeakers were left disconnected!

I tested several of the little transistors as best I could, with them still on the board. However, perhaps I should have taken them off the board (tedious) to be sure.

Thanks. The pre-amps are working OK.

Yes, there is a "DC Balance" preset and a "Bias" preset.

Oh, it all sounds too hard. Plan B beckons.

for the NPN, the MJE270 looks good-- same case and higher voltage. and

48 cents!

for the PNP try MJE172, a whopping 68 cents at digi-key.

Do check all the other transistors... there's usually a few more with dead shorts, easily found by an ohmmeter.

And do power it up gradually and check the voltages along the way to prevent another blowup.

If you do carry on make sure you check the bias pot for dead spots. If they go O/C they effectively switch the OP transistors hard on - BANG!

You are either a brave man or an idiot trying to repair it!

Was it the new resistors that smoked? Have your new transistors both gone short circuit or did you beat them to the switch off? I would try temporary insertion of much higher value resistors instead of the

0R27s e.g a couple of 1/2w 3k9. This should enable you to check through with a DC meter (no signal) to see which of the drivers are dead.

Geo

AIR One other way to disable the O/P transistors while testing is to simply short both bases together.

In light of people's responses, I looked again at the circuit and could see that it does all look rather fragile. Although I am an electronic engineer by education, I don't have all the test gear (Variac etc.) and stocks of components that I used to. So I have decided to take the coward's way out and replace the power amp by a ready-made module. I've been able to find one of the right rating that works with the same mains transformer as the amp already has, keeping the cost and modifications to a minimum.

I just wanted to thank everyone for their help and excellent suggestions. I'll be better informed for my next project, certainly.

As a matter of interest, which module did you use?

Ron(UK)

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(The description of a "200W" amplifier is marketing BS. It's actually a

70-100W amp, ideal for the Rick which was originally 75W).

This seems to be a quite modern VINTAGE transistor amp, as the schematic says "1977". A complementary symmetrical emitter-follower darlington circuit as output stage, input stages are quite modern-looking as well (differential pair, current generators etc...).

leaving the output transistors disconnected allows you to apply power to the circuit to make DC-measurements.

Using a light bulb in series with the mains supply has saved many output transistors when testing amplifiers. Even if the DC readings seem OK, there might sometimes be HF oscillation which can burn the transistors. Btw, check the R/C network in parallel with the speaker...

Replacing the output stage with a power module is a bad idea: all kinds of modifications drop the value of the amp and make it a "do-it-yourself electrical equipment". Repair the amp with proper components, and you'll be happy with the results.

Btw, a vintage transistor amp in my opinion has germanium output devices, both transistors are PNP, power supply is single-sided and negative, speaker is connected to the circuit via a big electrolytic capacitor, and there is a transformer between the driver stage and output...

Matti

It all depends on how you measure the power doesn't it? Are we talking of power input, RMS output or peak music power?

In the days of the amp' mentioned manufacturers (and sellers) could juggle with these figures all day long to come out with ridiculous figures and an ignoramus customer would often be shown the ac power input which due to the sheer inefficiencies of the day could be 3 times the RMS output figure. Peak music power could be quoted as 3 times this again! Fun eh?

Hmm CPC dont seem to stock them, I wonder how they stand up to abuse. I much regret the discontinuation of the Maplin 150watt Mosfet Amp which was perfect for resurecting old 70`s H=H and Carlsbro amps. I must have built well over 50 of those kits.

Ron(UK)

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Keep in mind, he's using it as a quitar amp, not a hi-fi or PA amp. We don't mind if it clips, in fact it's a desired effect.

The Velleman kit is also from Maplin.

I built one of the 150watt MOSFET amps about 15 years ago and it served me well for 10 years.

I can't give the same recommendation for this Vellman module, as it just went up in smoke after working (with good undistorted sound coming out and everything) for 5 minutes. No doubt I did something wrong, but I'm completely at a loss to know what.

Completely disheartened. Giving up on whole project.

Yes - but not a Maplin kit.

I've still got several in use around the country. ;-)

I really miss the genuine Maplin kits - they seemed to address holes in the market, although probably only at that time. I dunno what killed them off - perhaps the cost of CE approval, etc.

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*Some people are only alive because it is illegal to kill.

    Dave Plowman        dave@davenoise.co.uk           London SW
                  To e-mail, change noise into sound.

I dunno what killed them off - perhaps the cost of CE approval, etc.

Shortage of the output mosfets I believe, I found a supply of bare circuit boards at a ham radio fair rally some years ago.

Ron(UK)

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