SMPS help needed - Philips VCR

[[email protected]]

Hi,

I'd be grateful if someone could give me some clues as to where to go
next with a dead Philips VCR (VR750) SMPS.

The PSU died when the mains dropped to 30-60v for about half an hour
(in the UK so it should have been around 230v) before going away
completely for a few hours. I was on holiday at the time so don't
know how cleanly the power came back, although the VCR was on a surge
protector so it's unlikely that a surge has killed it.

Neither fuse is dead.

There is a short (~0.5s) squeal from the PSU when mains is applied.

I've found a circuit diagram and spent some time reading the various
FAQ notes.

I've checked the large electrolytics and the resistors I guess to be
the "startup resistors" and they all check out OK with a DVM (having
removed each component in turn). One electrolytic capacitor was a
little low (67% of its nominal value) and was replaced but this made
no difference.

I have measured various voltages around the circuit and quite a few
are different to those shown on the diagram. There is no voltage
present on the output side of the transformer. The PSU is integral to
the main circuit board so will be difficult to test in isolation with
no load.

In my limited knowledge, it would appear as though the switching
control transistor is holding the switching transistor in a non
conducting state but I would appreciate some clues as to why this
might be.

I have annotated the diagram with the measured voltages (in red) and
indicated the components I have pulled and checked; the diagram can be
found at http://www.ajuniper.f2s.com/psu.jpg (2MB image).

I would appreciate it if someone more knowledgable than myself could
give me some clues as to what to look at next.

Thanks,
Andy

 

[Ulrik Smed]

I have annotated the diagram with the measured voltages (in red) and
indicated the components I have pulled and checked; the diagram can be
found at http://www.ajuniper.f2s.com/psu.jpg (2MB image).

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

 

[Franc Zabkar]

There is a short (~0.5s) squeal from the PSU when mains is applied.

I have annotated the diagram with the measured voltages (in red) and
indicated the components I have pulled and checked; the diagram can be
found at http://www.ajuniper.f2s.com/psu.jpg (2MB image).

Thanks,
Andy

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

- Franc Zabkar

 

[Samuel M. Goldwasser]

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

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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[Franc Zabkar]

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.

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

 

[[email protected]]

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

Andy

 

[Samuel M. Goldwasser]

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.

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

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[[email protected]]

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

 

[Franc Zabkar]

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

 

[Samuel M. Goldwasser]

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.

Couldn't have said it better myself.

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[[email protected]]

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

Andy

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.

 

[[email protected]]

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

 

[[email protected]]

Another quick question on this - are all Zeners created such that they
go short circuit like this - or is it just luck - or do I need to
ensure that the replacement will fail in the same way under the same
circumstances?

 

[Samuel M. Goldwasser]

Another quick question on this - are all Zeners created such that they
go short circuit like this - or is it just luck - or do I need to
ensure that the replacement will fail in the same way under the same
circumstances?

Diodes do tend to fail shorted when abuse.

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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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

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?

On Aug 7, 1:44 pm, [email protected] (Samuel M. Goldwasser) wrote:

On 06 Aug 2008 19:16:33 -0400, [email protected] (Samuel M.
Goldwasser) put finger to keyboard and composed:

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.

Thanks for the followup.

I just remembered that I've seen faults in Sharp VCRs (PhilipsVCRs
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.

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

 

[[email protected]]

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...

Andy

 

[Franc Zabkar]

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...

Andy

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

 

[Samuel M. Goldwasser]

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...

Yeah, that could do it.....

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