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The failure

For the past few months I've been looking to repair my old-style Roland Cube 60 watt amp after it had been left out on a cold night. The point at which it failed was when I turned it on inside, and it started emitting a quite loud low-sounding hum through the speaker even though I had turned the master volume all the way down. I switched it off and back on a few times after checking cables, still humming, and after a few seconds it failed (blew all fuses).

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The problem

I opened it up, disconnected the speaker and after blowing a few more fuses, I isolated the fault to the two NPN power amps in red

Through some unknown means, they had become shorted out (I could test continuity between the pins). After removing them from the circuit, the fuses no longer blew and I could could get a good signal from the Preamp Out with no problems, so I know that every thing before the master stage is working as it should be. I checked a 44 and -44 volt power rails and they seem to be fine (not under load)

I ordered replacement 2SD845 power transistors, and after fitting them in the sockets and turning the amp on, the Q12 climbs to 60 C quickly and the loud low hum sounds so I quickly turn it off. Q9 also gets really hot. If I have only one of Q12/Q13 plugged in, there is no heating up, but a slight hum can be heard when only Q13 is plugged in.

I measured the DC voltage present at the sockets of Q12/13, while the transistors were removed:

Q12

B: -11, C: 44.3, E: -5.1.

Q13

B: -35.3, C: -5, E:-44.5

The now

I'm not at all experienced with amplifier design, and this layout is more complex than I've ever seen

1. What is it called, and what are the main things I should know about it?

MASTER STAGE

The only way I've been able to discern faulty parts is by testing for short circuits, to which there currently appear none, and even though Q9 and Q12 heat up, there is no tripping of the fuse

2. What could possibly be causing them to heat up so quickly?

I assume it is this heating damage that caused the original transistors to short out, so I'm not so keen on having to turn the amp on too many times at risk of destroying the new Q12/13 transistors.

3. What should I be testing for when it is turned on? Or even better, what could I test for when everything is powered off?

Thank you very much for reading, I would really appreciate any and all advice I can get.

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    \$\begingroup\$ Sounds like something is driving Q13 hard on. It will not get hot as Von low bit Ion high is dawn across top stage. So something around Q10 OR IF removing A10 removes heating the Q10 is being driven hard on and is driving Q13 and ... . Bases of Q9 and Q10 should be ABOUT mid rail wityh D1 2 3 holding bases about 2 Vbe apart so they are both at start of conduction to eliminate cross over distortion. Q8 drives the bases to the correct point and R48? 47K feeds the Vout result back to the Q6 Q7 differential amplifier (long tailed pair) .... \$\endgroup\$ – Russell McMahon Jul 27 '15 at 11:19
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    \$\begingroup\$ ... If anything in that loop unbalances the loop the diff amp ses an error signal and tried to correct it and drives the whole shooting match to one rail. OR if the diff amp is dead it fails to correct error and wholelot wanders over to one raul. So if eg C34 was short or very leaky it puts DC on Q7 base pulls it hard on so q6 off so R44 top to ground so Q8 off so q10 base high so whatever. | Basically system should be balanced and sit with Q9-Q10 emitters at about 1/2 point bwtreen supplies = groundish. If not why not and follow it back. Feedback adds fun as everything drives everything :-) \$\endgroup\$ – Russell McMahon Jul 27 '15 at 11:24
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    \$\begingroup\$ Those two 0.3 ohm resistors R56,58 are very useful : measuring the voltage across them tells you the quiescent current. \$\endgroup\$ – Brian Drummond Jul 27 '15 at 12:05
  • \$\begingroup\$ This is some wonderful information. I won't have access to the amp until tomorrow, but I will make sure to measure Q9/10. I'm not sure if it will be very helpful but I added the DC voltages I measured on Q12/13 \$\endgroup\$ – Al Longley Jul 27 '15 at 12:15
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By measuring the voltage drops from Base to Collector on Q9 and Q10, I determined that Q10 was massively faulty, with a ~0.3V drop in both directions (swapping probe polarities). Because this was causing a short, it massively unbalanced the rest of the loop causing a great load across Q12 and Q13 and resulted in my documented hum. I replaced Q10, and noted the DC voltage present at Q12's emitter was now ~0V.

Through great interweb research I have discovered this amplifier arrangement is called a "Quasi Complementary" amp.

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If Q12 and/or Q13 fail you should replace both of them, you should also replace Q9/Q10. Reason being when the output transistors Q12/13 fail they normally go short between collector and base and take out the driver transistors Q9/10 too. For the extra cost it is not worth messing about as even though they may test OK they will often fail soon after.

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