Panasonic pv-4820 VCR Power supply

[tblake05]

Hello,  Goodwill special for 2.99 I scored this VCR in great shape.  Plugging it in shows the clock turns on, but everything is unresponsive.  Will not power up, and nothing happens when you try to insert a tape.

Upon reading sounds like these machines have power supply issues, mainly leaky electrolytic capacitors.  Upon opening it, quick check of the power supply shows there may be one or two bulged caps.  So I plug the machine in and test the outputs of the supply to the main board.  Based off this schematic, here's the outputs I get for outputs:

https://www.repairfaq.org/sam/vcrps.pdf

1) +4.24v
2) +51.4v
3) +17.32v
4) -1.6mv
5) +4.28v
6) +4.28v
7) -35.6v
-28.02v
9) -25.84v

From what I read, if the 5v rail is any lower, the machine will have issues.

It appears someone has tried to repair this power supply in the past, however with the wrong components.  By the schematic C16 was originally 6.3v 330uF, was replaced with a 25v 330uF.  Which should be fine as far as I know.  However both C11 and C14 which should both be 50v 10uF were replaced with 50v 4.7uF.  Which could be the problem totally.  Plan of attack; Order all replacement electrolytic caps (C4, C11, C12, C13, C13, C14, C16, C17, C19, & C21).  Also replace Zener diode D15 which according to what I read is destroyed when caps go bad.

My next problem, and it was completely my fault.  So when I was testing voltage output, I accidentally touched my multimeter leads together when I was measuring between outputs 3 & 4.  Which dead shorted 17.32v to ground.  It was a split second, but I saw a spark, and damage was done.  ugh.  Now the power supply makes a terrible whine when plugged in.  What I have done:  Removed C12 and C13 for time being.  Also removed and tested the power diode D6 out of circuit.  When removed, the supply mo longer makes the wine sound.  However the diode tested ok I believe.  (dmm leads one way 57kohm, the other way reading bounces between out of limits and 16mohm.  Did I zorch the transformer?  Where should I text next?  I hate when I f something up more than when I started, but, i'm sure it can be fixed.

Thanks for any help!!

[mzacharias]

Check for a possible shorted zener diode across the +14V line. The zener would be 16V 1 watt.

Common Panasonic problem; usually caused by bad caps (high esr/low value) causing a loss of regulation.

[tblake05]

Sounds good.  The Zener D15 according to what I read is a failure point so I was already planning on replacing it.  It is an 18v according to the schematic.  Seems funny that that high pitch whine stops without D6 and C12 and C13 and leaving the zener.  Seems if the zener were a short to ground it would be also whining without D6, C12 and C13.

[mzacharias]

I think you're right - 18 volts.

My brain...

[tblake05]

If I play it safe and replace the diode D6 also, the schematic shows no part info just says "PWR".  its a rather beefy diode with nothing decipherable marked on it.  Any idea what i could put in there?

[coromonadalix]

I would put an 1n 4000 series diode  like an 1n4002 up to 1n4007, an 1n4007 is 1000v 1 amp

https://www.mouser.com/ds/2/149/1N4007-888322.pdf

[andy2000]

Start with the caps.  If the zener diode was bad, it would make a high pitched squealing noise, and the display would be blank.  It only fails when the power supply loses regulation, so unless it's shorted, it's fine. 

There's no need to replace any of the other diodes unless you want extra work.

[tblake05]

Andy, read about my little "F" up in my first post regarding diode replacement.  The power supply does make a high pitch squealing noise.

I did measure the Zener forwards and backwards with my ohmmeter.  Both directions show very low resistance.  So I'm betting I've found my problem.

All I can do is rebuild the power supply and hope the tape mechanism works.  LOL!  The belt seems ok and the unit is in very nice shape otherwise so someone took care of it.

Thanks for the info everyone!

[andy2000]

Andy, read about my little "F" up in my first post regarding diode replacement.  The power supply does make a high pitch squealing noise.

I did measure the Zener forwards and backwards with my ohmmeter.  Both directions show very low resistance.  So I'm betting I've found my problem.

All I can do is rebuild the power supply and hope the tape mechanism works.  LOL!  The belt seems ok and the unit is in very nice shape otherwise so someone took care of it.

Thanks for the info everyone!

Sorry, I missed that.  Hopefully the power supply shut down before any damage was done to the VCR.  It's a good thing the protection diode was there.

Back in the 90's, I remember seeing a lot of very low quality Funai VCRs that omitted the protection diode.  When the power supply failed the output voltages went up and fried the whole VCR.