"Mark Zacharias" wrote in message
...
"G B" wrote in message
...

"Arfa Daily" wrote in message
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"b" wrote in message
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On 23 ene, 04:51, "G B" wrote:
Hi,

I have a Sony SLVN750. It failed such that it had no time display
and
would no longer power up when pressing the power switch. I tore it
apart
and "shot-gun" replaced the nine capacitors in the general area of
the power
supply with capacitors of the same value. It now will power up and
even
play a tape BUT when it is powered down (in standby) the display
blinks on
and off. It seems the power supply is cycling on and off. Any idea
what
would cause this? Any help is appreciated...


"G B" wrote in message
...
The display isn't flashing per say...

The display goes blank after the supply apepars to shut itself down. It
returns after say 500 msecs.. then stays on for about a second.. then
goes blank and starts again. Also every time the display blinks, the
carriage motor appears to go through its position check...
I did replace the all the "big" electrolytic caps.. the largest were a
couple of 1000uF... I suspect that section of the circuit is working
properly.. I think there must be a standby power section of the circuit
which is toast... any ideas?


Have you replaced any electros in the front end or were they all
secondary side ? This sort of odd behaviour in switchers can be
symptomatic of small caps - often only from 1 to 47uF in value -
situated around the oscillator / control IC having gone high ESR. You
will often find them located close to some other component or heatsink,
which runs hot. Do you have an ESR meter ? Just as an aside, it makes a
thread and the replies much easier to follow if you bottom-post rather
than top. :-)

Arfa



I replaced 9 caps all in the area of the switcher, or at least where I
think the switcher is. I replaced the large regulation cap (82uF @ 200V)
and I replaced a small (4.7uF@50V) on what I would consider the primary
side. (prior to the large transformer). I replaced all the
electorlytics: 10uF, 100uF, 330uF, 470uF, and 1000uF values on what I
think is the secondary. Do you know if the switcher provides the
boot-strap voltage in standby mode? Or do they have another regulator
somewhere off the line regulation circuitr? I have also what appears to
be a hot-spot on the circuit board around QIP107, RIP110, QIP108, ZDIP05,
and DIP109. It doesn't feel warm to the touch after it has been running
for a while.. so the hot-spot is a bit of a mystery. I don't have an ESR
meter here. I guess I could probe the removed components at work... of
course I don't know what the ESR of the original caps. I thought I could
use a DMM to get an indication, but it must be marginal enough I can't
tell which one is the bad actor.

As for the posting at the top vs bottom... thank Outlook for always
starting at the top rather than the bottom of the post



Likely scenario:
The hot spot on the board was caused by the over-voltage at that point,
caused by the bad caps. You replaced the caps - now the voltage there is
normal. This presumes the zener diode, which is what got hot, did not just
short.

Mark Z.
That was at least part of the theory.. I was thinking this might be the
circuit that was used to regulate ~150VDC down to a standby voltage. I need
to trace the circuits to see if I can figure it out. If it is the problem,
I have no idea what to replace it with. I doubt I can see the numbers on
the diode, but
I will yank it all back apart to see if I can determine what it is. Any
idea what the standby voltage in these VCR's is?
Greg