I inherited a dead GE 31GT657 (CTC177CC) that someone said died while
running. The HOT was shorted. I replaced the transistor and after some initial
checks, powered the set up with lightbulbs in series with the AC line. It ran
for about half an hour, snapped a few times and failed again. To see if the CRT
was arcing, I ran the set with the anode disconnected and monitored the HV and
+140V line. The B+ was rock stable but the HV was all over the place. When it
arced over the top of the flyback, I shut it down quickly. There are no blue
caps bulging, all electrolytics ESR check OK, and all known failure points such
as the H drive tranny have been resoldered. The tuner had been reworked and
EEPROM replaced several years ago.
I cleaned and patched the fly (it still works), but before I replace it,
I'd like to know what's really happening. I replaced the LV reg IC anyway, just
for luck. What would make the HV bounce around (too low and too high) like that?
When the set was running, the picture never glitched (except when it snapped) or
lost sync (thinking corrupt EEPROM), and I never saw the sides pull in, so I
don't think the fly has shorted turns. It tests OK on my Dick Smith ringer. Why
wouldn't the overvoltage trigger shutdown? Something else must be going on here.
Bad HV diodes in the fly wouldn't produce HV too high. The CRT looks good and
the set has S inputs, so I'd like to save it if I can. Anyone seen this before?
I have not found much info on HV problems for this chassis. Thanks!

Ray
--
My return addy is spamblocked. To reply, remove the zeroes.

Almost all 31" ctc177 failures of the horiz output are caused by either bad
solder connections on the driver transformer, or a bad flyback. In your case
I'd bet flyback.
Ron

I inherited a dead GE 31GT657 (CTC177CC) that someone said died while
running. The HOT was shorted. I replaced the transistor and after some
initial
checks, powered the set up with lightbulbs in series with the AC line. It ran
for about half an hour, snapped a few times and failed again. To see if the
CRT
was arcing, I ran the set with the anode disconnected and monitored the HV
and
+140V line. The B+ was rock stable but the HV was all over the place. When it
arced over the top of the flyback, I shut it down quickly. There are no blue
caps bulging, all electrolytics ESR check OK, and all known failure points
such
as the H drive tranny have been resoldered. The tuner had been reworked and
EEPROM replaced several years ago.
I cleaned and patched the fly (it still works), but before I replace it,
I'd like to know what's really happening. I replaced the LV reg IC anyway,
just
for luck. What would make the HV bounce around (too low and too high) like
that?
When the set was running, the picture never glitched (except when it snapped)
or
lost sync (thinking corrupt EEPROM), and I never saw the sides pull in, so I
don't think the fly has shorted turns. It tests OK on my Dick Smith ringer.
Why
wouldn't the overvoltage trigger shutdown? Something else must be going on
here.
Bad HV diodes in the fly wouldn't produce HV too high. The CRT looks good and
the set has S inputs, so I'd like to save it if I can. Anyone seen this
before?
I have not found much info on HV problems for this chassis. Thanks!

Ray
--
My return addy is spamblocked. To reply, remove the zeroe