Marshall DSL issue

[Valvehead]

Hi guys - I have a Marshall dsl 50 here . I can power it on and play for 1 or 2 minutes, then the signal slowly fades to nothing. It will fade back in randomly , and eventually fade back out.
Swapped tubes .
cleaned all jacks / sockets .

I was able to measure some stuff and got some stragne readings when the amp is in "dead" mode .

This isnt right :

amp dead
V1 plates skyrocket to 423/423 vdc
V2 428/428
v3 433 / 244
v4 290 / 292

power tubes measure normal dc and bias .

also , when amp is dead i get 3.3 vdc at V1 and V2 heaters. But v3/v4/ v5 / v6 are normal at the heaters.

Could it be the bridge rectifier that feeds v1/ v2 ? Or does it sound like a dying cap problem ?

[52 Bill]

Cold solder joints on the rectifier bridge are pretty common on these.

[Valvehead]

they look ok , but i can re-flow them

[woodyc]

Sounds like a bad ground on the preamp board. One thing to check might be the wire that goes directly from that board to a spade lug on the chassis.

[Enzo]

LOOK at the heaters in the preamp tubes. Do the first ones go dark?

WHen you say you have 3.3v on the heaters, I assume you mean to ground. WHat matters is the voltage across the tube heater, not what the voltage to ground is. And with the heater itself being a low resistance, if you had that 3v on one end of it, you'd also measure it at the other.


I also vote for crummy solder at the heater rectifier bridge. Remove the bridge, scrape clean the legs to remove any oxidation, then reinstall it with fresh solder. DOn;t just reflow it in place.

[Valvehead]

Yes i meant 3.3 vdc to ground. Cant look at the heaters right now--the pcb is removed. I will re-do the bridge as stated...thanks Enzo

[Valvehead]

Ok, cleaned the bridge legs and got it back together---same thing signal fades out --and sure enough, V1 and V2 filaments go dark. The other tubes look fine.
Safe to assume its that bridge rect. ?

[Enzo]

Worth just replacing it, yes. But first, if you can, check the DC coming out of it, or for that matter the AC going into it, right at its pins, and then along the traces to the sockets. Won;t help to replace a bridge if the trace to the first socket is cracked.


In other words FIND the actual problem.

[Valvehead]

Ok....ill try to get to it.

is that a universal type part? never ordered one , Mouser might have it

[Enzo]

Yes, it is just a bridge, I forget the specs.

Schematic says: BR62 - looks like 6A 200v. Search BR62 at Mouser and find them.

[Valvehead]

Ok, here is what i got -

was able to have the amp on and measure.

these readings are when the 1st 2 tubes are dead =

On the + of the rect. , -3.8VDC to ground

on the - pin , -3.8 vdc to ground

from the + pin to the - pin i get 0vdc on the rect.

from the ac pin to the other ac i get 6.5VAC

there are 4 electrolytic caps right before or after the rect.
they all had 3.4 VDC on them if that helps ?

[g1]

On the + of the rect. , -3.8VDC to ground...
from the + pin to the - pin i get 0vdc on the rect.
from the ac pin to the other ac i get 6.5VAC

Replace the bridge rectifier.

[Valvehead]

Installed the new bridge rec tonight. amp has been on for 20 minutes and the filaments look normal now . i think the problem is licked ------- thanks fellas !!!!!!