Ashdown FA60 blowing mains fuse

[Tage]

I was asked to look at an Ashdown FA60 that had no out put. It had just had it's power tubes replaced, and worked for a while then quit. When I got to look at it I checked the fuses first, there is one located in the IAC jack on the back of the amp it's a 2A250 slow blo. I replaced i hoping it had burnt out from being old or just in a high stress spot, but no the amp still blew it. So now I'm going to look into what is drawing so much current, the tubes "look" ok and there are no obviously burnt components. any advice on a troubleshooting process would be apreciated. you can message me if you would like to look at the schematic, but I have noted errors in it already and it's realy not all that complex of a design. Thanks for any feedback.

[guitarmike2107]

Well you have a short circuit between one of the voltage supplies and ground, or could be between the high voltage rail and heater rail etc

Have you got a continuity tester at least?

My guess is its around the power section (plates and screens) and/or heater supply. if it was in the pre amp the dropping resistors would probably have cooked a nice golden brown colour

Does it blow the fuse with all the valves removed?

[Jazz P Bass]

Yeah, remove all of the tubes.
Then do the voltage checks.
My money is on one of the new power tubes took a turn for the worse.

[Tage]

I do have a continuity tester and I will check for shorts. I'll also check voltages, although I'll probably just be guessing at most of the voltages I think it runs the plates at around 490V, but yeah that is what I'll do provided the fuse isn't blowing due to current draw unassociated with the tubes

[stokes]

When looking for a short,which you have,it is best to use a light bulb in series current limiter.You plug the amp into the current limiter with all tubes installed and then remove,first the power tubes one at a time till the bulb dims and that will pinpoint the part of the circuit or the tube that is the problem.Without a current limiter you will blow a lot of fuses.It is easy to make with a couple of outlets wired in series.Do a search for "light bulb current limiter" on google or such and you will find directions.

[stokes]

Light Bulb Current Limiter Heres a link for one

[Tage]

Well I have determined that the short only happens when the amp, which is a combo, is in it's cabinet. So I am thinking that something is either shorting to the metal shielding tape, or something is being forced into contact with a ground/power that shouldn't.
Anyone else had an issue like this?

[Jazz P Bass]

That is a good one.
I just finished up an FA60 DSP.
Pretty much the same amp.
This one was a head.
I do not recall anything sticking up from the chassis.
Maybe you could peek in as the chassis is put back into the cabinet or does it happen when you tighten the chassis screws.

[Tage]

Oh yeah this one has the DSP as well, and some realy scratchy master volumes, I'll clean them up though. I think that may be why my schematic seems misnumbered, maybe there is a different one for the DSP model

[Jazz P Bass]

Ashdowne FA60 DSP Schematic

Here is the FA60 DSP schematic
Awesome Amp!