SMPS help needed - Philips VCR

Could it be Q003? It should not be possible to have 1.2V at its base when the emitter is grounded, if it works.

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Ulrik Smed
Aarhus, Denmark

On Wed, 6 Aug 2008 14:34:17 -0700 (PDT), snipped-for-privacy@freeuk.com put finger to keyboard and composed:

Check for shorts on the secondary outputs. I'd suspect a shorted 18V protection zener (DO15) on the AL+15V rail.

- Franc Zabkar

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Quite possible.

Some of the voltages don't make any sense around Q003. For example,

1.2 V across B-E of a normal NPN transistor, and +1.5 and -1.5 on the IC001. But this might be due to there being some AC as the PSU repeatedly tries to start up, not just DC voltages, and this is confusing your meter.

While you're at it, it would be worthwhile to check the electrolytic caps for ESR and uF. If the zener is shorted, the cause may be bad caps.

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On 06 Aug 2008 19:16:33 -0400, snipped-for-privacy@seas.upenn.edu (Samuel M. Goldwasser) put finger to keyboard and composed:

Thanks for the followup.

I just remembered that I've seen faults in Sharp VCRs (Philips VCRs are often rebadged Sharps) where the zener isn't actually shorted, but is protecting the outputs from an overvoltage caused by a small electrolytic cap on the primary side of the supply. In this case I'd change CO11 just for good measure.

- Franc Zabkar

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Thanks for all the suggestions, I'll work thru those tonight and see what comes up.

Andy

Good idea to change them all if you find one that is marginal or bad!

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Early days yet but it would appear as though both Q003 (infinite resistance both ways across one junction) and D015 (short circuit) are both dead. So a good call on both counts.

Unfortunately I can't get a replacement diode or a close transistor match until tomorrow so it'll be after the weekend before I find out what else has blown.

Aside from experience, what else led you guys to the above conclusions? Why do the symptoms point at these components?

Thanks, Andy

On Thu, 7 Aug 2008 12:41:18 -0700 (PDT), snipped-for-privacy@freeuk.com put finger to keyboard and composed:

A squealing sound from an SMPS often indicates a short on the outputs. A protection zener across one of the outputs, in this case the AL+15 rail, will sometimes sacrifice itself to prevent an overvoltage condition from destroying more expensive components.

An overvoltage condition can arise when the main switchmode regulator loses control. When everything is working correctly, the optocoupler (IC001) senses the output voltage on the secondary side and uses this feedback to control Q003 and Q002 in such a manner as to reduce or increase the drive to the chopper (Q001). If, for example, IC001 or Q003 fails, then the supply thinks that there is no output, which then causes the chopper to "run away".

In your case I would check all the active components (transistors, diodes, optocoupler). If any are faulty, then there is a real possibility that the supply will fail catastrophically and you'll end up changing the exact same components again. 2SC1815 transistors are cheap, as are signal diodes, so a shotgun approach may be advisable. I wouldn't change the chopper, though.

- Franc Zabkar

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Couldn't have said it better myself. :)

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My thought is Q002 has been switched on by the Diode OR Gate formed by D007 and D008. If D021 does not operate, then I think 6.4 volts will not be generated for IC001. My 2 C.

- Ravi.

Hi,

Thanks for all the help - turned out it was just D015 that had died (I initially messed up checking the transistor) - the video now lives again.

(although the video did get me worried when it wouldn't accept, or would just spit the tape out, before I'd put the lid on, but that would appear to have been my bench spotlights confusing the tape detection beam breaks...)

Thanks for the explanations of what went wrong.

Andy

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About your repair: Don't be surprised if the same thing happens again.

The zener likely didn't die on its own. Probably, some electrolytic caps in the supply are marginal. I wouldn't be surprised that if you unplug it for a day and plug it back in, the same zener will go belly up again.

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I did also replace the electrolytics anyway, since I'd already done that to find that they weren't completely at fault. :-)

They may have been at fault (although only around 5 years old), but I bet that the mains voltage dropping down to 30-60v for half an hour also had something to do with it...

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On Thu, 14 Aug 2008 06:07:57 -0700 (PDT), snipped-for-privacy@freeuk.com put finger to keyboard and composed:

A friend's Sharp VCR died exactly the same way (shorted protection zener) after a brown-out. The only problem I could find was that zener, so I assumed that it must have died when the output of the supply went into overshoot when the mains supply returned. I usually change the small electros on the mains side in any case.

- Franc Zabkar

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Yeah, that could do it.....

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