JCM 800 2203 1985 whistle, mods

[lamortemifabello]

Hi guys, congratulations on this amazing forum. I recently bought a 1985 JCM 800 head. It was giving me an issue on the high input with the preamp cranked. I replaced the preamp tubes and it’s much better now, but there's still a whistle when the guitar volume is turned down. Before making further changes, I opened the chassis and noticed it has been modified. Can you help me understand what modifications were made and what could be causing the whistle? I've been learning a lot from reading this forum and I want to thank everyone. Thank you from Italy!

 

[rtcook]

Looks like that pile of grey caps are replacing the large filter caps. I see some disconnected but can completely tell from the pics. Not sure if that is the cause of the whistle, but if it were mine I'd be changing the large filter caps on the chassis back to stock and look at the wiring.

 

[Purgasound]

Good god... Not only is that terrible work but it's incredibly dangerous to have exposed terminals on heavy parts that aren't properly mounted. That nonsense has to be removed. Once the filter capacitors and bias caps are replaced the amp should work fine. Of course now you will need to follow the correct schematic to ensure it's done properly. Looks like they cut some of the existing wires so you'll have to run new ones to avoid unnecessary splices. I would discontinue operation of the amplifier until this is corrected.

 

[marshallmellowed]

Looks like they connected several "generic" caps in series to replace the disconnected chassis mounted cap. Agree with replacing the chassis mounted cap and returning to stock wiring. May, or may not have anything to do with the "whistling', but it's not the right way to repair an amp.

 

[lamortemifabello]

Thank you guys, and yes, really a terrible job. Can any of you help me find a clear wiring diagram?"

 

[rtcook]

Check here.

Marshall JCM800 Schematics - Dr.Tube

www.drtube.com

 

[rtcook]

I recommend ARS 50uf/50uf 500V caps.

 

[Pete Farrington]

The lead dress between board and valve sockets is bad, wires far too long.
While you're getting the ecaps sorted out, at least get the V1a wiring away from both the heater and V1b wiring.
If it still whistles after all that, maybe consider screened cable from the high input to V1 pin 2, with the 1M mounted at the input socket and 68k mounted at the valve socket lug.