Engl Blackmore E650 crackling noise

[Zouto]

A friend's Engl Blackmore E650 makes annoying crackling noises, especially in the lead modes. Retubing the amp and resoldering the tube socket pins on the pcb brought no bettering. Since all parts on pcb are upside down hidden on Engl amps, servicing is a real drag. You also have to unsolder all power transformer leads on the pcb to remove the main board. I tried a whole bunch of different power and pre amp tubes without any success.
Any help much appreciated
Zouto

[nickb]

Try to narrow down the problem area by adjusting all the controls to see the effect on the noises. There is also the tap-board-with-non metallic chop stick method.

[Zouto]

Try to narrow down the problem area by adjusting all the controls to see the effect on the noises. There is also the tap-board-with-non metallic chop stick method.

I checked it, the crackling is always present in the signal path, tone controls modify it like a normal signal, high gain modes increase it's volume, tapping did not reveal any "hot spots". Seems to be some crappy resistor or bad connection.
Meanwhile I resoldered nearly all wires on the pcb with no results. When the guitar is unplugged, the crackling is dimed since the input is shorted, but it is still audible. I also resolderd the input jack connections.
Think I've got to bite the bullet and remove the board to have a look at the components.

[nickb]

I checked it, the crackling is always present in the signal path, tone controls modify it like a normal signal, high gain modes increase it's volume, tapping did not reveal any "hot spots". Seems to be some crappy resistor or bad connection.
Meanwhile I resoldered nearly all wires on the pcb with no results. When the guitar is unplugged, the crackling is dimed since the input is shorted, but it is still audible. I also resolderd the input jack connections.
Think I've got to bite the bullet and remove the board to have a look at the components.

Sounds like it may be the first stage. Since shorting the grid to ground reduces the noise maybe it's grid resistor is bad. The "lead" and "clean" controls are right after the first stage so if they kill the noise you've narrowed to that stage of less than half a dozen components for sure.

e650-ii-schematic.pdf

[Zouto]

Sounds like it may be the first stage. Since shorting the grid to ground reduces the noise maybe it's grid resistor is bad. The "lead" and "clean" controls are right after the first stage so if they kill the noise you've narrowed to that stage of less than half a dozen components for sure.

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Nick: Thanks for the schemo, I just checked and changed the grid and plate resistors on the first stage for no cure at all. The crackling also appears on plate (pin 6) of V1B when checking voltages with with my DVM's test lead's tip. So troubleshooting that amp seems to be quite tricky.

[nickb]

So, did changing the 'lead' or 'clean' controls reduce the crackle? If they did they you've narrowed it to the first stage.

When you touch the plate (pin 6) with your meter it will produce pops/crackles as that induces noise into the circuit. If, on the other hand, if you hear the noise as you push on the lead it might be the socket that is bad or whatever connection you are probing on. Waggle the tube with the power on and listen for a change in the noise. Check / resolder the connection and see if it affects the noise.

Try a different tube. Another possibility is the input socket itself - clean it. Try a different cable. Try unplugging your instrument from the cable in case it's there.

Failing these easy things and assuming it is the first stage, I would choose to replace the remaining components on the first stage (there's only three ) to save the time consuming step of taking the PCB in and out.

[Zouto]

So, did changing the 'lead' or 'clean' controls reduce the crackle? If they did they you've narrowed it to the first stage.

When you touch the plate (pin 6) with your meter it will produce pops/crackles as that induces noise into the circuit. If, on the other hand, if you hear the noise as you push on the lead it might be the socket that is bad or whatever connection you are probing on. Waggle the tube with the power on and listen for a change in the noise. Check / resolder the connection and see if it affects the noise.

Try a different tube. Another possibility is the input socket itself - clean it. Try a different cable. Try unplugging your instrument from the cable in case it's there.

Failing these easy things and assuming it is the first stage, I would choose to replace the remaining components on the first stage (there's only three ) to save the time consuming step of taking the PCB in and out.

Culprits found: replaced input jack plus cable and coupling cap C1 (47nF), resoldered V1 tubesocket again thoroughly, all noise and crackling is gone, just cool sounds finally. I took 4 Tung-Sol 12AX7 for preamp and mil.specs Russian NOS 5881s for power amp.

[nickb]

Culprits found: replaced input jack plus cable and coupling cap C1 (47nF), resoldered V1 tubesocket again thoroughly, all noise and crackling is gone, just cool sounds finally. I took 4 Tung-Sol 12AX7 for preamp and mil.specs Russian NOS 5881s for power amp.

Excellent!