Help Me Trouble Shoot Leach Amp

[ptah]

I built Leach amp V4.5 about 5 years ago and have enjoyed it without trouble until recently. Right channel intermittently produces reduced highly distorted audio. When I quickly vary the input level it recovers, for awhile, and then reverts.

What I have done to diagnose (not in the order shown):

1. Replaced the power supply fuses.
2. Replaced the power transistors (MJE15003/4)
3. Checked all wiring to the PCB board.
4. Switched the speakers.
5. Checked the source (DAC).
6. Checked the bias current.

I think the problem is with one of the components on the PCB but I don't know where to begin.

Any ideas or test that I can make would be appreciated!

 

[jwilhelm]

I built Leach amp V4.5 about 5 years ago and have enjoyed it without trouble until recently. Right channel intermittently produces reduced highly distorted audio. When I quickly vary the input level it recovers, for awhile, and then reverts.

What I have done to diagnose (not in the order shown):

1. Replaced the power supply fuses.
2. Replaced the power transistors (MJE15003/4)
3. Checked all wiring to the PCB board.
4. Switched the speakers.
5. Checked the source (DAC).
6. Checked the bias current.

I think the problem is with one of the components on the PCB but I don't know where to begin.

Any ideas or test that I can make would be appreciated!

Have you checked your volume pot?

 

[ptah]

Yes, I checked the volume pot with an ohm meter and could see no problems. That does not mean much though. I will swap it out, easily done. Although if I vary the volume pot, that has no effect on the right channel amp when acting up.

 

[jwilhelm]

Is your volume control separate from the amp? Are you able to switch input cables and verify the problem is in the amp, and not the pre-amp?

Pots almost always test good with a meter, they can still be failing under load.

 

[AndrewT]

When I quickly vary the input level it recovers, for awhile, and then reverts.

Jwi reckons you should be looking at the vol pot.
I agree.
Swap channels and listen if the problem swaps channels as well.

 

[ptah]

Is your volume control separate from the amp? Are you able to switch input cables and verify the problem is in the amp, and not the pre-amp?

Pots almost always test good with a meter, they can still be failing under load.

The volume pot is built into the chassis. I probably should swap the pot out. I have swapped the input cables (L->R, R->L) with no effect.

 

[ptah]

Okay, let me make the pot change out. Good idea guys. I will bump if the problem continues. Thanks much!

 

[jwilhelm]

It's worth trying to swap the cabling inside the amp, just so you can verify where the issue is. Protection relays can have similar failures.

 

[ptah]

Thanks JWI, I replaced the volume pot (cheap Alps) and that solved the problem. In fact it was an improvement for both channels.

 

[jwilhelm]

Thanks JWI, I replaced the volume pot (cheap Alps) and that solved the problem. In fact it was an improvement for both channels.

Nice easy fix!