Marshall JCM900/4100 PS and channel switching problem

[Enzo]

Well, there is hte other half of the test, plug a signal into the FX return and see what comes o0ut the speaker.

[jaz10]

Enzo -- I should have clarified that -- I DID run input into the return -- nothing out of the speaker except the hum.

[Gtr_tech]

About the zeners -- both you and Bill have mentioned the D3 and D4 zeners. so I replaced them again just to play it safe - As far as I've always known - a zener is a zener - so unless I've got that wrong -- they should be OK -- The schematic called for a specific zener -- but I never knew there was any difference -- so maybe I'm wrong on that point and maybe I've got 2 different ones in there causing some kind of inbalance in the ps dc.

A zener is not a zener....they're rated for a specific voltage and wattage.. In your case they'll be 15v 1w zeners, likely a 1n4744a type. Pay attention to polarity when replacing. If the pcb is not marked, use your "sharpie" and identify the cathode end so you don't put one in backwards.

[52 Bill]

A zener is not a zener....they're rated for a specific voltage and wattage.. In your case they'll be 15v 1w zeners, likely a 1n4744a type. Pay attention to polarity when replacing.

Exactly.

The send and return jacks are in the middle of the pre-amp in this model, so you still need to check IC5 and IC8, as well as tube V2.

Try this...turn up the reverb control and shake the reverb tank. Do you get reverb crashing? Remember that this will be controlled by the master volumes so turn them up too.