Thanks again Roly, managed to get it down to about 20mV. I think it'll be OK at that seems to work fine anyway!

Cheers
Shane

Hi Folks
Hving succesfuly fixed an Ashdown with some great advice from Enzo Roly and JM Fahey. I've now moved on to another repair, a Warwick xtreme 10.1. It had blown lots of stuff including most of the output transistors, the drivers and all the transistors to from the diff amp onwards (bias stuff etc). I replaced it all and it seems ok now except I have about 65mV of DC at the output. Is that too high or is it an acceptable amount? if not how do I adjust it out?

And yes I know the attached schem is from the tube path 10 but the power board in this amp is the same, with a few additions from the xtreme 10
Cheers
Shane

OK JM will do!

Absolutely Enzo, I use it every day, Just learning its limits! :tu:

Well, I tried doing as JM said and it played fine with the lamp connected, not even a flicker of the lamp!
So bit the bullet and tried it straight to the mains and again it was fine.. so I guess you guys were on the money! Experience shows!!!

Roly I think your reasoning is correct, a case of the helpful device not being helpful eh...

I'll keep trying it over the next couple of days but I reckon its OK now. Keep your fingers crossed.

Thanks so much for your help and advice I learn something new every day here!

Cheers
Shane

OK.. so as per Enzo's good advice I went over all the resistors and they all measure good. And out of curiosity I checked the old output transistors again and guess what.. none of em were bad they all measured ok with the diode test on my meter!
Guess that'll teach me not to jump to conclusions!
If the drivers were bad surely they would be bad all the time?

Amp still has the problem.
Turn it on with no load and there's no DC on the output.
Plug a load in while it's on, still fine. Turn it on with load connected and bam.. light bulb lights up and theres about 6.6V DC on the output, (Enzo definitly on the money there).--disconnect load and its back to normal.
Reconnect load while on, still OK...arrrghhh.


One thing I notice is that when its pulling the excess current the Fan doesnt come on nor does the meter on the front panel light up so it must be pulling all the voltages to the pre-amp board down sinc thtey all come off there.
I'm wondering if the problem is in the mains transformer in some way. Dunno why it would be so intermittent and load dependent though, maybe I'm clutchin' at straws! Just at the mo' I'll clutch at anything to save pullin my hair out...!

Cheers
Shane

Thanks Enzo
I'll go over it all again and check those parts you mention. I have checked the emitter resistors and they seem ok as do all the other transistors, but did only check em with a meter so...

I really take on board what you say about troubleshooting too. For year I definately had a blind spot over SS amps but have realised that I should 'get over it' and get to grips with them. Thanks for the advice and the gentle kick in the pants!
Cheers
Shane

Hi Roly
Thanks for helping out it's very much appreciated!

Yes thats what I mean by fine. As for the rail voltages they are as follows:

No load attached: -66.5V and 66.5V
with load and fault occuring: -17.5V falling slowly to -9.7V (tho previously it was about-35V) and 8V
Disconnect load and reconnect load: -66V and 66V

These readings have changed, as I said above, during fault conditions the neg rail was sitting at about-35V but now its reading as I quote above.

Do these readings agree with your ideas about the current sources not having enough voltage? if so what could cause that?

I have little experience with solid state power amps having been a tube amp guy for quite a few years and only doing this and another SS amp( a Warwick xtreme 10.1 thats eating output transistors like it going out of fashion!). for some friends.

Cheers
Shane

Hi Folks

I've been trying to fix an Ashdown MAG250 which a friend of mine owns. It had blown a fuse and was not producing any sound.
I replaced the output transistors and checked all the others on the power board but still it was showing an apparent short (light bulb limiter stayed lit up and the amp produced a loud hum).So checking more deeply I found that the positive supply was being pulled way down and the negative was pretty much OK.
So I did some more measurin' and figured I may have a shorted smoothing cap(one did measure low resistance) and changed the main caps.
Now plug her in (still with light bulb limiter) and hey she's OK...BUT... turn it on with the speaker connected and we're back to the positive side pullin loads of current...ahhh.
Weirdly if I turn it on without the speaker connected then plug the speaker in its fine! even if I then turn it off and on still with the speaker connected it's OK but if I leave it a while and turn it on with speaker it swings again and I have to disconnect the speaker and plug it back in to get it to run OK.

Ok so I'm stumped...(not difficult!).. Anyone got any Ideas? I know amps can tilt at turn on but dont normally stay that way!

Cheers
Shane