-27volts from a PC power supply

Sorry, just noticed that there are two more rails also, a -12v and an AC3.9v. I'm not sure if the AC is refering to Alternate Current or accessory. I'm pretty sure it would be a dc rail though.

The AC voltage is for the heater in the VFD display. You'll spend a lot more time re-engineering a PC power supply to work than you will just fixing the original.

Likewise, the -27v is also for the VFD. The 3.3v will be the CPU core voltage, so must be properly regulated. If you have done damage beyond the original PSU fault, it's probably not worth wasting more time and effort on. Just what additional components do you think you have fried ?

Arfa

Thanks for your input. Here is what happened:

After testing the output voltages on the PSU and finding them to be a bit off-target, I did a touch of soldering to eliminate the dry joint possibility. Unfortunately I think I may have killed a couple of the more sensitive components (all soldering was done on the low-voltage side of the board). Upon plugging it back in there were several flashes followed by a blown fuse. On closer investigation with my trusty multimeter, I found the live side to be completely shorted. Once I desoldered and tested a few components, I found three of the four rectifier diodes were shot. I imagine it must have been a pretty catastrophic problem on the low-power side to have caused this! (and no, there are no shorted tracks) I **think** I may have seen the L7812cv rectifier IC flash in the process.

Just to add to the problem there is a signal diode that has shattered, and is now unreadable. Other than that there are only a couple of sensitive components, so I guess it may well be easier to try and fix it. Does anyone have a schematic for this? I realise that these units aren't really made to be repaired, so there is a good chance that one doesn't exist.

Anyway, thanks again for your help.

messagenews:9os7h.95$mM1.63@trndny08...

the CPU core

Did you create a solder bridge somewhere? That could pop things like that.

Your best bet is probably to look on ebay or craigslist for an identical unit with some other problem, I'd guess the optical drive is the most common failure.

That could pop things like that.

A power supply is just a device. You can repair it like most other things. Sometimes you can't get the parts you need but often you can substitute. However you do need to have some experience and this may be a tough one to learn on.

3v could possibly come from 5v via 2 power diodes, which will drop apx 1v each. However the exact drop depends on the diodes and current draw, so feeding a cpu from this would be chancey. A 3v reg would be rather safer. Easier though would be a regulated 1.7v dropper, basically a transistor multiplying resistors/diode on the base side. Easy to implement.

The vfd filament is a single fixed load, so this can be run from the smpsu transformer output via a cap or resistor. IOW you need to either measure the vfd filament current draw, or start with a high value R and move down until you get 3.9v on the vfd. IIRC with vfds start by assuming 20mA, then adjust upwards as required.

VFD -27 rail will be very low current, so you can get this by voltage tripling the ac output from the smpsu transformer. It does not need to be 27v, 24-36 is fine. Because the smpsu runs at high freq you should be able to get above 27v no problem. Capacitor size determines how much of the notional '36v - diode drops' you get, so again start small and increase them to get enough V.

NT

Before getting into this any deeper though, I would strongly suggest that you get a suitable isolation transformer to run it from whilst you are working on it. Switch mode power supplies are potentially lethal - and I mean that in the literal sense of the word. Even when run on an isolated supply, they are still dangerous, so please be very careful. It should also be noted that they are extraordinarily fickle - not for the faint-hearted. They can have the most experienced of us tearing their hair out ...

Good luck with it !

Arfa

3.3V is supplied natively on ATX power supplies- no need for trickery, unless you want to use an old AT supply. -27V or close enough can probably be derived from the winding on the transformer which supplies the -12V rail. Voltage doubling may be necessary though.

Seriously, this is not a very wise idea- there are safety issues to consider. While it may well be made to work, until there's a problem somewhere- PC PSUs are capable of supplying massive currents and will dump

30+ amps into a short circuit before shutting down or self destructing. In the event of a fault on your DVD recorder's main PCB this will vapourise PCB tracks, explode ICs and maybe melt wiring etc, not good unless you like fireworks from your AV gear!.

Frankenstein projects like this are best avoided, if it's worth fixing then I'd advise you get it done properly. If not, scrap it or sell it for spares on eBay. It is quite possible that the power supply damaged the main PCB when it blew anyway.

If you're sure there were no solder bridges, I suspect the original fault was due to high ESR/failed caps, and when you soldered around the PSU the heat affected them causing the catastrophic failure. I've seen this happen before.

Morse

Oh, so we can add visualisations to our older gear? :)

NT

Yes, but leave the cover off or you won't get the full benefit! ;-)

Morse