Hello,
Mitsubishi TV Model CS-27403C over 10 years old
Problem: When TV is first turned on there is no vertical sync. As it warms up sync gradually returns. Initially the problem was minor since the sync returned quickly. Now it takes an hour or more. There are no obvious controls on the back. Not like in the good old days. Where to dig?
-- Boris
On modern sets, vertical sync is generated from the horizontal sync. The chip that handles sync separation is a likely possibility.
It's also possible the bias on the vertical output stage is drifting.
Bad caps
On May 8, 11:43=A0am, "Lance Dyer" wrote: > Bad caps >
I second the bad caps suggestion. I put the odds at 98% dried out cap(s). The 'warm up' description is exactly how 'lytics behave. they always measure better after unsoldering. I just changed 72 caps in Sony digital Betacam machines this morning.
The 'vertical from horizontal' description is pretty poor. Vertical like horizontal is part of the composite sync. First sync is recovered from the video and then processed to extract the H and V components. This can be done with counters and PLLs or simple RC networks.
G=B2
It turned out to be a 40pin proprietary chip that decided to fail. I found this out by waning up the whole board with hot air until vertical sync stabilized. Selective application of cold spray pinpointed the chip. I wish that it was the caps. Not having the replacement handy I glued a jacket of thermal insulation over the chip hoping that it warms up sooner. Seems shorten the "warmup" time.
-- Boris
"Heh, heh, heh", he muttered smugly.
Out of curiosity, what exactly was the chip's function or funtions?
OK, you made the 2%. Semiconductor failure are far less frequent compared to 'lytic caps. Personally I'd change all the lytics around the chip before changing the chip. They're easier to get, cheaper, more likely to fail and if more than 5 years old will probably need to go anyway. You have it open....
G=B2
d
=A0I wish
t of
I don=B4t know, but I can=B4t remember a case where a semiconductor improved after warming. Usually is in reverse: a damaged semiconductor will work while it=B4s cold and start to fail as soon as it heats. If you see any electrolytic capacitors in the vecinity of the chip, replace them first. Electrolitycs are cheap and easy to replace, and probaly will solve the fault.
Right now it is a thermometer;
-- Boris
Once you replace the lytics give the board a good scrub. Fluid which has leaked from the caps will cause this problem after all the caps are replaced. The resistance of this liquid will change with the ambient temperature. Chuck
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