Arhh yes .... Did you check transistors before soldring, have you checked if they are okay, diode drop.
Its frustrating yes..
Its frustrating yes..
Check bases of Input Differential Pair (Q1/Q2) Base Voltages:
Base of Q1 ≈ 0V
Base of Q2 ≈ 0V
If either is far off, check R1, R2, R11, and feedback connection to Q2 base.-
3. Tail Current Check (R11):
Measure voltage across R11 (536 ohms)
Expect ~0.5 to 1.1V
→ That corresponds to ~1–2 mA tail current.
If 0V, the differential pair isn’t conducting. Check Q5/Q6 and R11.
4. Trimpot VR1 / VR2 Functionality:
Power on the amp, measure output voltage
Slowly turn VR1 — does output voltage change?
Measure voltage between base of Q11 and base of Q12
Slowly turn VR2 — expect the voltage to vary (~1.5–2.2V)
If nothing changes, suspect open trimpots, broken tracks, or bad solder joints.
5. Q11 / Q12 Base Voltages:
Base of Q11: around +1.5V
Base of Q12: around −1.5V
If unbalanced or stuck, check surrounding resistors and current mirror stage.
6. Feedback Loop Integrity:
Make sure R8 and R9 are in place and soldered properly
Confirm Q2 base is connected to output via feedback
Look for broken traces or cold joints
Without proper DC feedback, output won’t self-correct and stays offset.
7. Input Should Be Shorted During Test:
Connect input signal pin to GND when testing without source
Prevents input stage from floating and introducing unwanted DC.
A short to check list.. let me know.
Base of Q1 ≈ 0V
Base of Q2 ≈ 0V
If either is far off, check R1, R2, R11, and feedback connection to Q2 base.-
3. Tail Current Check (R11):
Measure voltage across R11 (536 ohms)
Expect ~0.5 to 1.1V
→ That corresponds to ~1–2 mA tail current.
If 0V, the differential pair isn’t conducting. Check Q5/Q6 and R11.
4. Trimpot VR1 / VR2 Functionality:
Power on the amp, measure output voltage
Slowly turn VR1 — does output voltage change?
Measure voltage between base of Q11 and base of Q12
Slowly turn VR2 — expect the voltage to vary (~1.5–2.2V)
If nothing changes, suspect open trimpots, broken tracks, or bad solder joints.
5. Q11 / Q12 Base Voltages:
Base of Q11: around +1.5V
Base of Q12: around −1.5V
If unbalanced or stuck, check surrounding resistors and current mirror stage.
6. Feedback Loop Integrity:
Make sure R8 and R9 are in place and soldered properly
Confirm Q2 base is connected to output via feedback
Look for broken traces or cold joints
Without proper DC feedback, output won’t self-correct and stays offset.
7. Input Should Be Shorted During Test:
Connect input signal pin to GND when testing without source
Prevents input stage from floating and introducing unwanted DC.
A short to check list.. let me know.
Amp, I appreciate your time trying to troubleshoot this thing, but both of the boards have the same problems. They have -20v across r11 so serious negative voltage. I’m convinced the transistors are bad. I’ve pretty much moved on to other projects. I may come back to it later and use current transistors. Regards.