Marshall DSL-50 fried lineup of B+ resistors in preamp ?

[Iplayloud]

Hi everyone, my DSL50 had a huge volume drop and then no sound at all.

Schematic:
http://www.drtube.com/schematics/mar...60-02-iss7.pdf

So on the schematic just above V2, R24 is burnt crisp black (multimeter reads open), R61 and R62 before and after V3 are dark brown, and R63 before V4 is burned black, but still reads 4.7k.

-Does this look like a shorted preamp tube ?

-Am I right to assume it must have happened in a triode before the earlyest fried resistor (R24) (either V1B or V2a)

-If it's a shorted preamp tube, why is the corresponding plate resistor not also fried ? (R22 or R23)

Thanks !

[Iplayloud]

To keep things simple, can I just change those resistors, put fresh preamp tubes and fire it up ?

[Jazz P Bass]

I would not "Fire Up" anything yet.
You need to discern the cause of the failure. (which is most unusual)
You may be correct in assuming the path was through R28, as it is open.
I would suggest removing the tubes & the burnt resistors.
Then measure the resistance of the high voltage connection points to ground.
You may find that C21 is shorted.
If nothing can be found out of whack, replace the resistors but leave the tubes out.
Now you can "fire it up" while monitoring the B+ string.
If everything agrees install the preamp tubes one at a time.
Again, while monitoring the voltages.
If it looks good, install the power tubes & let er rip.

[Iplayloud]

Thanks P Bass for your answer. This happened while the amp was loaned to a friend, after the "problem" happened he changed all the tubes thinking it would fix the problem. So I don't have the original preamp tubes to check for burning signs. Maybe he played with a 12AX7 and reinstalled it in a bad position and bent a pin and shorted the plate and cathode connections...

I pulled all the tubes and measured the resistance of the high voltages points (both sides of R24 and R28)(while the amp is off) and it starts around 80K and then rises to 300-400K. That's the capacitors doing that right ? I suppose if one of the capacitors was shorted the resistance would go very low rather than rise, right ?

[Iplayloud]

So my multimeter is trying to gauge the resisance by passing a voltage or current but as the capacitors fill their resistance rises ?

[Jazz P Bass]

Yup.

[Iplayloud]

I changed the fried resistors, put new preamp tubes. Now I power up the amp and point my infrared temp meter on the new resistors and within 2 minutes they heat up to 200 degrees F. There is still a short. Assuming that the short must be before the first fried resistor R28 I disconnected Con3 to eliminate the possibility after that point and the resistors still overheat, the only remaining possibilities for shorts are C19 & C21 right ?

It is surprising because I had tested them and the resistance rises so they don't appear shorted

If these caps are shorted, should they get hot too, because they stay cold. Is it just the resistors that become hot because they are semi-conductors while the shorting capacitor is a direct conductor and doesn't heat ?

[Iplayloud]

Alright forget it.. I pulled C19 & C21 and now the B+ resistors stay cool. Even though they don't read shorted they were doing something wrong. I should have done it right away but you know how cramped it is in a DSL and having to pull everything apart. Thx !

[Jazz P Bass]

If you follow the schematic, C19 & 21 are the Only components that have a path to ground after R28.
That is why you would want to supect them.
I hear what you are saying about getting in there to work at stuff.
It's real easy to sit here & look at the schematic & say "well pull such & such".

[Alex R]

Amen.

It is hard to test caps, when they short it's often intermittent, so yes jou just have to pull em out.

I want an infra-red thermomenter like yours! I use my fingers instead and it's not wise at all.

[jmaf]

I use terfingermometers too, as long as the thumb isn't resting on the edge of the chassis, you should be safe from shock, though not from burns. Electrolytics form the electrolyte only when powered up, thus some appear fine and blow up in a mushroom cloud when you power them up. I use a 100W lamp current limiter in series with the DUT for that reason.

[Rick Erickson]

A DSL burning resistors? I'm shocked.... LOL
If you suspect the caps then pull the preamp tubes while observing the voltage drop across the overheating resistors. If the circuit continues to pull excessive current it will suggest the filter caps may be failing under load. Check all the tubes to be certain they are all 12AX7's. Chinese 12AX7's work best, and generally sound best in these amps. They are not all the same - some brands will pull more current than others, and some just plain sound terrible. A 12AT7 or 12AU7 in a location that calls for a 12AX7 will cause supply and plate resistors to go up in smoke. At best these resistors are usually running on the edge of sanity and replacing them with a higher wattage rating will not hurt anything. Check all the plate resistors and cathode resistors of CF stages too. I would be surprised if there aren't some of those smoked or drifting from overheating. My shop has pages of things to check & fix on DSL and TSL amps. This is a common problem - most players don't look inside their amps, so they only find out about overheating resistors when their tech informs them.