80's Randall RRM 2-120

[bbirling]

I have this amp here and it's one of the most basic Solid State amps I have seen. Good place to start learning some of this. The amp doesn't work. Plug in and turn on. The breaker kicks out almost immediatly. I tested the PT primaries and Secondaries. The PT is not shorted. Where to go from here is the question. Any and all help is appreciated. The schematic is attached.

Bruce

[sluckey]

Disconnect points A and B from the amp circuit so that the power supply rails are open. Then fire it up and check for -40 and +40 volts at the filter caps. If breaker is still tripping, suspect the bridge and/or one or both filter caps. If have proper voltage, check each of the power transistors out of the circuit.

[bbirling]

OK, good news. Disconnected the A/B connections. Amp stayed on. 54v and -54 respectivly. That made sense from a troubleshooting perspective after I did it. A little high according to the schematic.
Now, do you know of a good link, or an easy method for testing the transistors. This is uncharted territory for me.

Bruce

[sluckey]

I have a very good, quick and easy method but it requires an analog ohm meter. There should be plenty of youtube videos showing how to use a simple test circuit. Do you have an analog meter?

[bbirling]

Sure do. I keep all the old stuff around. Work on old stuff need old equipment. I did a quick search on Youtube but with limited knowledge of "true" technical terms it turns up an overwhelming response.

Bruce

[PRR]

> A little high according to the schematic.

No-load current is even less than idle current. Power rails un-sag.

> easy method for testing the transistors

"Disconnect points A and B from the amp circuit..." Get two 300 ohm 10 Watt (sized so the full 54V won't burn). Use these to re-connect points A and B to the amp.

Since you already know the amp is shorted, at least one of these resistors will get HOT. But with the added resistance, the amp will limp-along while you poke at it.

Read Collector-to-Emitter voltage on each transistor. The transistor(s) with <0.1V C-E is(are) shorted, permanently dead. Start with the big outputs Q8-Q11. If either is dead, remove all deaders and disconnect the maybe-OK. Try with just Q6 Q7. If the added resistors now run cool and Q6 Q7 show nearly full 50V C-E, and the OUTput is very nearly zero DC, then you may have found all the deaders. If not, yank Q6 Q7 and try to run with just Q4 Q5 driving the output.

If even Q6 Q7 are burnt, take em out. Connect 22K from Q3 Collector to Q2 Base (to maintain NFB). In this connection it should bias-up with Q3 Collector _AT_ zero (+/-50mV). And the Q5 Base point should be just 1.2V-1.4V lower than ground. If stuck high, probably Q3 busted/shorted a gut after a domino failure of Q8 Q6 Q4.

"RCA31C" is almost certainly a TIP31C made by RCA.

High-power amps are NOT "Good place to start learning". But with all the small jobs turned over to indestructable chips, it may be all you can find.

[bbirling]

Thanks PRR. I will run these tests in the morning. I can do Tube stuff all day long. So I am comfortable with the High power and High Voltages. The solid state (transistors) end is where it starts to throw me.

Bruce

[bbirling]

One bad output transistor. Replaced and all is working. Voltages look correct according to schematic. I only had a 2n4348 to replace the mj15003 with, but I ordered the correct one (plus extras) and will make the change when it arrives.

Thanks for the help. It was a learning experience.

Bruce

[PRR]

> I am comfortable with the High power and High Voltages.

It's not (just) that.

In tubes, a failure here won't (usually) cause a smoke-producing failure over there. The DC conditions are per-stage, isolated by caps. (In tubes or transistors, audio failures don't usually cause smoke.)

However in the classic audio Power amplifier, each stage depends on the next. In your plan, if Q1 burned open, the output would slam to the negative rail, 50V across the speaker, and loud buzz or burnt speaker. Open Q2 slams the other way with equally unhappy result.

Further there are domino failures. If Q8 fails but open, Q6 will try to drive the speaker. If this does not cause lousy sound, Q6 soon melts.

If both Q8 and Q6 have failed short, but you only find and fix Q8, the shorted Q6 will slam Q8 and probably melt it again.

In more complicated amps R10 R11 are another transistor, loading/fighting Q3. Now you can get into multiple failures where if you fix 2 out of 3, other good parts melt.

Which is why I outlined all the way to the basic low-power amp, Q1 Q2 Q3. All the rest just buffers from the 10K that Q3 can drive down to the 4 ohm load. If you cut back to here and get Q3 to force Q2 to match Q1 bias (0V), then you can build-back the output sections, pair at a time, with no load, and possibly with huge (100X) emitter resistors to delay burn-up while you poke.

Can you tell I've had way too much fun with these things?

[bbirling]

<Can you tell I've had way too much fun with these things?

Yes. And your knowledge and willingness to share that knowledge is above and beyond any other forum on the net.

Thanks again
Bruce