JCM 800 2204 - Died - No Current?

[jgab]

Hi,
My 86' Canadian JCM 800 2204 died on me. I turned on the power and there was no indicator light (6vac powered off the first power tube). Tubes didn't light up. No sound at all through the amp. Here is what I did:

- Shut off the amp and plugged it in through my light bulb limiter. With the amp fully on, I don't have any shorts.
- Took the amp off the limiter.
- Measured 120vac at my plug connector, through my mains fuses (Canadian Version has two HV fuses). - Measured 120 vac on both sides of my power switch.
- Measured 120vac out of my mains selector.
- Getting ~360 vac (2X's) on my high voltage secondaries measured at my diodes, and ~470 vdc rectified and on my plates (screens about the same).
- Measuring 3.30 vac (2X's) on my heaters (heaters don't light up on any of my tubes- cool to touch).

I am getting ~470vdc on my screens and then almost the same voltage after my two first 10K droppers. Usually I have about 327-330 vdc dropped after the two 10 droppers and going to my PI. Instead I have about the same voltage throughout.

If I had current, the voltages would be lower right? I don't seem to have any current. I tried plugging into my variac and direct into the wall. Also used a couple IEC connector power cords to rule out the cord.

Not sure what to do right now. Can a bad filter cap cause this? If a 50uf/50uf goes bad?

Any advice?

J

 

[Pete Farrington]

Measuring 3.30 vac (2X's) on my heaters

At every valve socket?

 

[Purgasound]

Bad caps won't affect heaters. Aren't there fuses on the heaters for Canadian models?

 

[jgab]

Yes all the fuses passed continuity tests on my mm.

I have 3.3vac at every valve socket v5 to v1. . 2/7, 2/7, 45/9, 45/9, and 45/9.

I don’t know wtf is going on. So strange.

Damn pilot light won’t even come on. 6 vac pilot. Metal toggles.

 

[Helmholtz]

Yes all the fuses passed continuity tests on my mm.

If you tested the fuses in the amp you are measuring the very low resistance of the surrounding heater circuit.
To test fuses they need to be removed from circuit.

 

[Helmholtz]

Another tip:
A fuse is essentially a zero Ohm link.
So, whenever you have a substantial voltage between the terminals of a fuse in a circuit, the fuse must be open.

 

[dtier]

You likely have one side of the heater circuit open. What do you read from pin 2 to pin 7 vac?

 

[dtier]

When you read from chassis ground you may just be seeing the one feed on both sides of the heater windings as there is no voltage drop across them.

 

[johan.b]

Canadian and Scandinavian 800's have two small "pico-fuse" mounted at the powerswitch end of the pcb. Check those. You can't really check for continuity since the whole circuit is low z. Power up and measure voltage across them. I bet one will read 6.3 volt, meaning fried.
J

 

[jgab]

Hi! Yes I screwed up. I had to pull the fuse as mentioned. One of the 6.3vac heater fuses popped. Just one of them. Now the question is "why". I just finished testing my tubes and didn't find any shorts or issues, but I think there could still be a tube problem. Once I get a new fuse tomorrow, I will fire things back up on my limiter. and bring up the voltage on my variac.

Anything else I should try? Should I fire it up without tubes? Is it possible I just had a current spike from my mains at the time? The fuses are T6.3vac so not a lot of room for current spikes.

 

[dtier]

You could plug the tubes in one at a time while on the limiter and on standby. Could have a heater to cathode short in a tube?

 

[jgab]

Just to close this off, it was a failing 12ax7. Drew too much current.

I know people talk crap about the extra fuses in the Canadian models, but I would rather have 5 extra fuses over a blown transformer any day.

Everything is good. Live to fight another day.

 

[PelliX]

Just to close this off, it was a failing 12ax7. Drew too much current.

Problem solved, cool. Incidedentally, you can measure the heater filament's resistance with a regular meter. It won't always give you a clear indication that something is wrong (filament may behave differently when heating/hot), but you can determine that it should come on (i.e. there is a low resistance instead of a completely open circuit) and measure to the other pins to check for any shorts to other parts of the valve.

 

[jgab]

Measure resistance from one 3.15vac secondary to the other, or each secondary lead to ground?

I really need a solid guide for testing power and output transformers. That is one area I suck at for sure.

Also, after I replaced the tube, the amp is less distorted and has a bit of a tighter bottom end. It was the first 12ax7 Mullard (i63 gen) that I've ever seen fail. I’ve never had a vintage Mullard fail on me until now. I’ve had the tube in there since about 2006. Had a good run.

 

[PelliX]

Measure resistance from one 3.15vac secondary to the other, or each secondary lead to ground?

No no, the tube itself. You can take it out and measure the resistance from one heater pin to the other(s). That should give you a reading that you can compare with others of the same type (it's an indication, ballpark figure). All other pins (plate, grid, etc) should have infinite resistance to everything else as far as you can measure on a multimeter.

Also, after I replaced the tube, the amp is less distorted and has a bit of a tighter bottom end. It was the first 12ax7 Mullard (i63 gen) that I've ever seen fail. I’ve never had a vintage Mullard fail on me until now. I’ve had the tube in there since about 2006. Had a good run.

Yup, it happens eventually. C'est la vie, eh.