Don’t be fooled by the power supply when doing that PA amp repair!
Recently I had a Crest Audio stereo 1200-Watt power amp come in for repair. In fact, I had a few of them with differing faults. However, this particular one was deceptive. It came in with what appeared to be a protection fault. Protection faults in PA amp repairs are mothers of faults in high powered amps and often difficult to diagnose. Many are engineered in such a way that you simply can’t work on them live, well at least not without making break out looms and this Crest is one of those.
http://www.musicelectronicsrepair.co.nz
The Protection Symptom
The amp came in for repair with the inrush mains relays clacking away. The protect LED and both ACL clipping LEDs on.
First things first as always, I checked the power supply rails as I do with all pa amp repairs and they checked out OK. Secondly checked for any DC on the output, both output power amp stages were good with no DC offset on the outputs.
Signal Path
Satisfied there’s no problem with the power stages, it was now time to pump in some signal. I scoped the output pre protection relays and all checked out fine, signal was present and clean.
Oh, now the fun starts. Protection fault you think and on further diagnostics and troubleshooting, sure enough there were dried electrolytic capacitors in the switch on delay circuit. This circuit is like a soft start for the power amp modules, reducing the surges that can be present in power output stages and part of the protection system in this amp.
The capacitors were promptly replaced in both channels, but alas of course it wasn’t going to be that simple, it’s hardly ever that straight forward.
More Protection Circuit Thinking
Arrrhhh, where to next? Well let’s start looking at the clip detector circuit, the attenuation driver circuit, limiter circuits etc.
Guess what…? All checked out and functioning as expected. All this time it has been in the back of my mind, niggling at me continually. It has to be something common to both channels and now it’s taking the fore. Well What?
Leaving power supplies out and not exactly sure why at that point in time. There were a few thoughts and one of them was over temperature. But how come, how could that be ACL LEDs, both channels on together with protect.
It was now sort of starting to dawn on me that the cooling fan sort of wants to start. Hmm ok let me do a visual check of the board and guess what? On this board I note tantalum electros, notorious for going noisy. However, I couldn’t quite see the values and circuit designator so decided to unplug the fan header which was obscuring the cap for a better look and hey presto.
But how come? The amp protection is released and I now get sound. Plug the fan back in and bingo, protect. What the!
OK, let’s check out this circuit, which is basically a Darlington Switch driven by a three-terminal regulator.
And guess what? All checks out to be working OK passively, but apply power and it just doesn’t behave. And to boot doesn’t make sense.
What About the Power Supply?
But hang on just one second. Have I overlooked the obvious and No1 rule in all electronics servicing? The Power Supply.
Previously I had measured the two supplies in question here. The +27v and the +36v. Both were down, at +20v or thereabouts, which I put down to my in-line surge protection/limiter supply that in the initial stages of diagnosis I power all high current and potential fuse blowing jobs from. There’s usually a drop in AC volts.
But something had just occurred to me. Even if down, the two supplies should be at somewhat 25% difference and they weren’t. Ah Ha. Rule No1. Always check the power supply.
The +36v supply was down at a level as if there were no filter caps. Hmm, out comes the scope. I scoped the +36v line and guess what? A nice AC signal.
Pull the power supply out and remove both electrolytic caps which are in parallel. It didn’t prove to be too difficult, not that it is, because there was only one lead holding them in. Both the caps had the +ve electrode corroded away.
Fitted the new caps and checked the power supply over while I was at it. Reassembled and guess what? The amplifier now powers up as normal.
The Moral of the Story
Wow, quite a ride on this one. However, the moral of the story for all electronics repair and PA amplifier repairs are no exception, is even if it doesn’t look like a power supply fault.
The No1 rule is always check the eFN power supplies. Protection faults can be tricky problems to resolve at the best of times. But, don’t make the job any harder that it needs to be by not thoroughly checking out the power supplies properly. Is it the power supply or not? Well it probably is.
