Marshall JCM 900 4100 blowing fuses

[Hapa]

Sirs;

I have on my bench a GCM 900 that is blowing fuses immediately on power up. I removed all the tubes and it still does it. The standby switch position makes no difference. I’m feeling this is a transformer issue, but there are no telltail Signs such as discolored windings or evidence of too much heat. Can anyone offer me some ideas of what to check?

Thank you

 

[V-man]

Possibly unrelated but years back my 4100 issue was traced back to some arcing at the 2-way impedance switch… an oddball part the 900s had instead of the 3-way rotary switch you typically see on Marshalls.

 

[Purgasound]

1. Should be posted in the Workbench section
2. Always give specific model numbers. There are three completely different JCM900 models, MKIII, DR, and SL-X, all of which could have a different failure.
3. It's good that you've got the tubes removed. If you don't have a lightbulb limiter or current limiting device troubleshooting will involve a lot of fuses. I would tell you to start disconnecting secondaries from the PT but I can't speculate on what else could be wrong unless we know the model. Generally speaking though, the 900 series shares the same PT so each will have 3 windings, one for the heaters, one for the HT "high tension" supply and one for the LT "low tension" which powers the switching IC's. If the standby switch is off but the fuse still blows you've likely ruled out the HT secondary. Although the standby switch only lifts one side of the HT secondary and the bias supply is tapped off this line prior to the switch. If there is a short in the bias circuit it could pop the mains fuse. The diodes for the high voltage rectifier need to be checked. Failed filter caps. Could be the heater winding. Where do you want to start?
I would disconnect the secondaries first and power the unit on. With the secondaries disconnected you can rule out if the transformer itself is shorted. Then you can power down and reconnect each secondary one by one until you've determined which one is bad. Noting that the heaters don't have any secondary connections in any model except the 900 SL-X so those might not need to be disconnected in the other models.

 

[Hapa]

I’m not entirely sure how to determine the exact model but I can supply the following;
It is a 100 watt hi-gain dual reverb
Chassis end sticker says 4001 U
Preamp board says 94 1-0
Speaker Jack board says A.I

It should be noted the unit came to me with EL34s that tested approximately 30,30,50,50 where replacement is indicated at 75 on my machine. It arrived with a blown mains fuse,and,get this……the first spkr fuse was 5 amp and the second was 20 amp which seems likely to have caused or contributed to the amps present condition. I have tested all diodes and resistors on the board and tube sockets with nothing remarkable appearing.

 

[Purgasound]

Take it to a qualified technician.

 

[Hapa]

That would be ideal,but there’s no one remotely qualified nearby,and I’d rather not endure the expense and risk of shipping.
Anyone else have any ideas?

 

[79JMP100]

ten bucks says the PT is gone, with some possible collateral damage from the hideously oversized fuses... It might be hard to obtain a replacement 100W PT for that unit at this point...

Do as suggested: take a good picture of the circuit board where the PT connects, then carefully remove all of the secondary wires from the board. Cap the wires, put the proper fuse in the holder, and switch it on. If it blows the fuse, you have your answer- the PT is gone. If it does not blow, then your trouble is downstream from there, and will require technician style troubleshooting...

 

[danfrank]

You are correct, the model number is 4100, which is the DR model jcm900. I don’t know if you edited that in or not…
Your PT is most likely blown because the bias “X” cap blew out and shorted, which basically shorts the HV winding to ground. Or better stated, what is supposed to be the “x” cap was replaced sometime in the past with a non “x” cap and that shorted… A common problem with jcm900 amps because the engineers at Marshall back then were numbnutz and designed a severely compromised bias supply in order to save a few pennies for the bean counters.
So replace C15 on the schematic and then power up and see if fuse still blows. If fuse blows, you have a bad PT.

Schematic:

https://www.thetubestore.com/lib/thetubestore/schematics/Marshall/Marshall-JCM900-Dualrev-100W-4100-Schematic.pdf

Replacement PT:

British 900 100W Power Transformer (Direct Replacement for the Marshall® JCM900)

British 900 Style 100W Power Transformer. (Direct Replacement for the Marshall® JCM900)Please use caution when referencing these wiring diagrams. Mojo transformers have undergone various changes over the years and these wiring diagrams are meant to aid in the purchasing process and represent...

www.mojotone.com

I had to do this about a week ago where the PT on a 4100 blew. I was fancy though and installed a small dedicated bias transformer in the amp because C15 is prone to going bad. Marshall should put me on their payroll for solving their fuckups.

 

[Purgasound]

Don't blindly replace parts.

Replacing C15 and still blowing a mains fuse doesn't automatically equate to a bad PT.

The OP is already lost in the woods. Let's not give them even worse directions.