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Technical Note
W. Curtiss Priest

October 6, 2003
Subject: Switching Power Supply Failure
Device: Panasonic PV-1560 VCR
Keywords: Fuse blows with bright light
Switching transistors
Problem, VCR completely dead
Repair a switching power supply

Preface:

While this repair story is about a fifteen year old
VCR, the proceedure is one to follow for everything
from PC power supplies to Audio CD to DVD players.

The front ends of switching supplies are quite simple.

We "make" 180 VDC from the AC line by using a "full-wave"
rectifier (i.e. 4 diodes, usually in a single package)
and storing that in a husky capacitor (around 300 ufd.,
200 volts). We tuck one or two MOV devices to suppress
transients, a fuse to meet UL requirements for fire
safety, and then one or two transistors in an oscillator
circuit to chop the DC and apply it to a small transformer
next to the transistor(s).

Introduction:

Typical reasons why a VCR (or most anything else) is dead:

1. Humidity sensor shuts down power supply
2. Switching supply transformer shorts
3. Switching transistor shorts
4. Bridge rectifier shorts

In that the fuse (1.6 amp) goes with a bright light, the problem
would not be #1.

An ohm meter check of the rectifiers indicated
semiconductor action -- i.e. -- leads one way (one
polarity via ohm meter battery) indicates a different
resistance than the other. Always use an earlier,
analog (meter) ohm meter -- not a digital one -- to
get enough voltage across the diode to conduct (i.e.
above .6 volts) Use range x1

Rectifers were fine, so not #4.

Checking again with an ohm meter, Base to Collector and
Base to Emitter indicated that the switching transistor
(only one in this design) was fused. So the switching
transformer appeared as a dead short, and thus the
bright light.

Now. Is the switching transformer ok? Or, did it fail,
and take out the transistor?

To distinguish this is the reason one has a small collection
of switching power supplies from various devices.

We check the NTE equivalent of the C3890 transistor (really
2SC3890 -- the first two characters are almost always
dropped). Cross is an NTE 379 -- 12 amp T-NPN Silicon,
Power Amp., high voltage switch -- 700 volts.

While we are only switching rectified line voltage, i.e.,
about 180 VDC, transients around an inductor dictate the
much higher transistor voltage. And, while VCRs tend to
run at 10-15 watts ("off" versus play) and so currents
on the high voltage side of the supply are quite low
(note the 1.6 amp fuse -- which is way over 15/180 amps --
having a 12 amp switcher also deals with transients.

Grabbing a power supply, it had a pair of C4242 (2SC4242)
switching transistors on a largish aluminum heat sink.

Crossing that, these are NTE 2312, same category transistor,
700 volts, 8 amps. Same TO220 case, same pattern of
base, collector, emitter leads.

***

The original transistor had absolutely no heat sink. This
is highly unusual and probably the reason it was susceptible
to shorting.

So, with a hack saw, I cut the C4242 from the heat sink,
leaving enough heat sink to "look right" (and fit). I
use a steel cutter in a rotary tool to shape the cut
edge, removing sharp edges and smoothing the cut surface.

I've cut the C3890 out, leaving 1/4 inch of leads coming
up from the circuit board. (I used to diligently unsolder
the leads from the board and solder suck the holes clear,
but overlapping pre-tinned leads is faster.)

So, with the new transistor in, we will now determine
if there is a #2 fault (for a similar kind of fault
in a CRT monitor/TV, there is an LOPT/FBT tester to
examine whether the high voltage winding is shorted
on the flyback transformer).

A third fuse (uum, running low on 2 amp fuses), the
"new" transistor soldered in, we plug the machine in
again -- wincing.

Ah! No bright white light. (Front panel is off, to access
the power supply -- but we hear a few motors start and
stop -- "good sounds."

Reinstalling the power supply (too many screws and steel
twist tabs -- plus -- multi-pin connector to the front
panel, we plug it in -- the machine blinks 12:00. Put
in a tape, hit play -- says it is -- this is proof enough
for me, finish by replacing the top cover.

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