First atempt at class D amp repair

[retgaz]

Hi
I'm about to start on my first repair job on a class D amplifier, (Orange Terror Bass 500) and I wanted to check with more experienced people, if there were any special considerations I should give to this type of amplifier, compared to a normal solid state amp? For example: should the amplifier be tested without a load and assuming that a light bulb limiter can't be used due to the smps, is there anything I can do to protect the amp other than fitting a small fuse?
Thanks in advance for any help.

[tedmich]

I'm curious here too, all I know is that you shouldn't expect it to be clean anywhere near its rated RMS output...

[The Dude]

The answer to this question could be a book in itself as most of these amps have switch mode power supplies. Google search "troubleshooting SMPS" and you'll find a host of tips and articles on the subject. I can't see a reason to repost all of the info here unless you have specific questions. My best advice is to not even attempt it until you have read about it and understand how the SMPS works and how best to troubleshoot it.

The first and probably most important advice I can give you is to note that the primary of a SMPS is not normally chassis grounded. If you attempt to ground it even with a scope probe to poke around, you're gonna see sparks and bad things will happen. I would not attempt repairing a SMPS without an isolation transformer.

[Jazz P Bass]

And if you think about it, what is a Class D output section but a 'switcher'.

[retgaz]

Thanks for the replies. I have done a lot of reading on smps and as much information on class D amps as I've been able to find. I understand the principles of operation of both units, but as with everything on the net there is a lot of dubious information about. One piece of information that I am questioning is that testing of the amplifier should be carried out with a load connected. This does not make any sense to me, but I want to make sure I'm not missing something. My main concern is that I carry out the testing safely. I understand that testing on the primary side of the smps without an isolation transformer is a bad idea. My plan was to lift the connections between the psu and the amplifier and check the output power rails. This psu has 2 primaries and 3 secondaries.. If the primaries are faulty I would expect to see incorrect voltages on the secondaries. All parts of the amp after the secondaries, including the class D amp itself are referenced to chassis ground. With regards to test equipment, I have a fluke 175 dmm, a hameg 35MHz scope and a velleman double insulated battery powered scope meter that I can use on primaries if I absolutely have to. I do not have an isolation transformer and I only have standard x1/x10 scope probes which I understand may not be suitable due to their loading effect. I work on valve and solid state amps everyday, but I know I'm at the edge of my knowledge on this one. Just hoped some body could give me some safety tips, this stuff wasn't about when I went to college.

[g1]

One piece of information that I am questioning is that testing of the amplifier should be carried out with a load connected. This does not make any sense to me, but I want to make sure I'm not missing something.

The amp does not need a load for testing. Class D amps can be used with no load so you can use their headphone out or preamp out, there is no problem.
Perhaps the info you refer to meant the SMPS needs a load, this depends on the particular unit, some will not run unloaded.
Here is a thread with some SMPS info:
http://music-electronics-forum.com/t23974/

[52 Bill]

I work on valve and solid state amps everyday, but I know I'm at the edge of my knowledge on this one. Just hoped some body could give me some safety tips, this stuff wasn't about when I went to college.

While it is best to approach all of these things with the kind of precautions you are taking, don't be so afraid of working on this that it changes your basic approach to working on electronics. Yes, the primary side of the SMPS has caps that are charged to high voltages, but so do large tube amps. Carefully measure and discharge all of the caps just like you should on a large analog tube amp power supply.

You don't mention what is wrong with the amp, but in many units that I have repaired the problems are often simple shorted diodes or transistors, just like the analog solid state ones.

Yes, please be careful, but don't be so cautious that you make yourself do something silly and/or dangerous.

[Jazz P Bass]

As to the 'Class D' part of the amp, I would look up the datasheet on the IC that is being used.

[retgaz]

Thanks for all the help. It's helped my confidence levels. g-one thanks for confirming my thoughts. The amp has a pair of class D amps running in bridge mode. The fault is loss of volume and quality, which occurred over the time taken to play a couple of songs. The drop in level was described to me as going from a 500watt amp to a 40watt amp. So once I've checked the power rails I want to check each of the amps. The owner swapped the pre amp valves for known good ones, which didn't help so he brought it to me. I've already checked for shorts but found nothing. I'll get it powered up tomorrow. Am I ok with x10 scope probes? Thanks again

[52 Bill]

The drop in level was described to me as going from a 500watt amp to a 40watt amp.

Did you test/service the effects loop jacks?

[retgaz]

It could easily have been that 52 Bill. I'd serviced / cleaned all the connectors internal and external whilst I was waiting for the first replies to this thread. When I got it back on test it was running perfectly. I've had it running for a total of six hours over the past few days with various test signals into various loads and it's been fine. The hardest test was probably a sine wave at 350watt output for about 2 hours.
The information and help has not been wasted though, I've learnt a lot and although I've never worked on one of these amps before, they're just like London busses... you don't see one for ages, then 2 turn up together. Sure enough I've just had another one arrive at the workshop.