That's all very good and shows that all TO220 drivers seem to be O.K.
Now go back to WH.1 WH.5. and turn the 500R pot to get between 2.0 and 2.4 Volt between these points.
Then, attached to these two points are R132 and R133, both 20K.
Connect the outer sides of these resistors with a wire.
Connect both +/- VCC voltages and switch on the amp, but eventually you could take extra precaution by inserting two bulbs in the +/-VCC lines.
No current should be flowing through R20 and R44 with the VCC's now connected.
If so, switch off the amp and remove the short between R132 and R133.
Switch on the amp back again and measure the voltage over R20 and R44.
Hans
Now go back to WH.1 WH.5. and turn the 500R pot to get between 2.0 and 2.4 Volt between these points.
Then, attached to these two points are R132 and R133, both 20K.
Connect the outer sides of these resistors with a wire.
Connect both +/- VCC voltages and switch on the amp, but eventually you could take extra precaution by inserting two bulbs in the +/-VCC lines.
No current should be flowing through R20 and R44 with the VCC's now connected.
If so, switch off the amp and remove the short between R132 and R133.
Switch on the amp back again and measure the voltage over R20 and R44.
Hans
We will look at the pot at a later stage, but for now connect both ends of R132 and R133 with a short, making the voltage between WH.1 and WH.5 zero volt.
Connect both +/- VCC voltages and switch on the amp, eventually you could take extra precaution by inserting two bulbs in the +/-VCC lines.
No current should be flowing through R20 and R44 with the VCC's now connected.
If so, this will prove that the end stage is O.K.
Hans
Connect both +/- VCC voltages and switch on the amp, eventually you could take extra precaution by inserting two bulbs in the +/-VCC lines.
No current should be flowing through R20 and R44 with the VCC's now connected.
If so, this will prove that the end stage is O.K.
Hans
Mystery solved. I checked resistance between WH1 and WH5 and measured an open circuit. I did some circuit tracing and discovered no continuity between R21 and R124. There is a trace that runs underneath the board between R21 and Q50. Somehow during disassembly of the board for a recap that trace got severed or damaged. I ran a jumper wire and now it works. I will do final bias and test tomorrow.
My sincerest thanks Hans for helping me understand the circuit and developing strategy for troubleshooting. Have a happy New Year. Also thanks to others who chimed in.
My sincerest thanks Hans for helping me understand the circuit and developing strategy for troubleshooting. Have a happy New Year. Also thanks to others who chimed in.
Everything looks great Hans. I measured WH1 to WH5 on both channels and they are the same at 3.5V. Bias is stable and within .2mV for R20 and R44 both channels (I set it for 26.5mV). DC offset 4mV on R, 9mV L. A 1K sine wave looks good on both channels and amplitude matched to .02V. Amp was running for over two hours.
