Marshall DSL hum

[Valvehead]

Working on a DSL 50 --it has a low frequency hum on both channels. If i turn off the master volumes---hum is gone.

Its not the tubes or any solder joints. All joints have been checked / re-flowed .

I was able to power on the amp and paralell the B+ filter caps one by one .. No change ...

checking grounds --all seems good ...

if i pull V2 = no hum.

if i pull V1 --still hums .

I guess that narrows it down---BUT.. i was assuming filter caps...

what should i start looking at now ?

[Enzo]

Go through the circuit systematically. So you determined that the problem is either AT or BEFORE V2, and AFTER V1. OK. What controls have ANY affect on the hum? DO any controls affect its volume? DO any affect the tone of the hum?

Here is one I bet you didn't think of, a way out thought, but what the heck. The channel switching JFETs and LDR3 run on +24vDC, which is from C15 and D1 D2 on the rear jack board. Is that clean smooth DC? You could remove LDR3 and see if the hum goes. A long shot I admit.

Hum is not generic, it can come from many things. If you are on 60Hz power, then power supply ripple will be 120Hz hum. 60Hz hum will come from grounding issues and pickup of radiated fields and tube heaters.

[Valvehead]

the gain pots just seem to amplify the hum. the volume controls kill it when on 0 . both channels have hum.

i can try removing ldr3 .

I dont own a scope---how do i know if the dc is "clean / smooth " ?

[Jazz P Bass]

I dont own a scope---how do i know if the dc is "clean / smooth " ?

Clean & smooth refers to the Vac ripple.
You can measure it with your meter set to read volts ac.

[Valvehead]

well, let me know if this helps---

i measured ac to gnd.
at the plates and grids of the first 3 pre tubes.


GRIDS of V1 = 1.8mv ac / 1.7mv ac

V2 = 4.7mv ac / 4.1mv ac

V3 = 9.1VAC / 1.2 mv ac (is that normal 9vac ? )

Now at the plates =

V1 = 11.8vac / pin 6 the meter goes nuts ....

V2 = 18vac / 24vac

V3 = pin 1 the meter goes nuts / pin 6 is 9.2vac


does this help / make sense ?

[Jazz P Bass]

It sounds like C32 is bad (V3 filter cap).
Tag another cap, same value, in parallel to see if the ripple stops.

[Valvehead]

tried that--i did that with all the caps from the rect down the line, all the filter caps. Heard no change.

I notice now that the hum is there with all volumes on 0 .

I have a new input jack and gain pot (clean ch) on the way. Not sure if that will help--but the gain pot is noisy ---even when on the dirty channel ! I tried cleaning it to no avail.

[g1]

I notice now that the hum is there with all volumes on 0 .

Originally you said turning down the volumes killed the hum. Was that an error or has something changed?
If you plug into the FX return with your guitar volume off, is the hum still there? When you said "all the filter caps" were you including the bias filter?

[Valvehead]

I guess something changed ---the volumes killed the hum--Now they do not.

going into the fx return = there is still hum

which is the bias filter---im not in front of the scem right now---i will parallell a cap there when i get a chance

[g1]

Not sure which schematic you are using, on this one, http://www.drtube.com/schematics/mar...%29%20iss3.gif
the bias filters are C42, C36, and C37, shown to the left of V7 (area C5 of the schematic).

[Valvehead]

ok, put a cap on those and all the filters again. no change.