For a while, I’ve been stuck on repairing a channel of my Accuphase P-266. Initially, I tracked the fault to a shorted dual transistor (Q3: uPA75V) in the complementary differential input stage. I replaced both dual transistors with matched KSC1845 and KSA992 transistors. Additionally, I discovered that the R23 68-ohm fusible resistor was open, so I replaced it with a regular metal film resistor.
When I turned the amplifier on, nothing seemed out of the ordinary. However, when I began measuring the bias on T.P.1, I noticed the reading was very unstable. According to the schematic, the correct reading for T.P.1 in class AB mode is 88mV, but the value fluctuated wildly. It would jump well above 100mV and sometimes drop all the way down to 14mV.
I tested all the semiconductors in diode mode with my multimeter and didn’t find any anomalies. I’ve also replaced all the electrolytic capacitors. At this point, I’m stumped and unsure of what could be causing the issue.
Here are a few possibilities I’m considering:
I’ve attached a photo of the repaired board for reference. I would appreciate any insights or advice!
When I turned the amplifier on, nothing seemed out of the ordinary. However, when I began measuring the bias on T.P.1, I noticed the reading was very unstable. According to the schematic, the correct reading for T.P.1 in class AB mode is 88mV, but the value fluctuated wildly. It would jump well above 100mV and sometimes drop all the way down to 14mV.
I tested all the semiconductors in diode mode with my multimeter and didn’t find any anomalies. I’ve also replaced all the electrolytic capacitors. At this point, I’m stumped and unsure of what could be causing the issue.
Here are a few possibilities I’m considering:
- Could the trimmpotentiometers be faulty? I’ve cleaned them with IPA and lubricated them with contact cleaner, but could they still be causing the instability?
- Could the new complementary differential input transistors I installed be contributing to the problem?
- Is it possible that a diode or a transistor, while testing well, is failing under load?
I’ve attached a photo of the repaired board for reference. I would appreciate any insights or advice!
Attachments
Not a lot to cause that problem, a few diodes and a transistor, that's all.
Q22 is normally switched on when in class B and switches off in class A. That eliminates VR2 so just VR1 and the diodes.
Monitor the voltage across D11 ... does it vary ... if it does, it is faulty.
Do the same with D12, 13, 25 & 26.
If all OK then that leaves just Q22, monitor its CE voltage.
Ensure they are thermally in contact with the power output heat sink, if appropriate.
Q22 is normally switched on when in class B and switches off in class A. That eliminates VR2 so just VR1 and the diodes.
Monitor the voltage across D11 ... does it vary ... if it does, it is faulty.
Do the same with D12, 13, 25 & 26.
If all OK then that leaves just Q22, monitor its CE voltage.
Ensure they are thermally in contact with the power output heat sink, if appropriate.
Thanks for the response!
I went ahead and made some measurements. When I powered on the amp, the readings on T.P.1 started around 60mV and immediately began to drop.
While the readings on T.P.1 were decreasing, I checked the voltage drops across the following components with my second meter, and they all seemed stable:
The voltage across the CE on Q22 was also stable, reading around 70mV.
Interestingly, I noticed that when I turned off the amp, the readings on T.P.1 actually started going back up for a brief moment.
I went ahead and made some measurements. When I powered on the amp, the readings on T.P.1 started around 60mV and immediately began to drop.
While the readings on T.P.1 were decreasing, I checked the voltage drops across the following components with my second meter, and they all seemed stable:
- D11: 2.4V
- D12, D13, D25, D26: 1.9V
The voltage across the CE on Q22 was also stable, reading around 70mV.
Interestingly, I noticed that when I turned off the amp, the readings on T.P.1 actually started going back up for a brief moment.
I believe you may have looked at the T.P.2 instead of T.P.1. The bias adjustment is done across the R63 resistor. The voltage across this resistor starts immediately dropping after powering on the amp. It is unclear why.
The bias current is measured across R63. Not set by.
The bias is set with the diodes combination as I explained and if any of the diodes vary, that will vary the bias current.
The bias current is set by a combination of VR1 and VR2.
Hence to accertain that there is no instability with the voltage set between the collectors of Q8 and Q9.
The bias is set with the diodes combination as I explained and if any of the diodes vary, that will vary the bias current.
The bias current is set by a combination of VR1 and VR2.
Hence to accertain that there is no instability with the voltage set between the collectors of Q8 and Q9.
