Marshall JCM800 4212/2505 PT

[paggerman]

Hi all, it's nice to be back. Hope you're all doing well. A friend has bought the JCM800 in non-working condition and has dumped it on me to fix. I do it for fun, so I don't mind. Anyhow, it arrived with a blown fuse. First thing - test the valves. All ok except one of the EL34 is 'bad' but doesn't indicate a short (I've got a simple tester). Changed in a quick blow fuse of the same rating, put the current limiter lamp in power, plugged in (valves removed) and the lamp glows brightly.
Removed chassis - all looks very clean and unmolested (except maybe reverb master pot).
Started to look for shorts to ground. Very quickly found the HT/B+ secondary had a short on one side (red wire on standby switch to chassis) DMM reads 26ohms on side of CT that beeps a short to ground , 35.9 on the other.

Having never had one before - can this be confirmed as a blown PT? Is there anything else downstream I should be checking for as a root cause?


Edit:
I think I have answered my own question. With the limiter in and glowing brightly and the secondary isolated (standby switch) there is almost no voltage from CT to either secondary.

When I switch standby off there is just 12v after rectification.

Thinking time is a powerful thing.
Many thanks
Jeff

[Jazz P Bass]

I would check the ac voltage coming out of the secondary with the leads disconnected from the circuit board. (flying leads)

PT replacements are available.
Marshall 50W Power Transformer for JMP and JCM800

[paggerman]

Thanks for that, Jazz P. If I disconnect the flying leads from the standby switch and put the limiter in line it shouldn't glow at allf the PT is good, should it? No load, no current = no light. And check voltages as well, of course.

Thanks for the link - great prices for the goods. Shame about the cost of shipping to Australia. It would be less expensive to buy a Hammond from Digikey locally.

[paggerman]

Hi Jazz P. Disconnected secondary leads from the switch. Each side to ground was 8vac and 16vac lead to lead. 0.1vac on the heaters. Limiter was glowing brightly throughout. I think my friend has an expensive problem! 🌵

[drewl]

Transformers are available from several places only around $100-$150 or so.
So not too bad considering the amp is worth at $500 to $1000 or so.

[g1]

Did you also have the heater wires disconnected during this test?