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zebra100

Faulty power supply

36 posts in this topic

zebra100 wrote:

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.

Good to know they're helping. :)

I'm glad I could help ..... I've quite enjoyed the sleuthing. :D

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D12 swapped around and power on........... waiting for loud bang. Nothing, no smoke, no bang. Now to check the voltage......... 15, 16, 17.... Stopped at 17.7 volts. Mind you, my volt meter hasn't been calibrated in about 20 years. The meter I have been using at work gets calibrated every year (a lovely Fluke scope/meter with a colour screen. I wonder if they'd miss it?), so I know which one I trust...

:zip:I'll test it one more time and then leave it on for a while. Let's wait and see..... If this is the problem, then the screen printing on the PCB iswrong... The swines!

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Well, it's been on for 5 minutes now. No excessive heat, no smoke. Only 0.2v rise on the -ve rail. Great sucess. Colin you are a geneus. It would have taken me ages to find that. Just goes to show, never trust the kit or the instructions.They mightbe wrong.

Now to connect the psu to the amp......:zip:

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zebra100 wrote:

Well, it's been on for 5 minutes now. No excessive heat, no smoke. Only 0.2v rise on the -ve rail. Great sucess. Colin you are a geneus. It would have taken me ages to find that. Just goes to show, never trust the kit or the instructions.They mightbe wrong.

Now to connect the psu to the amp......:zip:

That's great news, Chris. :)

It did make me wonder about all the other kits they've sold ........ or not. :D

Thanks for the compliment, but I'm still very much a novice at this.

I have done quite a few power supplies with 317/337's, though, so I had a pretty good idea how it SHOULD be. :D

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Not quite a happy ending. Just to be extra safe and avoid blowing any more components I decided to do one more check. Meter selected to ac and checked the -ve rail. Zero volts, so far so good...... Checked across +ve rail...... 36volts, WHAT?

:shock:

The only way I should be seeing 36vac in this cct is directly across the transformer (18-0-18vac, 120VA). Much swearing/shouting/head scratching/shaking of fistsetc......

:wall:

Pass me the cct diagram. Start again...........

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zebra100 wrote:

Not quite a happy ending. Just to be extra safe and avoid blowing any more components I decided to do one more check. Meter selected to ac and checked the -ve rail. Zero volts, so far so good...... Checked across +ve rail...... 36volts, WHAT?

:shock:

The only way I should be seeing 36vac in this cct is directly across the transformer (18-0-18vac, 120VA). Much swearing/shouting/head scratching/shaking of fistsetc......

:wall:

Pass me the cct diagram. Start again...........

Oh Dear ......... DON'T PANIC! :)

I assume you are measuring from pins 5 and 7 to ground?

If you have ac after the regulator, then the reg must have failed, as well as the bridge.

There might be other failed components, after what happened.

Can you measure the voltage on the input pin of the reg (pin3), to ground, and confirm the dc voltage. ?

(I was a little worried at the 17.7v output, before actually - it should be much closer to 15v)

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I'd be tempted to bin the lot and start again .....also if the curcuit diag is wrong you could get your money back ?

actually a psu is one of the simpler builds there are any number of simple and excellent curcuits available onthe web and elsewhere...

It also might help you understand the fault if you did a little diy psu building ?

rs do some relatively cheap modules that with a few tweaks can sound very good or

build a simple fullwave diode bridge [schottky based] using your existing tranny ..then any of the myriad voltage regulators to obtain the positve ands negative voltages you need .

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What a bummer. These sort of things really p**s you off. I firstly never doubt that the manufacturer has got it right, then when it keeps going wrong, you think is it me?!

I would be inclined to make a simple 7815/7915 supply.

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Coliam wrote:

Oh Dear ......... DON'T PANIC! :)

I assume you are measuring from pins 5 and 7 to ground?

If you have ac after the regulator, then the reg must have failed, as well as the bridge.

There might be other failed components, after what happened.

Can you measure the voltage on the input pin of the reg (pin3), to ground, and confirm the dc voltage. ?

(I was a little worried at the 17.7v output, before actually - it should be much closer to 15v)

Hi Colin.

I have calmed down a little now.:)

I'm getting 24vdc at the input's of both regulators.

I am going to do some more checking of the various components in isolation. If the rectifiers in the bridge are ok, I will remove the RC filters and try it without them. Also, if I remove the reg's and D9 &10 then, if the components up to the input of the reg's are ok then, in theory I should see about 24 volts of fairly clean dcat that point. Mind you, if I am going to have to buynew bits for it then I might as well build my own PSU.

Either way, I think Quasar can have the kit back. Still, on the bright side, at least I have learned something about regulated power supplies from this "experience".:)

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I've had a reply from Quasar. They have spoken to the manufacturer of the kits who have confirmed that there is a problem with the screen printing on the board. D12 isprintedthe wrong way round. Also, something which I completely missed on the PCB.On the track for C18/R18, R18 is connected to the -ve rail as it should be, but C18 directlyis shorted by the track and doesn't connect to the ground rail. That would explain a lot.

To rectify the issue (no pun intended), I would have to cut the track and run a link to ground, or solder the cap directly to the ground, neither of which are desirable if I want to keep the supply symmetrical. In any case, I think one of the Voltage regs has blown, andsome of the capsmay have given up the ghost.

I'll try soldering the cap directly between R18and ground as a temporary fix and see if I can get any life out of this kit. If that fails, which I'm sure it will, then it's going back to Quasar.

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Thanks, Chris.

At least, they've aknowledged there's a problem. :)

My aging eyes didn't see the cap problem.

It's very hard to make out the component numbers on the printed layout diagram.

If you decide on repairing it, I wouldn't worry too much about the effect on the symmetry, I doubt that modification will make that much difference...

You're using two different voltage regs, anyway, so it can never be perfectly matched.

If you decide to 'go it alone' , supplies similar to that are quite straight forward to build on vero-board.

It might even be better matched, as you could use 2 x 317's and make 2 identical supplies.

You then use the 'high' rail of one supply and tie it to the 'low' rail of the other supply. That gives you your common, circuit ground.

Whatever you choose to do, have fun. When you get the DIY bug, you soon realise that when one project ends, another has to begin .......... :D

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earlofsodbury wrote:

bloody poor that they haven't offered a replacement :x

I am waiting on a reply from Quasar. I tried the fix, and the board doesn't work, so I e-mailed them this morning. To be fair, they have been good (so far) and I have learned from this experience, so it's not all been bad. Anyway, if the PSU had worked first time, then I wouldn't haveput togethera passive pre (usinga 50K Alps Blue andthe case and sockets from the active pre)for my LM3875 Gainclone:)

I now wish that I had gone down that route to start with. I could have spent a bit more on a stepped attenuator.

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thats very bad ....it suggests they have not built one of there own kits .....otherwise they would have enclosed an erratum ....or reworked the boards.

It would make me think long and hard before I made a purchase from them !

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