VOX AC15CC Blowing Fuses - Help!

[JCMNelson]

My Vox AC15 has always worked great and hasn't been modified in any way. 2 Years ago I replaced the valves, and the 'new' ones have sounded great!

I played it a few nights ago for an hour or so, and everything was fine. Now it's just blowing fuses. It wouldn't power up at band prac and I figured it was prob a fuse that had gone. Got new ones today (T800, 250V) and the power light comes on for half a second before dying.

I've removed all tubes and tried, but it's doing the same.

HELP!!! I've done some work on amps and know how to discharge filter caps etc, where do I start with this?

[Prairie Dawg]

Get a schematic here: http://www.guitarpug.com/wp-content/uploads/guitarpug/2008/06/ac15cc1_schematic_only.pdf

If you have all the tubes removed, you have only three places to look: the filament string, the B+ (HT) string, and the low voltage supply for the solid state stuff. I would start by lifting one side of the HT, one side of the filament by removing FS2, and one side of the low voltage supply. This you can probably skip as the amp doesn't need this to function.

Then, power the amp up.

If the fuse still blows, your power transformer is bad.

If it does not blow, you can then connect the filament string with FS2 and power up again. If the fuse blows, you have localized the problem to the tube filament string and you need to start looking for shorted connections at the tube sockets.

If the fuse does not blow with the filament string connected, connect up the HT lead. If the fuse blows you have localized the problem to the HT circuit and you should start looking for things like shorted diodes or capacitors.

On the other hand it if looks like the power transformer is the guilty party the circuits in that are easy to check using the schematic and you will find one of the leads shorted out, most likely.

Best of luck. Tell us what the fix was.

[JCMNelson]

Thanks Dawg!

However, my inexperience is showing here a little. I've only ever worked on a 5E3 (other than replacing caps on my Traynor YGL3), so this schematic is a little intimidating.

I've found FS2, so no problem there. How would I go about lifting one side of the HT? Would that be FS1? The amp blows fuses even in standby, so FS1 doesn't seem right to me.

Would you lift the low voltage supply at W20/W21?

Thanks again for your help!

[wouldbeirish]

I had a problem with my Vox AC30CC2X blowing fuses as well. I ended up having to change the rectifier tube. I ended up putting a Weber Copper Cap in and haven't had a problem in over a year and a half. Don't know if that's what you've got going on but thought I'd mention it.

Good luck,

Ron

[Prairie Dawg]

Thanks Dawg!

However, my inexperience is showing here a little. I've only ever worked on a 5E3 (other than replacing caps on my Traynor YGL3), so this schematic is a little intimidating.

I've found FS2, so no problem there. How would I go about lifting one side of the HT? Would that be FS1? The amp blows fuses even in standby, so FS1 doesn't seem right to me.

Would you lift the low voltage supply at W20/W21?

Thanks again for your help!

Any way you can make the HT circuit "open", that is, not completed. lets you evaluate where the short circuit is, this applies to the other circuits too. If the amp is blowing fuses (FS1) in standby that localizes the problem. Just to be absolutely sure it's wise to be completely sure you've got the power transformer disconnected. They cost some serious money even here in the states without VAT and all that other stuff. Best to be absolutely sure that's what's wrong with it.

It's starting to look like a power transformer defect but you should eliminate the tube filament a/k/a heater section by removing FS2 and seeing whether that changes the mix. The low voltage supply for the solid state stuff could be interrupted at W20, assuming it is a connection of some sort.

Keep us posted.

[frquent flyer]

When I had my Vox ac 15 cc1 I experienced frequent fuse blowing. I finally called Vox customer service and was told that I could use a #3 instead of the stock #1 amp fuse. I know this is frowned upon but thats what Vox told me. Oddly enough the fuse blowing ended after I put some JJ power tubes in.Recently I sold that amp and everything was working fine.Nice amp. Good luck!

[JCMNelson]

So, I sent the amp to a tech and he found a short-circuit. Turns out that legs of two components were protuding so far through the pcb that they were touching the chassis!!!!! He thinks that they weren't originally touching, but that the board has lost a bit of rigidity of the years.

So, short circuit fixed, BUT it's damaged my PT :cry:

I'm going to replace it but I'm trying hard not to use a Vox part as I've heard that there have been a few faulty ones around. It's not easy to get alternatives in the UK, anyone got (reasonably priced) suggestions?

[oldhousescott]

I would stick with the Vox replacement. It's the proper physical size and electrical rating for the amp, and has the correct windings, especially the one for the SS stuff. A more generic replacement likely won't have that particular winding.