Marshall JCM 900 Tubes Red-Plating

[goldtop5]

Hello, I have a Marshall JCM 900 Model 4100 that's got a problem with power tubes red-plating.
Do any of you have a schematic?

Here are some numbers I've jotted down if this helps.
Item No: 4100-20-01
Issue: SLXa-60-00

[g1]

Remove power tubes. Is there any -DC voltage on pin 5 at all?
If none, check that there is AC on both sides of C12 (or C15 depending on version).
5881 version on pg.3, EL34 version on pg.4