Mitsubishi TV vertical hold

[Boris Mohar]

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?

 

[William Sommerwerck]

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.

 

[jerryg50]

Fixing Vertical Sync Fault, Thermo Sensitive

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

I can recommend a generic answer.

This is a thermo sensitive problem. Look for worn capacitors in the vertical sync separator and in the primary video amplifier processing sections. Also check the circuit areas where the vertical reference goes in to the sync gen/drive processor IC, or the equivalent.

When the chassis is jigged up you can use freeze spray to keep the caps cool. Use a scope to trace out the signal path to determine where the loss of vertical sync is occurring.

You will see the loss of vertical sync with the scope set at the frame (field) rate. Pay attention to the vertical serration and sync block portion of the signal to know where the vertical sync is occurring.

You may find a number of caps that will have to be replaced. On more rare occasions the fault will be with semiconductor, or resistor components.

This type of service could be too involved for the beginner. You may have to give the set out to a service center for this one. An experienced tech who knows the set model can most likely fix this with about 1 to 2 hours of labor.


Jerry G.

 

[Boris Mohar]

drifting.

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.

G²

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.

 

[William Sommerwerck]

It turned out to be a 40-pin proprietary chip that decided to fail.

I found this out by warming up the whole board with hot air until
vertical sync stabilized. Selective application of cold spray
pinpointed the chip.

"Heh, heh, heh", he muttered smugly.

Out of curiosity, what exactly was the chip's function or funtions?

 

[Boris Mohar]

"Heh, heh, heh", he muttered smugly.

Out of curiosity, what exactly was the chip's function or funtions?

Right now it is a thermometer;

 

[Chuck]

I don´t know, but I can´t remember a case where a semiconductor
improved after warming. Usually is in reverse: a damaged semiconductor
will work while it´s 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.

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

 

[jerryg50]

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

From age and use, there are some components in the vertical sync separator, and or in the vertical oscillator frequency-timing circuit that have become thermo sensitive. Most of the time it is capacitors in these areas.

An experienced TV tech can locate the defective parts, order replacements, and fix this for you. It should be about one to two hours of labor average.

I used to service many faults of this type in TV sets when I was in the service business.



Jerry G.