hum/buzz from a reissue marshall bluesbreaker

[acorkos]

this amp is relatively quiet (not silent) with the volume controls turned down, nothing plugged into the inputs. turning the volume controls up brings on a buzz (higher frequency than 60hz hum). pulling v1 and/or v2...no change. pulling v3 (phase inverter) kills the buzz (and all sound). tone controls change the vol/tone of the buzz. not sure where to go next with this? thanks!

http://drtube.com/schematics/marshall/1962ri89.gif

[Jazz P Bass]

[QUOTE=acorkos;228957]tone controls change the vol/tone of the buzz. not sure where to go next with this?
If the vol/tone changes the noise, then the issue is most likely BEFORE these controls.
I would do a static check of the coupling caps, of coarse looking for an "leaking" Vdc on the grids of the preamp tubes, coming from the previous plates high voltage.

[pdf64]

Maybe the shorting contacts on the input jack sockets aren't working?
Try plugging a guitar with the volume turned down to see if that stops/reduces the buzz.

[acorkos]

jacks are all good + working normally

[pdf64]

Is the link between chassis ground and circuit ground good?

[acorkos]

circuit grounds are all intact

grids of v1a, v1b and v2a all measure 0VDC

54 vdc on the input of the presence control....is that normal?

[pdf64]

So definitely excellent continuity between the circuit ground and the chassis metalwork?
54V - yikes!
That's at the node of the 27k feedback resistor R25 and 10k LTP tail resistor R2?.
There's an error on the schematic, as it doesn't show a dc ground return for the LTP.
I guess there may be a 4k7 fixed resistor between the above node and ground?

[acorkos]

definite solid continuity between circuit and chassis

the presence pot in this amp was wired like the schematic. i rewired it like a typical marshall presence pot w/wiper > cap > ground and output lug directly to ground...no change in the hum

the node where r25 and r2 connect w/the presence pot input measures 54vdc.
i think i'm going to replace c11

[pdf64]

What are V3 voltages; pins 1 and 6, 3?
Node B (supply to V3 plate resistors)?
Either side of R2 10k tail resistor?

Is the back panel screened with foil or something in those amps?
If so, has the screen got a solid ground connection (ie when the panel is screwed down tight)?

[acorkos]

What are V3 voltages; pins 1 and 6, 3?
Node B (supply to V3 plate resistors)?
Either side of R2 10k tail resistor?

Is the back panel screened with foil or something in those amps?
If so, has the screen got a solid ground connection (ie when the panel is screwed down tight)?

V3-pin1 220V
V3-pin6 217V
V3-pin3/8 77V
Node B 315V
R2 where it connects to 1M/470R/1M 76V
R2 where it connects to presence pot 54V

there is foil on the back panel....it looks to make solid contact with the chassis

[pdf64]

Wow, those amps must have been sent out as per the schematic, with the LTP dc ground return via the 27k feedback resistor then OT secondary//speaker.
It just goes to show that anyone can screw up, even after the drawing has got to issue 3.
See
http://www.webphix.com/schematic%20h...5_lead_45w.pdf
for how it should be, though that has the error regards the power supply choke arrangement.

But that obsevation isn't getting the issue fixed.
Have the heaters got a good ground reference?
Have you tried different tubes, especially V2?

[Zouto]

Have you checked the trannies' grounds at the mounting screws? There should be a solid connection between amp chassis and transformer chassis, which is sometimes coated by a clear coil-sealing lacquer.
Removing that stuff around the mounting holes could help.
Zouto