GE 9" under counter TV video in and out

Still might have been the HOT, you may now have just exposed a further fault. Bad capacitors can cause a HOT to blow, so can bad solder joints in the horizontal section.

and so can a bad integrated flyback!.. which is most of the time!

Since the old HOT still checked OK with an ohmmeter, that is with no emitter to collector short, I reinstalled it and the set played for about 1/2 hour before it again started its OFF and ON again recycling. With David's comment in mind, I have now removed the circuit board from its normal area and propped it up at a 90 degree angle so that I could attach an oscilloscope probe to the base electrode of the HOT. It appears to be the electrode which is driven whereas the emitter is at zero potential. I was hoping to see it going into linear mode. It has now being playing perfectly for about an hour. Since the circuit board has different mechanical stressed on it when positioned at 90 degrees, you may be correct James that this is a bad solder joint, but I must say I haven't found it as yet. I would think that if it is the flyback, Jamie, it wouldn't care that it was in a different position. But who knows?

Dan

Resolder the joints on the flyback. the weight of it sitting sideways is most likely baring down on a bad joint and keeping it going. It could be a top side joint that you can't see.

I have resoldered the bottom joints on the flyback at least twice. When the board was in the 90 degree position, I flexed the board in all directions to no avail. With the board in its normal position, I then pushed on the flyback in all directions so as to flex the board beneath it. The problem fails to appear.

Thanks for the input, Jamie. Dan

It only takes one joint to cause the problem, they can be really tough to track down.

Dan Dubosky ha escrito:

if it recurrs, could you post a video of this fault on youtube? that would help.

meanwhile, that HOT you bought could have been a fake. there are vast numbers around ,which fail quickly exactly like yours did.

After a short warm-up, the set still continues to perform as described. It is now often enough so that I was able to look at the waveform of the signal which drives the HOT. During the cycle to the OFF period, the waveform callapses and the HOT during this period of time is not being cut OFF. The waveform at the input of the driving transformer also collapses, but this may not be surprising since one of the B+ volltages is derived from the horizontal output transformer. In fact, any waveform with horizontal periodicity at many of the test points collapses. There is a rather high B+ voltage of over 160 volts which doesn't seem to be effected at all. As I said earlier, I do not have the schematic diagram so I don't know what circuit is supplied with what B+ source.

Perhaps as someone suggested it may be the horizontal output transformer, but if that is the problem, I cannot understand why spraying the HOT with a component coolent would seem to get rid of the problem at least temporarily.

I've gone over the circuit board with a jeweler's loupe in order to see if there are any suspicious solder joints. Yes, I know that is not 100% effective. Anything that looked even the sightest bit suspicious, I resoldered it.

As Sam Goldwasser states a problem with blowing HOTs may require substitution of components in order to solve it. I'm not sure how much more I am willing to invest in view of the age ot the set.

Thanks to all. Dan

Well you've come this far, gotta figure out what's going on now, if only for the curiosity.

Have you tried poking around with an insulated stick while the set is on in order to track down a bad connection? Could be a cracked trace as well, those can be hard to find. Is there an IC that supplies drive to the HOT? Perhaps the voltage to that is dropping?

Yes, James, I have poked and poked with an insulated stick of the type that I used to use for adjusting IF transformers. I have never been able to find anything that resulted in producing the problem. You are right that I am still intensely curious as to what the defect is, and I will probably continue to putter with the set in-between other projects.

Today, I found someone in a previous post who recommended a replacement for TP15. If you recall, this was listed by NTE as obsolete and no longer available. It is the transistor which has discolored the board due to heat. I bought an NTE 287 which was recommended as the replacement and installed it, but that has not resulted in any change. Tomorrow I will probably buy the Sam's Photofact Folder on this set just so I don't keep wandering around the board in the dark. I am fully aware of the fact that this may not help all that much if the defect is somewhere in the feedback loop involving the HOT, the output transformer, and the B+ supplied to previous stages. It sure would be nice to know what in the world I'm looking at on the board. Who knows ??? It might even help.

Dan

Success, at last!!! This is just to put an end to this thread of messages.

I finally bought the schematic diagram for this set and began to search for the problem. After the set was ON for a short while, the negative voltage spike at the base of the HOT would disappear. Sometimes it wasn't there even from the beginning. Accordingly, the HOT was not being shut OFF as rapidly as it should be, and the temperature of the HOT would rise to the point where it would shut down, and in some mysterious fashion cause the system control chip to turn the set OFF. The feedback loops in this set are numerous. At first I thought that the 100Mfd capacitor which differentiates the square wave to develop the spike was deffective, but replacing it did not solve the problem. I then resoldered the joints in this area particularly around RP21 and LP02, and this cured the disappearing spike and the turning OFF and ON.

So, James, you were right. It was a bad solder joint all along.

Thanks to all for your help. Dan