RCA TV blowing horizontal output

[glen65]

rca atc113 chassis

I am working with an RCA model 32F530T TV.
The chassis number is ATC113-CD4.
The unit makes a constant chirp noise when plugged in.
I checked it out and found that the horizontal output
transistor was shorted, its a 2SC5148 with no internal
damper diode. I checked the surrounding components
and solder joints all appear to check ok. I replaced the
transistor but when plugged the unit in it popped the
transistor and started chirping again. I was searching
through the forum and found a thread that talked about
a coil in series with the Base lead of the HOT, and how
the glue that they used on it could be at the root of the
problem.
https://news.elektroda.pl/rca-ctc203ca6-squeals-t879.html

I pulled the coil out of the set and attached a pic of it to
this thread.
Is the glue being referring to in the old thread the red blobs
on top? Or is it not present? Or is there something
completely different that I should be looking at?

Any help appreciated
Thanks

 

[eddwhatley]

rca blowing output

I am uncertain of the "glue" aspect of that inductor, as I have seen it used for SEVERAL chassis back in time prior to the ones mentioned. It was always a problem of one of the the coil wires having developed a floating connection at its solder connection joint. As for the "glue" aspect, I am glad that you submitted the photo as I might have been fooled into thinking that the manner of construcion was completely different. . . . . instead, it is as I have always seen it, with the only "glue" aspect to me seeming to be the two drops that are on the beginning and ends of the winding. I haven't a unit to inspect in front of me now, but would suspect the "dots" to be a hard setting epoxy rather than any hot glue or quasi-hard setting vinyl formulations.The purpose, being the locking down of the ends of the wire turns. Assuredly being put there by the coil manufacturer, and not after the installation of the component on the chassis.


I had personally always thought that problem being with those supplied coils having a build up of oxide upon their tinned leads and not initially taking a good and full solder adhesion to them at flow soldering time.

(Not a problem in Aerospace / NASA construction procurements, as they have a "use before" expiration date on tinned lead devices, and most are usually hermetically sealed in inert gas filled packaging for further assurance.)

In that manner, with less area than normal would being available, with time and the high frequency and high current passages involved, there would be cavitation away of the contact area even more, until the joint is eroded and floats, thus the loss of drive to the HOT, and its C/E punch through.

My usual procedure is to completely pull the inductor...which YOU have already done... and scrape the lead ends clean and then liberally flux up the leads and re-tin then, deflux / clean with MEK or Acetone, and then clean and re-tin the PCB pads that the coil was mounted into. Then an installation of the coil and reflow soldering with a just a wee bit larger than normal appliaction of solder to the joint, and a final de fluxing clean up of the board.
No further problems experienced with that unit.


BTW. . . .That HOT transistor does have an internal damping diode. . . .

Additionally there have also problems with "floating solder joints" on the low side of the HOZ deflection yoke components at Diodes DL03 and DL08 as well as poly caps CL12 and CL21.



73's. . . .Edd