leereynold56
New Member
Hello,
I have finished building a new Sansui F-2624 PCB for my 9090db. I used one of the new PCBs online with the fish icon and I trust the seller and PCB. However, after checking all the parts placement, values, polarities, and soldering, I put everything back together and one channel isn't working correctly. I've gone through it many times re-verifying and still haven't found any issues.
Here are the symptoms: the entire left channel is about 10 degrees C warmer than the right. The differential pair (Tr1/Tr3) is running about 20 degrees C warmer than the right (62 degrees C). But, all the dc voltages are very much identical to the right channel dc voltages. The resistors in the plus and minus voltage rails (R31, R32, R33, R34) all have comparable voltage drops comparing left to right channel. I removed the power transistors and the f4 and f5 fuses to remove the power output circuitry from the equation and the left channel remains the same warmer temperature with the left differential pair still hot. Btw, I matched the hFE for both pairs to 390, so they are matched pairs and both channels have the same hFE.
Setup: offset voltages and bias current set to specs and holding.
Dynamic tests: Here's where it gets even more confusing. Using a function generator, 100 watt load resistors and an oscilloscope. I khz input signal at 200 mVrms produces 156 watts rms with no clipping on both channels at full volume. From 100 hz down to 10 hz, the amplitude is fairly constant on the right channel, but badly fades in amplitude continuously down to10 hz on the left channel. At 10 khz, the right channel looks fine, but the left channel goes nuts and puts everything into protection mode.
I've been thinking maybe it's oscillation, but can't see anything on a 400 mhz scope.
Anybody have any idea what is going on here?
Thanks
I have finished building a new Sansui F-2624 PCB for my 9090db. I used one of the new PCBs online with the fish icon and I trust the seller and PCB. However, after checking all the parts placement, values, polarities, and soldering, I put everything back together and one channel isn't working correctly. I've gone through it many times re-verifying and still haven't found any issues.
Here are the symptoms: the entire left channel is about 10 degrees C warmer than the right. The differential pair (Tr1/Tr3) is running about 20 degrees C warmer than the right (62 degrees C). But, all the dc voltages are very much identical to the right channel dc voltages. The resistors in the plus and minus voltage rails (R31, R32, R33, R34) all have comparable voltage drops comparing left to right channel. I removed the power transistors and the f4 and f5 fuses to remove the power output circuitry from the equation and the left channel remains the same warmer temperature with the left differential pair still hot. Btw, I matched the hFE for both pairs to 390, so they are matched pairs and both channels have the same hFE.
Setup: offset voltages and bias current set to specs and holding.
Dynamic tests: Here's where it gets even more confusing. Using a function generator, 100 watt load resistors and an oscilloscope. I khz input signal at 200 mVrms produces 156 watts rms with no clipping on both channels at full volume. From 100 hz down to 10 hz, the amplitude is fairly constant on the right channel, but badly fades in amplitude continuously down to10 hz on the left channel. At 10 khz, the right channel looks fine, but the left channel goes nuts and puts everything into protection mode.
I've been thinking maybe it's oscillation, but can't see anything on a 400 mhz scope.
Anybody have any idea what is going on here?
Thanks