CTC203 "sacrificial" output transistors

on

sizes

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.

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

Ringing

windings,

dynamic

now

"Leonard Caillouet" wrote in news:q4kke.3409$5p.934@lakeread02:

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

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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