JCM800 Power Woes

[lpd]

Canadian JCM800 2204 50 watt Canadian amp.

Turned on amp standby on lights up look away and back no power. All fuses test good and then test ac on the main switch and it has ac on both poles.

I suppose pt is hooched. How do I measure impedance on the pt on this amp? I can post pics if helpful.

David Root mentioned there may be a thermal switch in the pt may be repairable? If pt is doomed any thoughts of what else to look at before a swap to ensure it isn't a faulty circuit?

[stretch2011]

You should make sure you have the pt wired up right. If you havnt checked the other voltages that'd be my first thing to do. Just to make sure you don't have the filament and HT mixed up. If it's a multi tap primary you might want to recheck which is the right tap.

I've never had a pt blow on me, you might be able to google power transformer winding calculators. Since you know your wall voltage, the rated secondary voltage and can read the ohms on both primary and secondary that may hint if it's blown or not.

[Stevem]

With just the main on / off switch in the on position you should have 6.3 volts AC across pins 2 and 7 on each output tube and on pin 5 you should have over 32 volts dc but under 45, the voltage on pins 5 will be minus!
Also on the input side of the standby switch you should have over 400 volts of ac, this AC voltage is read across the switch not to ground and with a normally working standby switch in the on position you will find this voltage on the input side of the rectifier diodes, is this the case when you check it out?

[lpd]

Thanks guys I will check out your recommendations. I should have indicated that this was until yesterday a perfectly fine working Marshall 2204 amp with no issues. It's not a new build

[lpd]

With just the main on / off switch in the on position you should have 6.3 volts AC across pins 2 and 7 on each output tube and on pin 5 you should have over 32 volts dc but under 45, the voltage on pins 5 will be minus!
Also on the input side of the standby switch you should have over 400 volts of ac, this AC voltage is read across the switch not to ground and with a normally working standby switch in the on position you will find this voltage on the input side of the rectifier diodes, is this the case when you check it out?

No heaters Or any voltage for that matter.

[Stevem]

Then turn the main switch on and check for contiuity thru the Amps power cable as you may just need a new IEC cable.
When set for 120 volts then resistance wise you should between 1.4 and 2 ohms if the cable is good and your reading thru the transformer.

[lpd]

Then turn the main switch on and check for contiuity thru the Amps power cable as you may just need a new IEC cable.
When set for 120 volts then resistance wise you should between 1.4 and 2 ohms if the cable is good and your reading thru the transformer.

The power cable is good as I get 120ac on the fuse and both poles of the main switch

[lpd]

Ok burned a few brain cells. Pulled all tubes and fuses and checked continuity and boom...popped heater fuse. Over to tubes and shorted heater on output tube. Sometimes things are really that simple.

[pdf64]

Pulled all tubes and fuses and checked continuity and boom...popped heater fuse. Over to tubes and shorted heater on output tube

So the short was on the output tube socket?

[lpd]

Pulled all tubes and fuses and checked continuity and boom...popped heater fuse. Over to tubes and shorted heater on output tube

So the short was on the output tube socket?

Yes. I put in a new pair of EL34s works fine Oh and a new fuse. The output tubes in the amp were old and looked haggard. I never checked bias when I picked it up either. The tube shorted on standby is this usual?

[Stevem]

Yes, a tube can give up with just the heater powered on!

[lpd]

Well here I was complaining about the amount of fuses in this amp to have a fuse pop on the heater circuit.

[lpd]

Well here I was complaining about the amount of fuses in this amp to have a fuse pop on the heater circuit.