Also crosses to Merit HVO-217. If I were you, Phil, I would look for a NOS replacement or a pull from some other predicta.

That looks bad, like a lot of Philco's from that era.

I'd try Moyers for one.

It might not hurt to melt all the old wax off and see what it looks like underneath but I'm betting on some turn to turn shorts, this is probably what was causing the drive line in the first place.

I had just finished removing the old wax and recoating it when John asked whether I had measured for continuity. It looked normal under the wax, without any obvious burned-looking holes on that side of the coil, but I suppose HV only needs the tiniest opening to escape.

I'll start with Moyers.

Thanks,

Phil Nelson

Out of curiosity, does anyone have a sense of what usually causes flybacks to fail?

I suppose overheating is never a good thing and it can readily lead to failure. But what's the typical scenario? A problem in some other circuit makes the flyback overheat and then croak? Or is there some internal factor (similar to old capacitors being imperfectly sealed against air/moisture) that means many of them are doomed to fail eventually, even if the rest of the TV works perfectly?

Just wondering . . . .

Phil Nelson

Besides too much current, likely age and insulation breakdown played a good role. That part of the circuit is a harsh environment for electrical components.

Keep an eye out for these Pomona Cathode Current Test Adaptors. They come with common cathode pins interrupted. Use an analog VOM to monitor current. I didn't have a clue what they were intended for until I started working on my first TV. :)

Kevin

Looks like a handy gadget. I'm often too lazy to unwire something to monitor current there unless I suspect there's a problem. I've also heard of using an HP millammeter (428B) with a clip-on probe that goes over a tube's cap lead, which sounds even easier. Of course, that would be yet another 50-lb instrument to find room for :)

Phil Nelson

No doubt. :D

Now that you mention it, I wonder how one of those small clamp on amp meters would work that I see them at the home center. I think some of those can measure AC or DC amps. You wouldn't catch the screen current on the plate cap wire, but it would be a very close reference.

Kevin

Then there's this thingy that I picked up at a hamfest two weeks ago. It has a tube socket adapter to measure HOT cathode current in addition to its other functions
http://i1216.photobucket.com/albums/...rson637003.jpg

I haven't tried using it yet. I'd like to find some documentation on it first. I'm curious to know how it checks flybacks.

-Clark

For sure the cheaper ones are just transformer coupled (the wire that it clamped over being 1 turn) so they would be AC only. The more expensive ones ($100-200) do have a "Hall effect" type of device and can measure DC. I have not seen one with better than 0.1 amp resolution...just not good enough.:no:

jr

jr, good point.

I think they are more aimed at electricians measuring up in the 10-100
amp range.

It would be pretty easy to make one of those Pomona dealies with a tube base and socket.

Kevin

Well I could indeed be wrong about the lower limit... there seem to be some "small clamp" units that advertise AC and DC capability down to 1 mA!
http://www.amazon.com/AC-Amp-Clamp-D...=pd_sbs_auto_1
not affiliated,
jr

Add: While poking around on the 'zon, I came across an interesting device that might be useful for detecting potential flyback trouble spots:
http://www.amazon.com/Raytek-MT6-Non...8418192&sr=8-7
Anybody here using something like that for TV work?
jr

Moyer had one in stock, so I ordered it.

Meanwhile, to satisfy my curiosity, I fixed the broken wire from the coil and gave the set a brief trial.

No sparks, and the picture had the same brightness as before. With a listening tube, however, I could hear faint ticking or sizzling noises seeming to come from the coil. Looking closely at the picture, I also saw very short, fleeting dashes (one scan line high) zipping horizontally across the screen. I suspect that the transformer does have internal shorts, as was suggested.

Earlier, the coil had taken several minutes to get hot enough to really misbehave. I didn't let it go that far tonight. While I wait for the new part, I'll power up the set just long enough to measure the HV level and the output tube's cathode current. If those are out of whack, I'd rather find out before I've installed a new flyback.

Phil Nelson

It'll be interesting to see what the before and after readings are. Interesting too will be to see if the new fly is same as the Predicta's. That is, the replacement consisting of only the primary windings and 'tire'. You have to reuse the whole frame and core and the paper spacers between the 'C' halves since they determine resonance.

As you have ordered a replacement, you could risk a few experimental fixes on the old one. I've used runny epoxy to fill in small cracks of the plastic surface of more modern flybacks. And there's a product called "Corona Dope" I think intended for the purpose of painting old flybacks. That the stuff, when dry, has a higher dielectric constant than air. And not allow arcs to happen. That it wicks itself into the flyback windings to fill air gaps. Presumably the original coatings and insulation has dried out and cracked, creating air gaps. This assumes that there are in fact no shorts.

Yes, I removed the old wax and recoated the coil with black "sensor safe" RTV, and then fixed the broken wire from the coil to the HOT cap. No sparks, no corona. It makes a great picture. Listening with a tube, I can hear faint ticking or sizzling from the coil.

Making some quick measurements, I got -45V on pin 5 of the HOT and 165V on pin 4. Neither of those seems alarming. Disconnected the cathode lead at pin 8, but my DMM wouldn't give a current reading there, for some reason.

I've used corona dope before, and it's good stuff. I didn't have enough left in the little bottle for this whole coil, though.

Time to set this aside until the new flyback arrives, I think.

Phil Nelson

http://antiqueradio.org/art/PhilcoF4...ticPartial.jpg