Repairing a Solid State Amp

[harryt]

Hi all,

All my experience thus far has been in repairing tube amps. There tends to be not much variations in the designs and I find faults quite easy to root out.

Solid-state amps are something I'm completely unfamiliar with. I have two that I'm trying to fix at the moment:

One is an Airedale S900 stereo power amp. One channel works, one doesn't (eBay). It blows the external fuse on that side's output. The power MOSFETS are 3x J200/K1529 combos. Using a multimeter from gate to source gets a measurement around 300k on the side that works. On the other side, gate to source is open. All the other components seem to measure similar values - I know this isn't the ideal way to do it, but it should at least give me an indication of what's going on.

On the working side the circuit board shows signs of burning under a resistor a few components down from the speaker jack (it's a low value, about 7 ohms). I guess this has been replaced because the non-working side has a slightly different resistor (about 6.8 ohms, different colour codes). I am treating this as a red herring.

My question is: say I splash out for three new J200/K1529 combos; what is the best way to fix this amplifier without damaging the new ones? I have read about the light bulb current limiter. This is a good idea for any kind of amplifier, no matter what topology, so I will be building one of those. The thing that worries me is that I heard that failures in solid state amps tend to take out a lot of neighbouring components. The resistors measure alright, but after that I'm in the dark. Would it be prudent to replace the surrounding transistors? There is a TIP47, which I'm assuming is a driver, a couple of BC212LBs and a BC549.


The other problem is an Ashdown MAG300H that blew an internal fuse. I replaced the power amp fuse; it didn't blow but it makes no sound apart from 50Vdc on the speaker output and about 7 or 8 V ripple. I'm after schematics for that so I'll save that for another day.


Any help with how to go about this would be much appreciated. As I say I'm familiar with fault-finding so I'm just after pointers and advice. Thank you all,

Harry

[nigelwright7557]

I have found from experience if a MOSFET has gone it is usually shorted drain to source. However, I have had other strange fault conditions but not as often, I had one with source-gate short.

I usually remove all the output transistors and route the VAS output back into the LTP feedback. This means i can test the driver without risking output transistors.
If the driver works ok i set the bias to zero and then reinstall tested output transistors.
If the driver doesnt work it is unlikely to blow fuses so i can prod around looking for silly voltages in the driver stage.

[harryt]

Thank you for the reply, Nigel, some good advice duly noted.

I have to admit I was a bit confused - I was expecting to see something shorted, but in actual fact the transistors on the problem side seem to be open-circuit.

The amp has fuses on the positive and negative supply rails going to the transistors (2x fuses on each side) - these hadn't blown.

What is the aim of routing VAS output back into the LTP feedback, is it to avoid damage to the VAS stage?

[gmphadte]

If only the output xtors have blown, and everything works, then you will get zero(near zero) volts at the speaker terminal. Do not connect the speaker.

[toprepairman]

Hi Harry.
The Ashdown is almost certainly the toroidal mais tranformer.
I have replaced two in the last year. The primaries burn out due to shorted turns caused by overtightening on assembly.
Sometimes it will be visible but not always.
Phone up Ashdown and they are very helpful and the replacement Tx is cheap.
Regards
Henry

[toprepairman]

Oops, didn't fully read your thread and jumped to a conclusion, might not be the transformer after all......