CTC203 "sacrificial" output transistors

I recently did a CTC203 that was squealing. I found the horizontal
output transistor shorted and replaced it. I then dug out the infamous
coil and cleaned/ resoldered it as well. I fired the thing up only to
have the flyback arc and take out the new transistor. I then replaced
the flyback and of course the new transistor again and the set worked
fine. I don't have a ringer but I have been told that this type of
insulation breakdown is somewhat common on these 32 inch and larger
chassis and very often won't show up on a ring test anyway. I don't
want to just replace a flyback as a prophylactic measure every time one
of these squealers comes in but is there any way to predict this might
happen before powering up a new repair. Thanks, Lenny Stein, Barlen
Electronics

 

Sad to say, either a ringer to test the LOPT before replacing the output
transistor or estimate to include the LOPT as a nominal repair. IMHO the
cost of the output transistor is small price to pay to do a dynamic test on
the LOPT. Seen a lot of these transformers fail not only in the larger sizes
but also the 27" versions.

 

Did I read that right? You think that blowing outputs is a reasonable way
to diagnose a bad LOPT? Please finish this line of reasoning...

Leonard

 

I really don't know any other way to do it other than powering up the
set. Ringing a flyback is not going to reveal a cracked case arcing
problem. What I have done is make 2 test transistors, one with a
damper, one without out of old obsolete TV outputs such as old 2SC1172
metal case outputs, and old outputs from TVs like the GE PC chassis.
Use the original metal mounting plate from an old set that had a socket
like a mid 80s Zenith. Run a foot or two of wiring to the socket. Now
when an output transistor is shorted, after doing all the pre power up
checks, instead of replacing the transistor, I simply run the wiring
from the test transistor to the board and solder. Turn the set on with
a HV probe that has min/max memory in the anode then quickly unplug and
see if HV normal. If arcing happens and the test transistor is shorted,
takes 30 seconds to replace it with another junk TV output. All free.
Works for me. Saves some work, and I've found these old transistors
pretty much will work any TV for a test anyway.

 

Leonard: Unless you have an absolute failsafe way of testing the LOPT
without sacrificing a transistor we will continue to do it our way. Ringing
will tell us if indeed there is a fault in one of the transformers windings,
or maybe a shorted diode in the rectifier portion. However, without dynamic
testing we will not find the cracked casing unless there is the brown ooze
already coming out of it. Even with these nice Sencore 325's that we are now
using. I like Ron's idea for bench testing however in the field it may
become a bit cumbersome.

 

I never said that there was a failsafe test. I simply question the routine
use of output transistors as test devices. We test LOPTs by ringing and
with the Sencore drive test which is simply a H pulse at 30vpp applied to
the primary. Get 500-700 vdc out and it is likely good. Rarely do we find
a bad one that passes both tests. Sometimes a bad one will cause
intermittent failures or overheating, but these are rare. I paid $200 for a
used VA62 on Ebay and the flyback test and ringing tests alone have made it
a worthwhile investment.

It just seems wasteful to me to consider output transistors to be
sacrificial test devices. It seems dangerous as well, since you never know
what else you will take out when it fails. Well, in the case of an AA1
Sony, you do actually...

Leonard

 

its a lot cheaper than the hi-pot test equipment you would need...

 

maybe we can improve on ron's idea....

get a really large test transistor and add some extra zener protection
and current limiting so that if there is a arc, the test transistor
will survive.


Mark

 

What about the power supply?

Leonard

 

Knowing the history and specific brands tendency to kill HOT
instantly, had no time to react. That how instant: Push (power
button)CLICK-POW. Either a fuse blows instantly or switching power
supply goes Eeeeeeee!

In generic monitors if I see shorted HOT and checked the driver
circuit if it has electrolytic cap, otherwise JUNK. Cost of flyback
is too costly in repairing out of warranty monitors.

In RCA, Citizen and some other models, when I see a blown HOT
depending on chassis, I usually replace both flyback and HOT and they
work. A failed cap (plastic ones) is extemely rare in many TV except
those few that had small blue disc or had to replace many caps in like
in CTC169. It is all based on experience to make the judgement call.

Cheers, Wizard