Re: SMPS help needed - Philips VCR

On Thu, 7 Aug 2008 12:41:18 -0700 (PDT), ajuniper@xxxxxxxxxx put
finger to keyboard and composed:

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

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
--
Please remove one 'i' from my address when replying by email.
.

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