CTC197 typical FBT symptom but FB rings ok

[Michael]

RCA F35676MB, CTC197CD, ser 649560677, mfr Dec 1996.

Have you had one of the CTC197 FBT failures where a ring test shows good but
the flyback is shorting or arcing internally only under full power?

This set has the "four strikes you're out" symptoms of a bad FBT reported in
this group in the past.

Power supply seems okay - in standby have 144 V, plus the lower voltages and
regulation appears good. When the power is turned on, the 144 drops to
about 100 V each time the set trys to fire up, then returns to 144 or so
back at standby.

In previous postings this symptom has been diagnosed as a faulty flyback and
most reported that changing the transformer fixed the problem. However,
this flyback rings good, and the yoke winding also show no apparent fault.
The HOT is okay.

I'm wondering if the CTC197 problems with the FBTs that were repaired by
replacement in the past were confirmed with a ring tester, or if the
failures showed up only when the HV comes up, as it might be in this case.

Michael (C.E.T.)

 

[AJ]

Seen this happen on numerous occasions, the ringer can not subject the LOPT
to sufficient voltage and current to test for arc over or isulation
breakdown. BTW seen so many RCA LOPT that are defective that almost any time
the h-output device is blown the LOPT is changd also. That is if a viable
replacmentg transformer is actually available.

 

[Michael]

Thanks AJ

My experiences with bad LOPTs has been that they either showed at least some
visible signs (cracks, bulging, discoloration, dark carbon traces, pin-point
arcing to core etc) and/or they fail the ring test. But I'm not discounting
the possibility that in this one there could be an internal breakdown only
at HV.

When the set came in, I cautioned
the owner that it could be the FBT and the instruction was "if so, don't
bother repairing". I'm trying to avoid having to bring in a replacement
transformer to prove the point as I won't recover the cost of the part in
this round. At the same time, I want to be reasonably certain that the
diagnosis is correct.

Knowing that this has been the experience of others (failed LOPT but no
visible sign s and passes ring test) is reassuring.

Michael (C.E.T.)