Randall rh200sc head repairing power amp board gs-200s-b

[MeekeRed]

Hello everybody,

Can someone please help me out to figured out what is going on with my beloved amp?
(Sorry in advance for this long post. Need to explain all my steps and it may be helpful to somebody else with the same problem.)

I have recently purchased a broken Randall 200watt stereo chorus amplifier head model RH200SC, where its previous owner apparently did not have time to repair it and he just decided to sell it. He claimed that the amp was turning on, but no sound was coming out from it. Nevertheless, I decided to purchase it since I love to repair and bring to a new life all sort of guitar related equipment.

After endless online research to find out the correct attached schematic regarding the damaged power amplifier board, labelled GS-200S-B, I have started replacing all the components that were damaged. In attached the “commented” schematic version where in green highlighted are all the components that I have replaced so far.

The main issue appeared right after its first start up. For precaution I have placed a current limiter light bulb at the 230VAC power source, and nothing connect into it, no speakers or guitar.

The light bulb was OFF.

The firsts voltages checks and components temperatures with thermal camera I have noticed that PNP transistor Q21 was overheating with no apparent reason. The temp was rising as long the amp was on, to avoid damages I was shutting down the amp when ~90deg C was reached from Q21. All other transistors apparently seem to be off, not even rising from the ambient temp. this weird behavior led me to further investigate. At the same time R28 was heating up and stabilizing at ~70deg C.
At this point I have founded all 4 5w wirewound resistors R24,R25,R48,R49 where dead too.

Once replaced this resistors, same thing happened but this time was Q27 instead of Q21 with the same unexplained overheating, the light bulb current limiter flashed a couple of times, after some power restart remained on and slowing fading to OFF state.
At this point my head start overheating.
Without having a deep knowledge in transistors in general I did not know what else to try. After a night of thoughts, I have come out with something new for the “to try” list.
Somehow, I thought that some ground pins were not making a good contact, so I clean and re-soldered all PCB contacts for all the components.

From here some success.

The board now was a bit more stable, voltages across the board where nominal and all smaller size transistors were heating up and stabilizing around 35degC. R28 still at 70degC. The voltage across R48 was, if I remember correctly, ~10mV and light bulb OFF. After some temp monitoring all components remained at constant temperature. Impatient to try it out I have connected my Randall Cab 4x12 and play with no issues at all for about 2hr with intervals in between to monitor temperatures.
At this point my overflown happiness last just over the day after when by restarting the amp I got shockingly blinded by the light bulb current limiter turning ON.

At this point I don’t know what else to verify.

With few “amp startups” I have noticed that the bulb light from ON start to fade and shuts OFF and all the power board stabilize at nominal conditions allowing to play for hours with no issues.
When light bulb turns ON after the amp is powered after few days without using it, no +-50V is supplied at the power amp board and the Fullbridge rectifiers Diodes D10,D11,D12,D13 are overheating.

Right now, I still keep using the current limiter as a warning since that this problem is not constant and sometimes doesn’t occur but at the same time I want to full repair it and consider it reliable with no fear to damaging it back once again.
Thanks for reading up to here and I am hoping someone can advise me how to solve this issue and release this beast amp to its full potential, in additional R28 for me is still a mystery, I don’t know if it is normal that is constantly running current through it and heating up at 70deg.

Thanks in advance and best regards.
MeekeRed

[glebert]

It is normal that R28 will always have current running through it because it is a ballast resistor for the -15V that is generated by ZD5, but 70C does seem pretty hot. If the mail rails are 50V then it needs to drop 35V across R28, which would be 1/2 a watt. I'm surprised it isn't a little higher wattage resistor there.

As to the rest of it, I hate intermittent problems, especially intermittent short circuits.

[g1]

Some amps won't start properly on the limiter lamp with speaker connected.
Disconnect speaker and turn on amp. If bulb is not lit, while amp is on, connect speaker.

[MeekeRed]

It is normal that R28 will always have current running through it because it is a ballast resistor for the -15V that is generated by ZD5, but 70C does seem pretty hot. If the mail rails are 50V then it needs to drop 35V across R28, which would be 1/2 a watt. I'm surprised it isn't a little higher wattage resistor there.

As to the rest of it, I hate intermittent problems, especially intermittent short circuits.

thanks glebert for your reply and your explanation regarding the R28, it make more sense for me now, thanks for your help.

[MeekeRed]

Some amps won't start properly on the limiter lamp with speaker connected.
Disconnect speaker and turn on amp. If bulb is not lit, while amp is on, connect speaker.

thanks for your suggestion g1, i will then try next time it will happen as you propose and i will let you know. if it is related to the limiter lamp, do you think should be safe to briefly remove the lamp limiter when this problem occur and turn on the amp just to verify that all voltages and temperature are nominal. would the amp, without the lamp limiter, get damaged immediately in case of intermittent short circuits? thanks

[J M Fahey]

No, itīs not safe.

Turn amp On WITH limiter WITHOUT load, what G1 said.

If amp is fine, voltages will be there, only somewhat lower.

Say normal rails are 40V; with limiter they may fall to,say, 25 to 30V; no big deal, as long as both are about the same, thereīs 0VDC at the output, etc.
You may even have normal+/-15V rails for preamp, because those are regulated, but worst case you will have,say,+/-10V.

Preamp and even power amp WILL still work with reduced voltage.

After you reconnect speaker, you may even play some Guitar, at low power of course, say 1 W or so, which is surprisingly loud.

Only AFTER you checked full functionality you can safely test without current limiter.