SMPS help

[Jumbaliah]

Hi, I've got a dead powersupply from a JVC VCR (HR-S4700U, pwrspply #
PB10559). I've replaced the electrolytic caps last year and recently
it came to a dead stop.

I opened it up and noticed that there were some scorch marks on two
caps due to heat from two power resistors (R8 - 220ohm and R13 -
180ohm). These resistors are in series between the base of Q1 (C4161
NPN high speed switch in TO220) and the collector of Q2 (C3616 NPN
high gain switch TO92).

What I found strange is that these resistors are bypassed with a trace
that goes from the base of Q1 to the collector of Q2. But the
resistors must form a type of voltage divider because there is a tap
taken between the resistors that leads to some other components and
into a Sharp PC817 photo transistor and also into one of the primary
inputs on the large transformer.

I thought that maybe R8 or R13 were blown but I removed both and
measured them and they have resistance close to their rated value.
Does anyone have some tips for me that would explain why R8 and R13
would see so much current they'd blacken some neighbouring caps?

Jumbaliah

 

[Mark D. Zacharias]

I know you replaced the caps, but did you get the one in the PRIMARY
circuit? About 1uf at 250 volts if memory serves.


Mark Z.

 

[Jumbaliah]

I know you replaced the caps, but did you get the one in the PRIMARY
circuit? About 1uf at 250 volts if memory serves.


Mark Z.

Hi Mark, thanks for the reply. Here's the list of caps I changed (all
are 105C rated)

C8 200V 100uF
C13 50V 2.2uf
C14 6.3V 120uf
C15 6.3V 100uf
C16 50V 47uF
C17 35V 33uf
C19 25V 270uf
C20 25V 220uF
C22 16V 1500uf
C23 16V 1200uf
C24 63V 33uf
C25 10V 1500uF
C26 10V 1000uf
C41 6.3V 100uF
C42 6.3V 100uf
C43 16V 100uf
C44 16V 100uF
C45 63V 10uF
C47 16V 100uF
C48 16V 100uF

Is this 1uf 250V your talking about a ceramic?

 

[Mark D. Zacharias]

Not a ceramic - a smaller electrolytic near the main switcher transistor
(the primary area).

Fixed my HR-S5800 which is almost identical to yours.

Mark Z.

 

[Jumbaliah]

Hmmm, the closest I've got is C13 to what your talking about. I
replaced every single electrolytic big and small in the powersupply
cage. And after reading suggestions I used 105C caps to boot.

I just checked R9, a 1W resistor, in the primary side and it's marked
"R33 J" and has a triangle with dots on it's vertices. It's blue in
color and is vertically mounted. I figured it was a 33ohm resistor so
I measured it in circuit and it read a touch above zero ohms. I
pulled it and it still read 0.4ohms. But not that I think about it
I'm thniking that it must be 0.33ohms. Damn, I thought I had it.

Jumbaliah

 

[Mark D. Zacharias]

Well, if it just won't run, and all the caps are known good, you could look
to see if there's any shorts, especially shorted zener diodes.
Often there's a regulation problem, the voltages go too high, and one or
more protective zeners will short out. This throws off the "Q" of the
circuit so far that it just won't run at all, thus (hopefully) saving other
parts.


Mark Z.