Marshall DSL 100 JCM 2000 only 2 preamp tubes light up

[Djizix]

So awhile back, I took this baby to a tech to have the board drilled out for the bias drift issue. Well everything has been good since until just the other day I went to fire it up and it wasn't getting any sound.

After I didn't get any sound it of it, I looked at my tubes and noticed V1 and V2 lit up but none of my other tubes. So I assumed it was a loose V3. Wiggled on it a little bit, powered it back on and it played. So it didn't give me any more trouble for a couple weeks.

When the issue reoccurred, I tried the same thing and the amp wouldn't come out of it. So I had read that the small fuses on the motherboard might have blown. I turned over the head. Unscrewed the chassis, took it out and checked continuity of the fuses and they were fine. I want to stress that that is all I did while the amp was open. I put the amp back together and turned it on and it worked again. So the next time I went to play, the tubes didn't fire up so I simply turned the head upside down and powered it on and I'll be damned if the thing didn't turn on... So my diagnosis is I need to refill the solder joints on V3 but I wanted to get some professional experience before I started anything.

[Leo_Gnardo]

The first two pre tubes are run from a DC supply derived by rectifying & filtering the filament line. I've been able to fix this situation a couple times by resoldering the rectifier bridge pins where they poke thru the circuit board. It's worth a try.

[Djizix]

The first two pre tubes are run from a DC supply derived by rectifying & filtering the filament line. I've been able to fix this situation a couple times by resoldering the rectifier bridge pins where they poke thru the circuit board. It's worth a try.

I suppose it's probably a stupid question. But before I plug all this back together and try it out, how likely is it that this bridge is bad? It has a faint discoloration on the grid connecting to the pins so I wonder if it didn't get too hot somehow and melt the component?

[Enzo]

it probably is fine. I have many times had to remove the bridge, shine the legs - usually by scraping with my Xacto blade - and resoldering. If you doubt the part, they are cheap enough to replace.

My personal theory is the bridges, wherever they came from, had some surface contamination causing the original solder to wet poorly to the leg surface.

[drewl]

I actually had one go bad a couple weeks ago in a 50w combo.
Cleaned the legs, resoldered it real good, it cut out after an hour.

Had to tear it all apart again to replace it.

[Djizix]

Resoldered it and it's working fine so far. Thanks guys.

[g1]

I'm glad you got it straightened out, but I'm a little confused by the description.
The bridge supplies only V1 and V2 heaters.
So all the tubes should have lit up except V1 and V2.
This seems the opposite of what you said in the initial post, but is that what you meant?

[Djizix]

I'm glad you got it straightened out, but I'm a little confused by the description.
The bridge supplies only V1 and V2 heaters.
So all the tubes should have lit up except V1 and V2.
This seems the opposite of what you said in the initial post, but is that what you meant?

Nope. V1 and V2 were the only two lit up. I'm not educated enough to tell you how or why, but she's purring again.

[The Dude]

I'm glad you got it straightened out, but I'm a little confused by the description.
The bridge supplies only V1 and V2 heaters.
So all the tubes should have lit up except V1 and V2.
This seems the opposite of what you said in the initial post, but is that what you meant?

I thought that was the case, also. If so, the bridge wouldn't have much to do with it. I suspect some other connection problem that was tweaked with disassembly and reassembly. I wouldn't be surprised if the problem crops up again.

[Djizix]

I thought that was the case, also. If so, the bridge wouldn't have much to do with it. I suspect some other connection problem that was tweaked with disassembly and reassembly. I wouldn't be surprised if the problem crops up again.

If that's the case, what do you suggest my situation is pointing to?

[J M Fahey]

CanŽt suggest anything, since that would be predicting the future like with a crystal ball or a stack of Tarot cards.

Does it work now? Cool.

Will it fail in the future?

Maybe/perhaps.

If and when it does, post symptoms and weŽll try to help.

[The Dude]

BR1 bridge rectifier is supplied by the same AC as the rest of the filaments (if I'm looking at the right schematic- none posted in this thread). You had filament voltage to V1 and V2. That means that the transformer is good and the rectifier is good and working. As I understand it, it's the other tubes that didn't light up. Since we know the filament winding and the rectifier are working, it would almost certainly be a connection issue elsewhere (in line with the other tubes)- intermittent jumper, cold solder, etc.

[Enzo]

Since it appears to be backwards of possibility, my first thought was V1,2,3 was counting from the wrong end. Just a thought.

[Djizix]

CanŽt suggest anything, since that would be predicting the future like with a crystal ball or a stack of Tarot cards.

Does it work now? Cool.

Will it fail in the future?

Maybe/perhaps.

If and when it does, post symptoms and weŽll try to help.

I have my order in for a crystal ball. I'm not sure why my symptoms would change from the prior symptoms if I didn't fix anything this time around.

[J M Fahey]

We hate those intermittent problems because nobody can accurately predict when/if they will happen again.

But as Dude said: check connectors, soldering and tracks, you have a poor connection along the path.
use good light, reading glasses or a loupe and inspect every inch of the path.