Watkins Super Mersea Crackle

[Ted]

Got a Watkins Super Mersea here that is taking some time. (See Custom 15 Schematic enclosed, which is a very similar, if not identical amp)

It has a persistent low intensity crackle.

The two channels go into opposite sides of the PI.

Have tried different tubes. With V1 and V2 pulled, the crackle is still present. This made me suspect the power supply. I tried swapping the 1K5 and 10K power supply droppers and that made no difference.

With both sides of the PI disconnected, the crackle goes away. With either one of them connected, its still there regardless of the presence of V1 and V2.

Earth continuity is good.

Advice appreciated.

[Jon Snell]

Check/replace the anode load resistors 100k and 56k.

[Mickey]

First of are you conversant working on HIGH VOLTAGE valve equipment, if not take it to someone who is !
With both volume pots turned all the way down is the crackle still there? if so I would replace the resistors coming from HT to V3 as well as th coupling capacitors to the output tubes coming from V3 anodes.
Have you swapped V3 with V1?
Probably a good idea to replace the capacitors feeding the grids of V3.
I haven't done any work on these amps but have repaired numerous Copicats which use similar quality (?) components and would strongly suggest replacing all capacitors especially if they are "hunts" branded.
Good luck.

[Ted]

Tried changing all the anode/plate resistors. No difference.

The caps are all either mustard or more modern yellow poly caps, with the exception of the 680pf/1000pF in the tone circuits which are ceramics.

A bit of chopsticking revealed that the ground connection on the big smoothing cap didn't have any solder on it. But soldering that didn't make any diff (Although it did stop an alarming crackle when I tapped the chassis)

With both volume controls down there is still a bit of popping and crackling audible.

Disconnecting both grid capacitor connections to the PI makes the crackle go away. (Sorry, wasn't clear in earlier post)

[Chuck H]

Has the amp been stored in a humid environment? Perhaps it's just humid where you live and the amp has been sitting longer than usual? Sometimes moisture in things like board material, paper insulations or even accumulated crud on surfaces can cause this. Leaving the amp on with a fan blowing on it for a while can help. Or you may need to can get more aggressive, cleaning contacts and the board surface in the open chassis with denatured alcohol and then blow drying it for a half hour.

[Mick Bailey]

+1. The board material is likely to be SRBP, which can absorb moisture and cause random noises.

In addition, both volume controls ground the inputs to the PI, so there shouldn't be any noise coming from either preamp channel with both on zero. This would then leave the PI grid coupling caps and associated circuit traces, and given the noise stops when the PI inputs are removed (by disconnecting the caps from the PI) I would be looking at either the caps or traces.