DSL 401 no sound/weber mini mass

[MarkB]

Hi all, we spent about an hour with out 401 two nights ago. We were using a Weber Mini Mass (16ohm) and the amp worked fine. We shut it down after an hour or so. The next morning we have no sound out of the 401 (straight connection). I am electronics savy, but not on amps. The output tube (the ones braced) light up. The fuses are good. My first thought is we damaged the speaker, but I don't have a 40watt or greater speaker to swap in to see. Perhaps we are at the end of tube life and pushing the amp with the Weber pushed them over the edge?
Any help where to start would be appreciated. The 14 year old is now forced to practice on a solid state!

MarkB

[rf7]

You can try any old speaker that you know works, and don't crank the amp. Just see if it makes sound with another speaker. If it does not make any sound with a different speaker, then it's time to dig into the amp.

[Enzo]

Or you can unplug the speaker and touch the speaker wire plug to the terminals of a 9 volt battery for a moment. Every time you touch, the speaker will go pop or thump if it is working. No pop means bad speaker.

[MarkB]

Speaker trick...

Thanks will try...I have never heard of the 9volt trick....

MarkB

[MarkB]

Plenty of POP

Did the 9volt trick, plenty of pop from the speaker.
Popped the amp head and F4 good (whatever that protects)....

Any thoughts on where to go now?
MarkB

[Enzo]

Yes, do the preamp tubes also light up?

I would go inside and start looking for voltages on the tubes.

[MarkB]

Preamp tubes...

The three tubes to the right (looking at the back of the amp do not glow. The one below the power tubes glow OK. I swapped tubes into that slot and they glow OK. I am assuming the tubes are good at this point.


MarkB

[Enzo]

Those tubes run the heaters on DC. Bridge BR1 is euther dead or lost its conncetion to the circuit board.

[MarkB]

BR1

Yep, I see that on the Schematic, it's common to V1, V2, ,V3, and V4.
But V4 lights up. Thoughts?

MarkB

[Enzo]

Oh? My schematic shows it supplying V1,2,3, not 4. V4 runs off FIL1/FIL2 along with the power tubes. My thoughts are still with that bridge.

And if I may hazzard some advice, never think up reasons NOT to check something.

[MarkB]

Br102

On it..I ordered a couple of rectifiers and I'll check the solder joints tonight. After checking several sites last night it looks like that component gets pretty hot. Many owners of the older 401's heat sink BR102. And yes you are correct, BR1 is common only to V1, V2, and V3. My bad.

MarkB

[MarkB]

Caps..

Enzo, thought.
I had some high voltage on W10 referenced to ground. I discharged C9 by shorting W10 to chassis. Am I OK to work on this beast at this point? All of the caps are surface mounted so I cannot check the DC on them without grabbing the PCB and going to town on it to get to the underside.
I remember on an Fender I used to have I would take a a power tube pin 3 to chassis and since all of the big caps were in parallel that was all that needed to be done.
Any thoughts would be appreciated on the discharging of all the big caps.

MarkB

[Enzo]

By surface mount I assume you mean radial leads on the caps. Surface mount is a specific term that means parts are soldered to the solder side without the leads running through holes to get there. Tiny little fucking things...

There are lots of places to check voltage without exposing the foil side of the board. Tube socket pins for plates and screens, screen resistor leads, HT fuse clips, rectifier pins, output transformer primary leads, plate load resistors, etc.

C9 doesn't hold much of a charge, it is just the small cap across the switch. C32 is the main filter cap. Then any cap on "HT1" through the circuit, like C42. Then on down the B+ string.

If you are not sure, FIND OUT. Never assume clipping something to something else will leave everything discharged unless you know it is the case. If you are not already confident that the thing you grounded makes it safe, then get out the volt meter and FIND OUT.

[MarkB]

Wrap Up.....

Enzo, you were indeed right on the rectifier call. Although it tested the same as a new one, I popped a fresh one in and V1, V2, and V3 glowed and the amp fired up without issue. I have read that just replacing BR102 is a temp fix. I installed a heat sink and the temperature on the front of the amp when pushed is no where near as hot as it was. I also will install a 110vac fan to help as well.
I took the time and brought the Conn 5 to banana jacks at the rear of the amp to aid biasing. Two bottom lines on this adventure. Number 1: The pre 2003 Marshall DSL 401 is a kick ass amp when pushed as long as you deal with the bad design with mods. Number 2: Using a Mini Mass or other attenuator may not be the best idea with these amps. More on that later as I will probe the temp of the amp when cranked in conjunction with the mini mass.

Thanks for you help Enzo,
MarkB