Sharp VC-H985 power supply issues

[YamahaFreak]

Decided to take a chance on a Goodwill VCR yesterday for $10. It has no power, totally dead with a tape inside. I get it home and open it up to discover the power fuse blown. Replaced and connected to my Variac, I got up to about 70 volts AC when smoke appeared from the choke shown in the attached image (L901), and I immediately removed power. Despite smoke I don't think I destroyed the choke, it wasn't open according to my multimeter. Upon closer inspection I had placed a 6 amp fuse in the clip when the original was a 1.6 amp, oops! I applied 12 volts DC to the transport motor to get the tape out. My question now is, where should I look to solve the problem? I assume something in the power supply is shorted. Can I just test components in-circuit with my meter to find the short? Where would be the most likely culprit? (Transistor, capacitor, resistor, etc.)

[Ed in Tx]

Originally Posted by YamahaFreak

Can I just test components in-circuit with my meter to find the short? Where would be the most likely culprit? (Transistor, capacitor, resistor, etc.)

Yes. Any of those you mentioned.

Repeat tip: NEVER apply unlimited power to a failed SMPS always use a series incandescent light bulb, I recommend 40W in this case, when first testing it out.

You'll probably find the drive transistor shorted, cannot tell from your pic either a shadow or blackened circuit board in that area. May find a protection Zener diode on the secondary side, maybe a shorted diode or two, too. Don't take that as gospel! I said "probably", as in might, may...

And check all capacitors for ESR. Replace those with higher than normal resistance.

my 2¢.

[YamahaFreak]

Originally Posted by Ed in Tx

Yes. Any of those you mentioned.

Repeat tip: NEVER apply unlimited power to a failed SMPS always use a series incandescent light bulb, I recommend 40W in this case, when first testing it out.

You'll probably find the drive transistor shorted, cannot tell from your pic either a shadow or blackened circuit board in that area. May find a protection Zener diode on the secondary side, maybe a shorted diode or two, too. Don't take that as gospel! I said "probably", as in might, may...

And check all capacitors for ESR. Replace those with higher than normal resistance.

my 2¢.

Many thanks for the tips and sense of direction! I'll get right on it. There are darkened areas on the SMPS part of the board, though I admit to assuming those are just 'baking' of the board from residual heat over the years.

[Ed in Tx]

Originally Posted by YamahaFreak

Many thanks for the tips and sense of direction! I'll get right on it. There are darkened areas on the SMPS part of the board, though I admit to assuming those are just 'baking' of the board from residual heat over the years.

Under normal conditions there shouldn't be enough heat to discolor the board. Might happen though if one or more capacitors mostly on the secondary side were failing. Sometimes that can make things run hot until they fail.

[YamahaFreak]

Did some preliminary testing. In the attached image, the circled diodes test shorted (same readings both ways) and the one circled resistor is open. I haven't looked up how to test caps and transistors with my meter yet.

I'd assume a totally shorted rectifier is a pretty bad thing, hopefully the whole unit isn't toast.

[Ed in Tx]

Originally Posted by YamahaFreak

Did some preliminary testing. In the attached image, the circled diodes test shorted (same readings both ways) and the one circled resistor is open. I haven't looked up how to test caps and transistors with my meter yet.

I'd assume a totally shorted rectifier is a pretty bad thing, hopefully the whole unit isn't toast.

The 4 diodes make up the full wave rectifier circuit you will read something both ways. Probably OK but to test you'd need to de-solder one end of each and test individually. The resistor you have circled is a very high meg-Ohm resistor can't quite tell if the 2nd band is brown or violet, but regardless it looks like either 2.1 or 2.7 meg Ohms and provides a static electricity path to ground from the cold side to the hot side. Never seen one there fail. It's going to read open unless you have your meter set on the highest Ohms scale.

The diode though, and Q901 and Q902 I would be suspicious of. And anything else in that cooked area. It's been getting way too hot there.

You need an ESR meter to test all the caps. If you plan on doing this sort of thing as a hobby (or a living☺) you need to get one, not just to fix this machine. Said it before: Electrolytic capacitors are the bane of electronics!

And yes if those 4 diodes that make the full-wave AC rectifier are all shorted it probably took a nasty power surge or lightning hit.