Marshall JCM800 2203KK Hum issue

[ct88]

Hello everyone, i'm looking for help!

i have a JCM800 KK in for repair, everything looks fine and triple checked almost everything but cannot figure it out what generates hum in the preamp section.
I removed all the preamp tubes one by one, and it goes away once i remove the V2, but stay there if i remove V1 (slight reduction)

I found this video on YouTube, same noise, but i can record something to post if needed (the hum is at 0:30 sec):
https://www.youtube.com/watch?v=sT7N...sorGrahamLeach


Resoldered all the parts on V1 and V2 section, the "the Beast" circuit" and the Eq/Gain/Master pcb without success (no bad solder joint at this point)
Tried several other preamp tubes, didn't cure
The amp is almost new, have been used sometimes in a couple of years, tubes seems pretty new too

Any suggestion would be very appreciated!

Ct

[The Dude]

How about a schematic?

[ct88]

Of course, here you have it

[The Dude]

I get errors trying to open either of those files.

[ct88]

Should be fixed now

[Enzo]

Opens for me.

[The Dude]

Yes, they open for me now, too.

What DC voltages do you have on the 15V rails? Also measure ACV on those rails to make sure they are clean.

[ct88]

Ok, going to measure those today and post them

[ct88]

ok, so the voltages are:

9.47 Vac on WP117 and WP118
9.48 Vac on WP118 and WP119

and after the regulators:

+14.90 Vdc on + of C126
-15.04 Vdc on - of C135

All seems pretty stable, they arrive correctly on the othet side of the CON4 ribbon cable

[strato56]

What voltages do you have on HT1, HT2 and HT3 ?

[ct88]

Ht1 367.7 v ht2 237.6 v
ht3 229.5 v

[g1]

Those seem kind of low. What DC volts do you measure on each side of R127 in the main supply cap. area? (also check AC volts there)

[ct88]

R127 has 438.8 V and 431.5 V. WP109-WP112: 341.1 VAC
WP105-WP106: 98 VAC

[ct88]

Any clues?

[g1]

What is AC volts on each side of R127 ?