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zebra100

Faulty power supply

36 posts in this topic

I have just built a regulated PSU (+/- 15v) for a pre amp. Quasar kit 1216 for the CCT diagram,http://www.quasarelectronics.com/1216.htm. When testing the voltages wrt ground, I initially got +/- 15. But the voltages started to creep up slowly until one of the caps went pop at about 24v :x. I checked and replaced the 15v zenner's and the damaged cap and checked again. Still the same, but this time I turned it off before the voltage got too high.

I have thoroughly checked the board against the CCT diagram and also checked for cold/dry joints. All the diodes and electrolytic caps are the correct way round and no short CCT's. The CCT uses LM317/337T voltage regulators.

Any ideas? Could one or both of the voltage reg's be faulty?

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Correct me if I am wrong, but the zener diodes are there to limit the voltage to the variable voltage regs ?If that is the case then there must be something wrong a) with the diodes (you say you replaced these and the problem persists) or b) the circuit. The LM 317 is designed to go up to 37v.

This may be a stupid comment, but why not make the PS with 7815/7915's ?

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Puffin wrote:

This may be a stupid comment, but why not make the PS with 7815/7915's ?

I would have built it differently, but I bought it as a kit. I may just have to salvage what I can from this kit a build my own. After all, I've spent ages on the enclosure external/internal's.

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zebra100 wrote:

I have just built a regulated PSU (+/- 15v) for a pre amp. Quasar kit 1216 for the CCT diagram,http://www.quasarelectronics.com/1216.htm. When testing the voltages wrt ground, I initially got +/- 15. But the voltages started to creep up slowly until one of the caps went pop at about 24v :x. I checked and replaced the 15v zenner's and the damaged cap and checked again. Still the same, but this time I turned it off before the voltage got too high.

I have thoroughly checked the board against the CCT diagram and also checked for cold/dry joints. All the diodes and electrolytic caps are the correct way round and no short CCT's. The CCT uses LM317/337T voltage regulators.

Any ideas? Could one or both of the voltage reg's be faulty?

Which cap popped ? I'm guessing c19 or 20 .......

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Well ........ it had to be an electrolytic and it was well within voltage, so it must have become reversed somehow ........

If it hadn't started correctly, i'd have said the voltage regs were wrong way round. ie 317 on negative supply and 337 on positive.

Edit: Is the phase of the negative transformer secondary correct ?

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Coliam wrote:

Well ........ it had to be an electrolytic and it was well within voltage, so it must have become reversed somehow ........

If it hadn't started correctly, i'd have said the voltage regs were wrong way round. ie 317 on negative supply and 337 on positive.

Edit: Is the phase of the negative transformer secondary correct ?

The voltage regs are in the correct positions. There is a difference with the voltage regs though. The 317 has the part no. 317T, and the 337 has the part no. 337SP. Could that be the cause?

Also, I didn't think the secondary on the transformer would make a difference, being AC (or am I being stupid?). I will swap the terminals anyway.

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No you're not being stupid ......... it shouldn't. :)

But I thought the circuit diagram looked a bit odd wrt the polarity of the bridge diodes.

It looked to me like it was perhaps designed for 2 x 317's rather than a 317 and a 337.

Maybe the circuit diagram is wrong ........

(the sp just denotes a TO220 case )

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Well, changing the ac inputs over on the -ve rail made no difference. I even tried it with the ac input disconnected on the +ve rail. Same problem.

Just to be sure, I re-connected the ac to the +ve rail and disconnecterd the -ve rail ac input. Still the same......Bizare, surely both voltage regs can't bedead?

:dunno:

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zebra100 wrote:

Well, changing the ac inputs over on the -ve rail made no difference. I even tried it with the ac input disconnected on the +ve rail. Same problem.

Just to be sure, I re-connected the ac to the +ve rail and disconnecterd the -ve rail ac input. Still the same......Bizare, surely both voltage regs can't bedead?

:dunno:

They could be if it was bad enough to blow a cap. There could be other bad caps too, especially the 2 big ones.

It's pretty weird, though. :?

I see the circuit diag. IS wrong ........ The layout drawing appears to be correct, so far .....

still looking ..... :)

Edit: D12 looks to be wrong on the layout, but correct on the circuit diagram ........

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Coliam wrote:

I see the circuit diag. IS wrong ........ The layout drawing appears to be correct, so far .....

still looking ..... :)

Edit: D12 looks to be wrong on the layout, but correct on the circuit diagram ........

I thought D12 looked odd.... That mean's that the PCB screen printing is wrong then?

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zebra100 wrote:

The transformer kicks out 18-0-18, 120VA. I'm getting about 23.5vdc after the rectifiers.

:wall:

That's quite normal. The Transformer is putting out 18v RMS, which translates to 25v peak-peak. Less the rectifier voltage drop (1.4 V for a bridge) gives 23.6V - which is what you are getting.
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zebra100 wrote:

Coliam wrote:
I see the circuit diag. IS wrong ........ The layout drawing appears to be correct, so far .....

still looking ..... :)

Edit: D12 looks to be wrong on the layout, but correct on the circuit diagram ........

I thought D12 looked odd.... That mean's that the PCB screen printing is wrong then?

Yes, looks that way.... It does make sense ....... if D12 were reversed, then C20 would be unable to discharge, hence the pop :).

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It looks like D12 is dead anyway. 27ohm's (R18)either way through the diode wrt -ve. I'll change it when I get home. I hope it works. I also have Quasar on the case. They are going to speak to the kit manufacturer to see if they have any ideas.Quasar have been most helpful.

Thanks Colin,

I'll keep you posted.

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