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.