Repairing a vintage solid-state guitar amplifier

[[email protected]]

Hi, I'm attempting to fix a Rickenbacker TR75 GT amplifier, circa 1971.
The power amp is dead. The output power transistors have gone CE
closed-circuit and power resistors in the output are open circuit. I
think the previous owner shorted the output. Other components seem OK,
as far as I can test them.

Luckily, there's a schematic: -

http://www.rickenbacker.com/pdfs/19345.pdf

The 2N5988 and 2N5991 transistors are unavailable except at great
expense from vintage parts suppliers. I'm not gonna pay big bucks for
them, because (a) I might blow them and cry, (b) it would be cheaper
just to buy a ready-made power amp module and use that.

I hoped that some cheap BD441/442 transistors might be sufficient for
me to find out if the rest of the circuit was OK. They are at least
rated for the voltage (73 volts rail-to-rail). But putting them, and
new 0.27R resistors, in the circuit and powering-up resulted in smoke.
I guess their characteristics (higher gain, lower current) are too
different, or there's something else wrong with the circuit. At least I
only blew up a few pence/cents worth of components.

Any suggestions about equivalent transistors at sensible price? Any
ideas how I can try things out while minimising the chance of more
fireworks?

I suspect I'll end up with Plan B - replace the whole power amp with a
modern one. But it would be nice to fix the original instead.

 

[Dave Plowman (News)]

I hoped that some cheap BD441/442 transistors might be sufficient for
me to find out if the rest of the circuit was OK. They are at least
rated for the voltage (73 volts rail-to-rail). But putting them, and
new 0.27R resistors, in the circuit and powering-up resulted in smoke.
I guess their characteristics (higher gain, lower current) are too
different, or there's something else wrong with the circuit. At least I
only blew up a few pence/cents worth of components.

Any suggestions about equivalent transistors at sensible price? Any
ideas how I can try things out while minimising the chance of more
fireworks?

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

 

[TimPerry]

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.

 

[Just Another Theremin Fan]

The power amp appears to be direct coupled so I'd guess a few blown up

transistors back down the line too.

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!

 

[[email protected]]

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.

 

[[email protected]]

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.

 

[Ancient_Hacker]

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.

 

[Just Another Theremin Fan]

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

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

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!

 

[Geo]

On 22 Feb 2006 04:01:40 -0800, [email protected] wrote:

I hoped that some cheap BD441/442 transistors might be sufficient for
me to find out if the rest of the circuit was OK. They are at least
rated for the voltage (73 volts rail-to-rail). But putting them, and
new 0.27R resistors, in the circuit and powering-up resulted in smoke.
I guess their characteristics (higher gain, lower current) are too
different, or there's something else wrong with the circuit. At least I
only blew up a few pence/cents worth of components.

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

 

[Just Another Theremin Fan]

I would try temporary insertion of much higher value resistors instead of the

0R27s e.g a couple of 1/2w 3k9.

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

 

[[email protected]]

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.

 

[Ron(UK)]

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)

 

[[email protected]]

http://www.velleman.be/ot/en/product/view/?id=360356

(The description of a "200W" amplifier is marketing BS. It's actually a
70-100W amp, ideal for the Rick which was originally 75W).

 

[Matti Adolfsen]

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

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

 

[Just Another Theremin Fan]

(The description of a "200W" amplifier is marketing BS. It's actually a
70-100W amp, ideal for the Rick which was originally 75W).

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?

 

[Ron(UK)]

http://www.velleman.be/ot/en/product/view/?id=360356

(The description of a "200W" amplifier is marketing BS. It's actually a
70-100W amp, ideal for the Rick which was originally 75W).

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)

 

[Michael Ware]

http://www.velleman.be/ot/en/product/view/?id=360356

(The description of a "200W" amplifier is marketing BS. It's actually a
70-100W amp, ideal for the Rick which was originally 75W).

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.

 

[[email protected]]

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.

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.

 

[Dave Plowman (News)]

The Velleman kit is also from Maplin.

Yes - but not a Maplin kit.

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

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.

 

[Ron(UK)]

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.

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)