03-12-2016, 12:33 PM
For the time being, let's stop doing the power tests. They're not needed, and don't do anything other than waste electricity. We know that the original designer has got the thermal design right - so once the actual problem has been fixed, the behaviour at high power will take care of itself.
It seems like we have a problem with quiescent stability. One channel is taking too much power even when the amp is silent. If you leave the amp powered but doing nothing, you should find that one channel runs much warmer than the other. Is that right?
So, Q3 is the Vbe Multiplier than sets the quiescent current. It has to maintain a constant voltage between its collector and emitter. Actually, that's a slight lie - the voltage actually depends on the temperature of the heat sink (quite deliberately). All of that you probably know already
What voltage should be between the collector and emitter? We have drivers and pre-drivers to consider, so that's 6 BE junctions in total, so about 3.6V between C and E of Q3. In practice, it might be slightly less.
However, your readings don't make sense. It is impossible for B to be less than E. We need to double-check...
R25 appears to be providing an additional feedback path between the output and the first of the transistors. I'm not sure it'll affect the quiescent current, but either way, it sounds like it has failed high.
Trouble is, the schematic has notes that refer to VR1, 2 and 3, but the presets aren't labelled as such. So my best guesses:
I reckon this one is R12A
That's clearly R22, and as we established earlier, that should be something closer to 0.3V. The 900B schematic says 0.37V.
This must be R25.
Looking again, the transistorised part of the amplifier has a voltage gain that is determined by the ratio of (R24+R25)/R11. The gain of the whole amplifier is fixed by the global NFB (R9 and R6), so the preset shouldn't alter the overall gain, but it decides the gain structure of the amplifier. With that preset failing high, the output from the op-amp will be lower in level than intended, and that could lead to all sorts of interesting problems. So, definitely replace those presets (sounds like both might be wrong, but check the markings on the part itself) and re-align them according to the instructions.
I see what you mean about R24 being missing. It's possible it was added to later models to prevent a "twiddler" winding it down to minimum.
It seems like we have a problem with quiescent stability. One channel is taking too much power even when the amp is silent. If you leave the amp powered but doing nothing, you should find that one channel runs much warmer than the other. Is that right?
So, Q3 is the Vbe Multiplier than sets the quiescent current. It has to maintain a constant voltage between its collector and emitter. Actually, that's a slight lie - the voltage actually depends on the temperature of the heat sink (quite deliberately). All of that you probably know already
What voltage should be between the collector and emitter? We have drivers and pre-drivers to consider, so that's 6 BE junctions in total, so about 3.6V between C and E of Q3. In practice, it might be slightly less.
However, your readings don't make sense. It is impossible for B to be less than E. We need to double-check...
R25 appears to be providing an additional feedback path between the output and the first of the transistors. I'm not sure it'll affect the quiescent current, but either way, it sounds like it has failed high.
Trouble is, the schematic has notes that refer to VR1, 2 and 3, but the presets aren't labelled as such. So my best guesses:
Quote:a: Adjust VR3 for symmetrical output clipping
I reckon this one is R12A
Quote:b: Adjust VR1 for 0.56V across R35 (TP1)
That's clearly R22, and as we established earlier, that should be something closer to 0.3V. The 900B schematic says 0.37V.
Quote:c: Adjust RV2 and check that pin 6 of IC2 is clipping at the same time as the output at 8 ohms
This must be R25.
Looking again, the transistorised part of the amplifier has a voltage gain that is determined by the ratio of (R24+R25)/R11. The gain of the whole amplifier is fixed by the global NFB (R9 and R6), so the preset shouldn't alter the overall gain, but it decides the gain structure of the amplifier. With that preset failing high, the output from the op-amp will be lower in level than intended, and that could lead to all sorts of interesting problems. So, definitely replace those presets (sounds like both might be wrong, but check the markings on the part itself) and re-align them according to the instructions.
I see what you mean about R24 being missing. It's possible it was added to later models to prevent a "twiddler" winding it down to minimum.
