Huges and kttner AST 120

Hughes and Kettner ATS120

Customer came in with the amplifier, telling me it needed new volume and tone controls etc. Powering the amp up produced very strange sounds, hissing, rumbling, random noises which changed into oscillations when any knob was altered. Now this was nothing to do with the potentiometers and so on. The owner did not believe me, getting to the point where I was going to tell him to take it somewhere else if he was not going to trust what I say. Eventually he agreed to leave it with me. The amp had obviously been to see other repairers as screws were loose and screws from the front panel being undone and the nuts behind left rattling around the insides. Great.

With the chassis out and a scope hooked up to the output and a signal fed to the input, showed a small half wave output, badly distorted. Feeding a signal into the main amp using the return jack socket, showed a similar condition. This mean there was something fundamentally wrong. Having a snoop around, I found that there was a missing power rail. The +/- 15v line had a missing +15v, with the series regulator LM7815 running very hot. Lifting it’s output leg off the board proved the regulator was okay and a dead short to ground. Now there are many ICs and OP amps etc dotted about all over the place sitting the +/- rails. Anyone or more of these could be the culprit. The PCBs are not the easiest to remove in this amp, Now, the fact that there was a dead short on the rail, zero Ohms suggested that this wasn’t an IC causing the fault. Unusual for them to go totally short. I could see that this amp used Tantalum capacitors dotted around to decouple the power rails. I was certain that one of these was going to be the issue. The easiest was to find out the problematic chap was to leave the feed into the PCB disconnected and run a line into the board from the bench supply with current limiting. As you wind up the current, not exceeding 15v of course may show you a component getting hot. So, away we went, 3 amps for 20 seconds should show something. One little Tant tucked away in the corner trying to hide was breaking into a hot sweat. Snipping him out and replacing it brought the amp to full working status. Not a potentiometer being changed. The owner came to collect it, still not believing what the route cause was. Should have saved a box of old faulty controls to give him. Would have been an easier way to get paid.

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