PSU Primary side issue (non QL)

[Chr$]

I know there are some expert electrical gurus here so thought I'd post this issue...

This schematic is the primary PSU side of a 1993 Grundig GV280S VCR. Mains comes in at the top, optocoupler bottom right and you can just about see the secondary windings along the bottom.

I bought it as broken, had a poke around and was able to get it working again (the complete VCR was lit up and active). The issues were the mains filter caps (like RIFA but different brand) and mainly the control IC, a TDA4605. I replaced those and also the other 2 electrolytics and it and I were happy.

Then, after being on for a few hours it died again. No voltage at all on a couple of EPROMS on another board, no voltage on another EPROM that is on the PSU board secondary side. No 5v anywhere.

The winding between 1 and 2 (lower right) shows a steady 17v. The other winding, between 3 and 5 is all over the place - my multimeter shows around 7v sometimes, then has a little think and then shows 7v ish and repeats that process. I'm not sure what voltage that winding is supposed to be and although I do have a scanned PDF of the service manual, it's massive and there is no search facility.

I've checked everything that is connected to the winding between 3 and 5 and have changed things that obviously were fine and didn't need changing, e.g. the BUK455 600 FET. I replaced the TDA4605 again, made no difference.

If you were having this issue, what would you do/check?

[Pr0f]

Do you have the service manual for it? Those circled numbers probably have expected waveforms or voltages to measure on the primary side, so if they are ok, the issue is likely on the secondary side of it.

It's rare but not impossible for rectifiers on the secondary side to fail, which will obviously kill your 5v if it's that winding. Which ever winding is considered the main winding on the secondary side will feedback via the opto, which is what regulates your voltages, the others should all track that winding, so for instance if one was 12V, then the 5V would be a given ratio of that, so provided the voltage for the feedback circuit was good, the others should be fine, but they will all have separate rectifiers and smoothing caps.

I've also seen crowbar circuits on the secondary to prevent over voltage on the logic supply - so check for a thyristor across the supply.

[Pr0f]

Looks like the 12V output from the PSU is used to regulate the PSU primary circuit, and the 5V circuit is further regulated by another IC, and also has a crowbar circuit across the input to that IC.

Full service manual here: https://elektrotanya.com/grundig_gv280s ... ad.html#dl

[Chr$]

Thanks ever so much Pr0f.

There is indeed an illustration of what to expect at those circled points:

Before I dig out the oscilloscope, circled no.1 should be approx 400v and it's one of the winding ends (the 5 between 5 and 3). But I assume it should be 400v between circled point no.1 and 0v, so the negative side of the big electrolytic cap (C395)/minus side of the bridge rectifier?

[Pr0f]

Yes - provided the main capacitor on the primary side is ok, and the bridge rectifier - you should see the full wave voltage from the mains across it - so it will around 380-400v - for a 230/240 V mains supply. It's not that critical as the power supply works down to 110V by the looks of it.

The capacitor voltage will drop whenever the winding is driven by the FET, but this will be short cycles for a light load. That ripple across the capacitor will be at the switching frequency - but probably won't be that many volts.

[Chr$]

I get 320v across the big cap (C395) but from the cap negative to pin 5 of the transformer (the circled test point no.1) the multimeter throws a fit but did briefly show around 11v. Not 400v!

So that seems to confirm that the issue (assuming it's the only issue) is on the primary side?

[Pr0f]

I think the wave forms and voltages may not read well on a multimeter. A scope will be the best way to read these voltages - assuming your probes and or scope inputs have the correct rating to check these - as you are dealing with quite high voltages here!

[Chr$]

My 'scope isn't rated for that high. I wonder if that new big cap is crap.

[Pr0f]

If you have a multimeter with a DC input voltage rating of 500v or higher - you should be able to measure across the main capacitor - on DC voltage range - you should see some 350-400V.

You could also check the stripe (cathode) side of the rectifiers on the secondary side - these should give some indication of the voltages present there - the 5v feed comes from a further regulator - so it's input is going to be at least 2v higher than the 5v output - which I think you observed - I wonder if the crowbar circuit is getting tripped, cutting the output. There is what looks like a 5V zener diode that's fed from somewhere to operate the crowbar - it might be worth checking the voltage on that too - D512 on the diagram.

[Chr$]

If you have a multimeter with a DC input voltage rating of 500v or higher - you should be able to measure across the main capacitor - on DC voltage range - you should see some 350-400V.

You could also check the stripe (cathode) side of the rectifiers on the secondary side - these should give some indication of the voltages present there - the 5v feed comes from a further regulator - so it's input is going to be at least 2v higher than the 5v output - which I think you observed - I wonder if the crowbar circuit is getting tripped, cutting the output. There is what looks like a 5V zener diode that's fed from somewhere to operate the crowbar - it might be worth checking the voltage on that too - D512 on the diagram.

Many thanks.

I went straight ahead and removed the TY510 Thyristor. Tested it in my Chinese multitester and it shows as an 80ish ohm resistor between cathode and gate. I don't think the tester is able to detect it properly because it's gate ignition current is more than 6 mA. With my multimeter between anode and cathode I get about 19Mohm in both directions then when I short anode to gate it goes down to the 80ish ohm but doesn't stay at that value when the short is removed.... doesn't seem right, but then reading up on testing such power thyristors I'm not entirely sure! I'll probably order a replacement.