W-Audio PSR-8A

[midibob]

Hi,

Picked up 3 of these units not working as I thought they might be useful for other projects once repaired. They have 2xTDA7294's one for low frequency and the other for high. The active crossover board with compressor is the small daughter board.

Lots of cap problems and all dodgy makes at 85c. It's not too easy to see in the photos but some of the caps are situated behind the ceramic power resistors. At 85c too I think it's fair to say it's built in obsolescence.
The main smoothing caps on the left (2x6800uF) on two of the boards had also dried up which was unusual. Their ESR was unmeasurable ie high resistance.
As the IC's need +- 30v (ish) if one supply line fails either the IC's explode or I would expect the speaker coils get burnt out. In one case both IC's had blown apart. Should have taken a piccy but I've already replaced them.

All caps now changed to Panasonic 105c so they should be somewhat more reliable.

If anyone comes across a schematic for these units I'd love to see it as I have a fault on one of the daughter boards, possibly one of the 072 op amps playing up but that's for another day.

Midibob

[Longbow]

Re: W-Audio PSR-8A

If anyone comes across a schematic for these units I'd love to see it as I have a fault on one of the daughter boards, possibly one of the 072 op amps playing up but that's for another day.

Schematics - probably not going to happen. But, looks like you've found the main culprits. A powered speaker with microphone input? Would that be for Kareoke? Never seen that. The op amps are standard and the fastest way to find a bad one is to look at the d.c. offset on the output pins. With +/- supplies the d.c. should be near 0 which can be quickly confirmed with a DVM.

[midibob]

Re: W-Audio PSR-8A

Hi Longbow,

Spot on with the op amp diagnosis just haven't got around to it yet.
As you say the schematic is a bit of a long shot, pretty much always the same these days with this Chinese gear.

I think these were probably more DJ type speakers? All I need now is some 100va 24-0-24v transformers and then a project to make use of the amps. (fixing them is the easy bit!)

One thing I forgot to mention was the remnants of the gluey stuff that was used to secure the ceramic resistors to the board. I've managed to clear off most of it. After a while and a bit of heat this stuff turns in to a brown conductive film leading to some bizarre faults. I've seen this so many times over the years I can't understand why manufacturers still use it?

Midibob

[Longbow]

Re: W-Audio PSR-8A

Yes, the burned glue syndrome. This burning seems to happen more easily if it contacts a power supply B+ or ground. If you are correct on your supply voltages to the amps, the +/- 30 volt supply would make these very unusual high voltage op amps. Most would take +/- 15v. Even if they are rated at +/- 30v, I would use +/- 12v. Why would you need a 50 volt p-p output swing on a low level device?

[Th3_uN1Qu3]

Re: W-Audio PSR-8A

The +/-30v supply is for the TDA7294s. They are 75W RMS power amplifiers. The opamps most likely have a 7815 and 7915 for their supplies.

[midibob]

Re: W-Audio PSR-8A

Correct, the +/- 30 is for the o/p chips. The two 220ohm ceramic resistors are used to drop the supply lines for the op amps but there aren't any 78(9)15's. It would appear to be zener diodes. I would have thought regulator chips would have been a better design though?

Midibob

[Th3_uN1Qu3]

Re: W-Audio PSR-8A

Sure, but resistors and zeners are cheaper.

[midibob]

Re: W-Audio PSR-8A

Well the fun and games continued.
Two out of the three were now working OK but the last one had a strange fault. At switch on with no input signal there was high frequency oscillation occurring from both IC's well out of the audio range. If the input signal was increased over a certain amount the oscillation would stop and stay stopped? As soon as the power was recycled it kicked off again.
One other thing I also noticed was that both chips outputs went to square wave if the volume was increased further.
Thinking the IC's either work or they don't this was a tad confusing.
I removed them anyway and on closer inspection noticed they were TDA7293's!! Somebody had obviously already replaced them and although quite similar to the 7294's weren't happy in this circuit.

So the morale of the story is check everything and don't assume the right parts have been fitted.

Midibob

[Longbow]

Re: W-Audio PSR-8A

Forget the "either work or they don't" - that theory was passed around when we first got introduced to the uPC and it is not true. If I understand your comments, some of your op amps oscillate and some do not. Let's focus on the no-signal condition. The output lines should be 0 volts d.c. These amps run in linear mode, with negative feedback to set the beta. Positive feedback will make the amps oscillate. Since you don't use positive feedback on linear op amps, then you probably have an unfiltered supply line to the amps. Oscillations are passed down the supply to other ic's and so on. First scope the supply lines - replace filter caps on both + and - op-amp rails, and look for board contamination that could be supplying positive feedback (burned glue, etc.). BTW whenever you have a positive feedback situation (or, negative feedback is disconnected) it is normal for the amp to operate in open loop (maximum beta) and you WILL see square waves at the output (rail-to-rail peak voltage).

[dood]

Re: W-Audio PSR-8A

Not that I can help with the diagnosis. But the speakers are still sold - http://prolight.co.uk/item/spea30a/

[midibob]

Re: W-Audio PSR-8A

Interesting feedback Longbow.
Would agree with everything you said.
Caps have all been changed and all of that nasty glue stuff removed.
Both supply lines are the same voltage and the outputs of the amps and op amps all sit at 0 volts.
Perhaps as you say my old thinking of the IC's either working or not doesn't hold true any more but one thing is certain, since changed to the new (correct) ones, both channels are now working normally.

These are still current models but the design leaves a little to be desired especially in the reliability department.

Midibob