Ashdown Klystron 500 blown fuse

[txstrat]

Hi everyone,

my bass players amp blew the main fuse a couple days ago while on stage.
Reading this forum (thanks to everybody) I found that in most cases the power transistors are the culprit.

Since my experience with transistor amps is even lower than with tube amps I thought I just beg for help.

Can I test those power transistors while they are in circuit or do I have to unsolder them.
I believe the latter would mean way more work on the amp because of unmounting the PCB etc.

thanks

Matt

[Jazz P Bass]

Shouldn't that be a warranty repair?
The amp is not that old, is it?

At any rate, I would try contacting Ashdown.
At least get a schematic.

[The Dude]

If it's not under warranty:

Output transistors are usually configured in parallel groups. (Without a schematic) Commonly, each channel of an amp will have a set of PNP transistors and a set of NPN transistors each amplifying their respective half of the waveform. You can do an initial check in circuit and usually figure out which bank of transistors has the short. This will narrow down the problem to a specific set of output transistors at which point you will need to pull them to find which one(s) is/are shorted. That is a START. It's common when amps fail for emitter resistors to burn open, driver transistors to short, etc., etc. You'll need to check the associated circuitry also. Always do this work without a speaker hooked up and check the amp for output DC before connecting a speaker. It's also a good idea to build a light bulb limiter (search forum for details).

[txstrat]

Thanks for your posts-

I've already contacted Ashdown and they were very kind. Got all the schematics I needed just like that.
Don't know if the amp is still under warranty, I suppose not, cause otherwise my bass player would have brought it in for warranty check.

So far I've never needed a light bulb limiter, but I could build one if necessary.

Please note that I haven't opened the amp yet. My plan was to ask if I can make a quick check, before I open it up.

Where do I have to check with my DMM?

[Jazz P Bass]

Post the schematic?

[txstrat]

attachment

Post the schematic?

Sure, here you go...

Oh and I might add, that I can't measure DC on the output cause it blows a new fuse at once (see me building a LBL).

[Jazz P Bass]

First off, you do not need 10 amp fuses to get this going.
1 amp is fine.
There are two ways that the amp can blow fuses. (basically)
A shorted bridge rectifier and/ or shorted output transistors.
Either can be found without power applied, with an ohmmeter.
The key word is shorted.

[J M Fahey]

1) if it instantly blows fuses, you already know all you need at this stage; next is opening it.
And of course, no fuses will blow using the lamp current limiter, but the bulb will glow *BRIGHT* .

2) take a couple nice readable in focus pictures of the PCB and output transistors area, one above (components), one below (tracks).
Try to reasonably match size and orientation, so looking at both at the same time, we can get a rough correlation between parts and wiring.

[olddawg]

Just a thought. If the amp is out of warranty, reasonably new, and he is a working musician. You might want to call the manufacturer and complain. Say it failed at a gig, you bought the amp and spent a lot of money because it was highly recommended, and that you are very disappointed and could not in good conscience recommend their product to your peers. You never know. They just might give you an accommodation for a one time repair. Happens all of the time. Squeeky wheel and all of that. Just don't take it apart first.

[Mick Bailey]

I don't see the PT or mains side in the schematic, or the preamp board which may have it's own PSU. Is there a MOV anywhere on the primary side?

A shorted PT or filter cap will also cause a fuse to blow.

I'm not familiar with that exact amp, but most Ashdowns can have their power amp isolated by unplugging the preamp and disconnecting the power leads from the power amp board. If it still blows fuses (or the limiter glows brightly) then the chances are it's a power supply failure.

Just take care that the disconnected power leads are insulated. Also, some Ashdowns use non-grounded heatsinks; one will typically be +80v, the other -80v. You don't want to get across these.

[txstrat]

Just take care that the disconnected power leads are insulated. Also, some Ashdowns use non-grounded heatsinks; one will typically be +80v, the other -80v. You don't want to get across these.

Thanks for the information. Will take care of that.

BTW, my bass player bought the amp used and I haven't asked him yet if it's out of warranty.
Still busy with other things at the moment. Give me some more time to post the pics.