Sunn coliseum 300 pre amp trouble

[pontiacpete]

I am tracing through the pre amp and finding that the signal stops at IC5 SSM2012a. I could  be wrong about this but it looks like this chip is bad. Since these are long unavailable what are people doing for a substitute?

The schematic is :
https://www.prowessamplifiers.com/schematics/images/Coliseum_300_11_x_17.pdf

[J M Fahey]

It might be wrong of course, but remember SSM2012/13 are current output ICs, so scope who reads voltages only can show "no signal" there.
Try output of Next op amp, pin 7 of IC6b.

From what I read modern substitutes are certain THAT ICs but are not pin compatible and require redesigning.
MAYBE the Synthesizer guys make a replacement board based on modern stuff. Remember Peavey made a replacement board for their obsolete analog switching chip.

[pontiacpete]

Unfortunately there is no signal on IC6b. Pin 5 and 6 just a flat line and pin 7 has a small wiggly line.
Is that it for this amp unless I can fit another chip in place of IC5?

[J M Fahey]

Look what the cat brought:

https://britishaudio.com/products/ssm2013-precision-monolithics-pmi-vca-nos?srsltid=AfmBOoqTUnd39el9qiksIqwS56QvfSlzfY9EzT4xgYLJcs6vbn7CBclC

[J M Fahey]

Absolute worst case, IF your amp is muted AND you need it NOW for a tour, a rehearsal studio, etc. you can fully bypass the compressor section by lifting the right leg of C1, the left leg of C7 and joining them with a piece of wire.
Amp will work fine, except compressor.
Right or left as seen on schematic,  they can point anywhere on PCB

[pontiacpete]

Look what the cat brought:

https://britishaudio.com/products/ssm2013-precision-monolithics-pmi-vca-nos?srsltid=AfmBOoqTUnd39el9qiksIqwS56QvfSlzfY9EzT4xgYLJcs6vbn7CBclC

How did you find that??
Thank you very much!



I think I will do the temporary fix for now, also to see if it's working

[J M Fahey]

Yes, you may also have other problems.
Is this from a standard customer so limited on how much time/money can you spend on it, or is it yours or with sentimental value which merits extra?
N
Because you can build a gain cell replacement with modern ICs, THAT or others, but it will require protoboarding it to be safe, designing and building a small PCB or maybe on Veroboard but that still requires creating a Vero layout.

This is THAT 4305 block diagram.

As you see, it is functionally equivalent to yours.



Top half is the gain cell, basically same: input signal through a 20k (24k in Sunn) resistor, gain cell proper, signal current output converted back to voltage in an Op Amp 20k NFB instead of 24k.

MAYBE you could make them pin compatible using a mini board, because modern is SMD.

Maybe you still use the original RMS rectifier , not sure whether control voltage generated is proper level or polarity but a single Op Amp can take care of that.

When amp works, inject signal and move threshold pot around, measure what is being generated and sent to SSM pin 9.

There is a control voltage pin: Ec-

[pontiacpete]

This amp belongs to a customer. A year ago I replaced the output transistors and fan, and he says it has worked great until this issue arose. I don't know how much more he wants to put into it but I did order the new ssm2013. If this works I think it will be reasonable enough for him.
So, I went and did the jumping over that chip, disconnected at C1 and C7 and I'm still not getting sound. This seems strange that nothing is coming out  but I'll have another look at it today to see if I did it right