Electronics

Guest · Posted: Wed Dec 07, 2005 1:35 am Post subject: Fixing a Toshiba scan converter

Has anyone done one ? I forget, but the ones in the fdx or hdx series.
I suspect they're all the same.

They were on backorder for quite awhile and we finally got it and the
set is fixed and gone, and the money is probably spent, but I was
looking at the old one.

There are several SMD caps on the module that check bad and some are
not even recognized by the cap checker. Some of these are near two
identical ICs that each have two crystals nearby. I reckon these are
digital clock"ing" chips and with the symptom I think it may be
repairable.

In this particular specimen there appears to be no capacitor piss, or
at least the resultant corrosion.

The symptom is that there is constant piecrusting in the main image
only, the PIP is good and the OSD is good. It is not a sweep problem.
It is because the set does not run 480i even on an NTSC input, and the
PIP uses a seperate scan converter for obvious reasons.

The problem used to be only when the set was cold.

Two plus two have at least come up to three here. Thus my plan; a
friend has a Toshiba with exactly the same problem, so I made a deal
with the shop, I will replace all the bad caps on company time, the
only way to see if this worked is for me to go install it in my
friend's set. These things are $280 list, so we agreed, if it works
I'll bring $100.

Now a question, who owns my buddy's old board which will be abandoned
to me ?

I don't have the tools at home anymore to fix it, the boss sold me a
board which was a pull, they pulled it, cust didn't want it, it is
theirs until I give them money.

I will append this later with more info, and later when I know if the
fix worked. I'd like your input, especially if you've fixed one, but I
also post this to share the info.

For those of you who want raw data : I found at least 6 marginal
capacitors, 3 bad and 2 that weren't even recognized as caps by the
checker. The symptom used to go away when warmed up.

2+2= ?

JURB

John-Del · Guest

I've done at least ten of these boards for various video\piecrusting
issues. I believe the 10uf caps were the culprits, but in any case
there are two at the top rail that need to be replaced. Not one came
back.

John

Guest

Thanks, that's encouraging.

So now I took the ESR checker to it again and am now replacing 29 of
them. Most of the 10s are bad and I think all of the 22s. I wonder how
the thing ran so long, I mean at this point some of those caps could be
so non critical application wise they could probably be removed, but
who knows which ones.

I have some theories about how to service stuff like this in the future
as the bottom and top lines put the financial squeeze on us.

To start, think about this; just what makes them use a 22 instead of a
10 ?

1. If HF needs to be bypassed they won't use those shitty performance
caps unless they have a smaller cap as an HF shunt.

2. The capacity tolerance makes it so they almost never use SMD caps
for any timing.

The only exception to #2 might be that they are trying to manage
startup and shutdown. I mean they want certain sources to come up or go
down faster than others. I have indeed seen Mits PIP modules that
seemed identical except for SMD cap values. Of course that doesn't mean
they were in an identical chassis, prehaps it has something to do with
the set having a different power supply. Choosing the wrong values
could possibly hurt reliability by causing unwanted surge currents
during starup or shutdown.I will tread carefully when messing with
other people's designs, but I will indeed tread.

Not in rebuttal to #1, but there is another factor. Even with HF help
from a ceramic or other decent cap the need to filter the current
fluctuations on the actual wires from the power supply, and foil
traces. The lower frequency components of this hash do not need to be
supressed completely, so they can get away with the cheapo caps.

Might take a little experimentation to figure out the best way to do
these.

JURB

Shoreline Electronics · Guest

Just replace all of the 10uf ones. That is the standard rebuild on these

==========================
Jeff Stielau
Shoreline Electronics Repair
344 East Main Street
Clinton,CT 06413
860-399-1861
860-664-3535 (fax)
jstielau@snet.net
========================

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