[TechAssist] Sony CTV KV27EXR20 (Chassis ANU-2) H. O. T. Shorts after 20 Minutes.

I have a Sony Model KV27EXR20 Chassis ANU-2. The first time this set was in 
the shop it had a bad STRS6301 Regulator IC but the H. O. T. Was Ok. I checked 
and resoldered any questionable connections. The customer had the unit home 
about 2-weeks and it died again.
This time the 2SD1886 H. O. T. had shorted. I checked the horizontal circuit 
using our Sencore TVA62 and everything checked ok. I replaced the H. O. T. and 
it ran for 20 minutes and shorted again. 

This time I thoroughly rechecked the regulator circuit using a resistor load 
and everything checked perfectly. I thoroughly went over the chassis for any 
questionable connections. I resoldered everything is the base drive & collector 
circuits of the H. O. T. I replaced the 2SD1886 again. I monitored the base 
drive and the collector waveform using our X100 scope probe. Everything looked 
textbook perfect. The Collector pulse width was 14us, which is normal. I did 
start monitoring the temperature of the H. O. T. Using an Infrared Thermometer. 
The temperature started rising and the transistor shorted at about 165 
degrees. 

Does anyone have any ideas what might be causing problems? I am using RCA SK 
2SD1886 which I haven't had any problems in the past. I checked RepairWorld 
and someone had used a 2SC4729 as a replacement. Do I need to try an exact Sony 
replacement? Any thought would be greatly appreciated.

TIA,
Dale

Dale A. Lubow... HarrisTV50010@xxxxxxx ..515-232-1109..(Fax)515-232-3356
Harris TV & Appliance..........335 Lincoln 
Way.....Ames.....Iowa.....50010-3384


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