"Arfa Daily" wrote in message
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"G B" wrote in message
...

"Arfa Daily" wrote in message
...
"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


Yeah, that's a problem with it. Nobody gets too upset on here, but on some
groups they get *really* arsey if you do it. Top posting can sometimes
confuse a thread, especially when specific questions are being answered,
and the majority are posting one way, and someone is posting the other ...

As to your question, I would expect that the standby supplies for the
system control micro are derived from the switcher, but I couldn't say for
sure. It's been a while since I did any work on any VCRs, Sony included.
Knowing what the ESRs of the original caps should be is not really an
absolute. As I have commented a number of times with regard to using ESR
meters, a lot of it is intuition, with the meter just acting as a backup.
In general, the smaller the value of the cap, the higher its ESR will read
on a meter. The small ones, such as your 4.7uF, should not read higher
than a few ohms. 47uF up to say 220uF should not be higher than a couple
of ohms. Above this capacitance value, ESRs will generally be in decimals
of an ohm, and a factor of 10 lower than that for the 'big' values of
2200uF and up. In general, the higher the voltage working, the lower the
range of ESR values that would be 'normal' for any particular cap.
Usually, if a cap has reached a point where it is causing trouble in a
switcher, its ESR value will have gone off with the fairies, and be easily
spotted as the bad one. Interestingly, its capacitance value will often
read near enough correct on a C - meter.

What your remaining problem is, I really don't know without looking at a
schematic for the power supply, but one thing I would say is that it seems
odd that you have managed to cure the primary problem with replacing
whichever cap it was amongst the ones that you changed, and then been left
with this slightly 'odd' problem. A faulty cap is a good call for the
original problem, and was almost certainly the correct diagnosis, but it
is very rare, when this is the case, for there to be any other issues. If
it were me, I think that as a first move, I would go over all my work
again very carefully, making sure that I had got the right values in the
right places (for that you would need a schematic), and that I had not
accidentally put any in backwards (easily done :-\ ). Also, make sure that
the replacement types are 'suitable' i.e. the correct or very close value,
similar voltage working, low ESR type. They should also be 105 degree or
better types for future reliability. Also, I would not trust any caps that
you have had lying around in a drawer for years, or any radial leaded
types as being suitable.

Stating the obvious maybe, but also be sure that your soldering is all
good. If it's not something which you do all day, it is easy to get a poor
joint on a component, without realising it ...

Arfa

I'm going to tear it apart again. I will check the cap's polarity, but I am
not expecting I messed that up. I did draw a diagram before removing the
original capacitors so I think I have the right capacitors in the right
holes. I did substitute one value, I think a 100uF for an 82uF. I tried to
buy duplicate values so worse case I can try a 2nd version of the
replacement capacitor. I will reflow all the solder too... I'm pretty good
at seeing a bad joint, but I've been bitten by that before too.

Thanks for all your help... This would be so much easier if we just had a
schematic or a similar schematic to get an idea from

-Greg