Marshall JCM900 Preamp Tubes not lit

[hashonboat]

Hi, I just found this forum. I hope someone can help.

I have a '92 Marshall JCM900 SL-X 2500sl-x 50W head. The only sound I can get out of it is a pop when I turn the standby to off.
The heaters on the 3 preamp tubes V1-3 are not lit. V4 preamp tube & the 2 Power tubes are lit. A tech suggested I order a retube kit so I bought new JJ's . (4) balanced ecc83s's & a matched pair of 6L6GC's. I still have the same problem. I found the schematics here http://www.schematicheaven.com/marsh...x_50w_2500.pdf
I can read them a little & after taking the amp out of the cabinet I can see that the C8 1000mf 16V cap is rounded/bulged on the top, not flat. I'm going to replace the cap Monday.
Is there anything else I should check? Is there anyway I can verify this (with no power applied). I do have a multi-meter & know how to use it but I don't really know enough about what NOT to touch when it's powered up.

Thanks in advance!

[Don Moose]

This is just a guess, but I'd look at BR3 - it sounds like it may be toast - and the wires that lead to it and away from it to V1-3.

Here is a good set of safety tips.

Hope this helps!

[SWEINC]

replace bridge rectifier BR3

Your problem is absolutely BR3. This bridge rectifier converts the 6.3 VAC filament voltage to DC for V1-3 to reduce hum. I see these blown quite often. Replace C8 also. It is the filter cap for BR3 and got stressed also.

[hashonboat]

Thanks alot! This is a great forum. I had problems finding someone where I live that works on amps full time but this has been a great resource & I think you all have defineitely nailed it. I'll report back tomorrow night after I get these parts in.

[Amp Kat]

I would also use a higher rated rectifier when changing it but I too agree BR3.

[hashonboat]

Do I just need the number off the top of the br3 when I go to get a new one?

[SWEINC]

br3

BR3 is a W005F 1.5 amp 50 volt bridge rectifier. Any bridge rectifier with equivalent or larger specs will work.Higher is better, but make sure if you use a higher current bridge that it doesn't use heavier leads that won't fit through the PC board holes.

[Mars Amp Repair]

one last suggestion;
mount the bridge rect off the board about 1/2" or so...that allows the heat of the bridge to dissipate into the air before it reaches the solder connections.

Depending on the thickness of the wires on the bridge, you should also add a slight kink in the wires of the bridge where they go through the ckt board (top of ckt board) added physical strength.

Otherwise you only have the solder on the clad side to hold the bridge in place. Kind of a recipe for future connection issues.

glen

[Enzo]

On my Marshall heater bridge repairs, I don't so often see bad bridges as I do the bridge wires never wetted in the solder, so the solder never flowed onto the legs, it just flowed up against them.

[SWEINC]

marshall bridge repairs

Enzo,
I usually do encounter what you are saying. The larger style bridge they use in the DSL series is either not soldered or has desoldered itself, but I have seen a number of blown bridges of the 1.5 amp style in the SLX series.

[Amp Kat]

The ones that are bad usually leave a dark brown spot underneath the BRD and the pc board. I have also seen a cold solder joint on the socket causing the tube/tubes to drop out after a minute or so of playing fine.

[drewl]

On my Marshall heater bridge repairs, I don't so often see bad bridges as I do the bridge wires never wetted in the solder, so the solder never flowed onto the legs, it just flowed up against them.

Had 2 this month exactly like that.

[hashonboat]

Well, I am still waiting on parts I ordered from Mouser. The BR3 I have is actually a 2.o amp 100v Bridge and of course no one around here has anything above 1.5 in the same small package. That area of the board is really cramped. I also couldn't find the cap in anything but a 25wv which is huge & covered the holes where the bridge goes. Anyway I'll update you all when I get the parts & get em in.

[Enzo]

Is there room to lie the cap on its side? That would clear the other holes.

[hashonboat]

OK, All. My amp is back up & running!
I kinked the leads on the rectifier so it's about a half inch off the board, all new tubes, new cap.
Thanks for all the help & suggestions.