VOX AC15HW1X blowing fuses

[Wavelength]

Gang,

I have Rob Fetters AC15 in here at the shop (I design and make amps, don't usually fix other companies stuff). He was recording a track. Turned it off, came back later turned it on and it started eating fuses left and right. I have 1A and 1.5A 5x20mm here and they go immediately on power up.

This thing blows fuses on power up. I removed the rectifier tube and check the caps there are no shorts in the HIGH voltage area. Choke is good, nothing on the secondary looks funky. The filament supply looks ok.

The power transformer at least from the back side looks like a 117 only version. Is this typical for this model?

Schematics anyone? Thoughts? Power transformer dead?

Thanks,
Gordon

 

[lkft]

You say nothing regarding the power tubes...are they good? B/c this is the usually the first suspect when troubleshooting blown fuses.

 

[mscaggs]

From the amp debug page

• fuse is the wrong rating - replace it with the correct rating
• power tube shorted
• rectifier tube shorted
• power supply filter cap failing
• Carbon trails on the output tube sockets between the plate lug and the other electrodes, especially the heater electrodes.
• power tubes have lost bias or biased incorrectly
• power section of the amp is oscillating at too high a frequency to hear
• there is an ac wiring short or high leakage
• power transformer is faulty
• choke (if present) is shorted/leaking to chassis
• output transformer is faulty
• AC power wiring or B+ power wiring is faulty/shorted

 

[Cirrus]

Yeah, power tubes is the no.1 suspect here.

 

[Wavelength]

Thanks all, but with the rectifier tube out the output tubes would not be getting any HV and the fuse is still blowing. I will pull them anyway and report back.
Thanks,
Gordon

 

[Wavelength]

Output tubes test ok, but not deadly. All tubes out fuse still blows. HT fuse is ok, Filament fuse is ok. It's really looking like a shorted transformer. Does anyone make the straight 100/117 input transformer or do I need to get an international and wire accordingly?

Thanks,
Gordon

 

[Wavelength]

A quick update, as I am totally confused about this sucker. I have the chassis out and I do not see any shorted wires or anything. The HV secondary is weird, PIN1 side of EZ81 is 36ohms to center tap and PIN7 is 58ohms???

I have all the tubes out and it's still blowing fuses. With the chassis out I don't see any burned areas or anything looking faulty. The HV caps all test fine, not shorts on any of these. I am going to get some slow blow fuses in from DigiKey and see if it's just the way this thing is designed.

Anyone else have any other ideas as to what it might be?

Thanks,
Gordon