Repairing a Goodmans PVR

[Resqueline]

A ripple with a repetition rate of around 3Hz indicates the IC enters a protective mode or has an insufficent supply voltage/power.
Check the voltage on the 47µF cap' (careful, mains voltage present) Notice that you can't scope there - without using an isolation transformer.
Also check the resistor in the vicinity (to the right). Recently one other member found it had broken. (Most small PSU's are made the same way.)
A transformer winding is rectified by a diode in series with a resistor and then fed to the capacitor. Once it gets going it should be self-sustaining.
The IC is good for delivering some 15-30W iirc. So I figure using a couple of test bulbs of up to 5W each should be ok.
Nice buy with the 'scope btw..

 

[nobrob]

Forgive my ignorance: how should I be measuring the voltage on that capacitor?

I've managed to revert the problem back to what it was: the 1-2V spike has gone away. I think I used a too low- current rating resistor for the one that sits just north of the IC.

With this, I replaced the two capacitors which were leaking (as I'd reverted them to original).

Then, I could only find some 1.4W bulbs (the ones from my car's speedo ). I strapped three together in parallel on the 12V output.

Voltage measures 10.70 under this load and the ripple looks very similar to without load (different shape but still around 20mV in height)

Bulbs are a good tip, thanks. I'll get some assorted ones in for the future.

 

[Resqueline]

You just set your DMM to 20V DC and put the probes on the capacitor connections on the underside of the PCB (or on the zener pins on the overside).

The datasheet suggests using a 100k resistor (= 3mA) to pin 5 on the IC. It needs at least 0.45mA, preferably 1mA, to charge the 47µF capacitor sufficiently to start.
What value did you put in there?

Read the Datasheet and the related Application Notes. There's a lot of information to be had, and even a troubleshooting guide.
The schematic diagrams presented are very similar to your PSU, with your circuit being even less complex.

 

[nobrob]

I decided to scope the working power supply so I could have a reference of a working unit. Much to my surprise I found a ripple of just under 0.8V at 50Hz. Way more than what the failed power supply has (20mV @ several hundred kHz)

Embarassingly, though, I accidentally blew the fuse by shorting something with my probe. The current would have had to come through the IC, so I'm of the opinion that I've blown it

Thanks for the suggestion to read those datasheets. I knew of the first one, but not the troubleshooting one which appears to be quite valuable. Once I revert the board back to "not blown apart by a short circuit" I'll get straight on the suggestions in that doc.

 

[nobrob]

I accidentally blew the fuse by shorting something with my probe

I realise how this might prove confusing: I shorted the PSU that I'm trying to repair.

I do think I need to take a step back here though. Despite my next step being going through the troubleshooting guide for the IC, the symptoms seem rather odd to me:

- The main board will power up but the hard drive won't
- The main board and the hard drive both have 5V and 12V supplies
- Replacing the hard drive doesn't change a thing
- Replacing the supply to the hard drive fixes the problem entirely
- The power supply measures 5V and 12V in the right places
- Ripple seems acceptable and even luxurious compared to the working power supply
- The supply can power bulbs rated in current above the rating of the hard drive

Am I looking in the wrong place?

 

[Resqueline]

It sure sounds confusing. It's hard to say what might be going on, in particular without "hands-on". There may be some small detail missing.
Some of the glitches that can reset digital circuitry can be so narrow that they're hard to capture & see on an analog 'scope for example.
There were two +5V's from the PSU, one straight and one extra filtered. Where did they lead to?

 

[nobrob]

There were two +5V's from the PSU, one straight and one extra filtered. Where did they lead to?

They both lead back to the three legged monster with the big heatsink (which seemingly has no markings). They have separate but identical capacitors and inductors all the way along.

The only difference between them is the T5V has a 1000ohm resistor to GND.

The main board is fed by +5V and T5V. The hard drive only gets +5V. They both get the same 12V feed.

 

[nobrob]

I've ordered a replacement PSU board. I came to the conclusion that

a) this board has probably got the better of me, and
b) £4 for a replacement board was probably not going to break the bank.

I've certainly learnt a lot from this project attempt, so thank you to everyone who helped

My next goal is to build an ESR meter. They look like they could come in handy for when the next PSU goes

 

[nobrob]

After waiting a while for the replacement board to arrive from China, I hooked it up only to find the same problem It would appear as though the power board may not have been the problem all along.

So here's a summary:

- The unit will power up but is unresponsive with "0000" on the display and the hard drive will not spin up
- Connecting a molex-terminated power supply to the hard drive solves the problem
- The power supply has been replaced
- The hard drive has been replaced
- The cable between supply and hard drive is good

I'm happy to try any ideas anyone might have, but for my own sake I'm happy to admin defeat at this particular project. I fear the unit will end up running with two power supplies for the remainder of its lifetime