Phil, have you ohmed the flyback from HO plate cap the HV plate cap to see if it is still continuous? What is the Philco and cross part numbers for the flyback from Sams?

No continuity between the 1B3 rectifier cap and 6DQ6 HOT cap. Bad sign, I guess.

The Philco part number from Sams & the transformer frame is 32-8799-1. No equivalent numbers are listed in the other columns for Holldorson, etc.

Phil Nelson

Crosses to Thordarson Fly 265.

Kevin

Hmm. Looking more closely, I see that the wire from the coil to the HOT cap is actually two very thin wires, and one of them is broken where it comes out of the coil. From that broken end back to the 1B3 rectifier cap I measure 541 ohms. The schematic seems to indicate 530 ohms between those points.

I wonder if it's worth trying to fix that break. I'm not good at soldering hair-thin wires, but if I had to replace the flyback, there would be some of that, anyhow.

Phil Nelson

Was that 541 ohms between the break and the 1B3, or you mean 541 ohms from the break to the HOT? 541 is close enough to 530.

Worth a try. Oh, and be sure that the new glob of solder has no pointy ends, but is rounded. Pointy ends may produce corona.

IIRC, solder iron heat and molten solder should burn off the enamel coating on the wire, if so, no need to scrape it.

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

Hmm, maybe that fix worked after all. It's been playing continuously for hours with nary a blip. I turned off the set to feel the flyback and it's not even warm.

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

http://antiqueradio.org/art/PhilcoF4...edFlyback2.jpg

Vertical linearity still needs improvement, but that's about all I've got left to fret over.

Phil Nelson

Picture looks super sharp from here. :thmbsp:

Kevin

Looking Good! :yes:
Did the ticking / dashed lines quit?
jr

I recommend Aervoe Industries Insulating Epoxy Coating for coating Flybacks and HV coils, perhaps prior to coating them with Wax or Silicone.

It has a rating of 2100 volts per Mil of thickness, probably well in excess of the voltage difference between individual turns of a Flyback. Also it's watery thin so it can flow deep into the windings and seal them.
http://www.aervoe.com/paints_coating...y-Coating.html

Hello Phil ,
what great set
If I marry my sweatie I know she would like that tv

No dashed lines, no ticking or sizzling. I wheeled it into my office and played it for six or seven hours, sitting two feet away from me. I haven't stuck a listening tube back into the HV cage, but when you turn the volume down all the way, it's as silent as any other tube TV.

I had actually given up on this flyback and just tried the repair out of curiosity. Good practice for a time when I might want to repair something that's not readily available.

Since I've already ordered a new transformer, I may as well install that and keep this one as an emergency spare.

An interesting experiment, anyhow. And now I won't be so nervous about putting a new flyback in this TV. If this funky old thing didn't go up in flames, I don't know why a new one would (knock on wood).

If I ever do another one, I'll check out that Aervoe stuff.

Phil Nelson

I guess I don't understand how the broken wire would have cooked the core? :scratch2: Was it the HV zapping outward from that brooken connection that melted all that insulation? Looking at the meltdown picture, I'd of guessed it heated from the inside out.

If it were mine, I'd leave the repaired old part in there and keep torturing it. :D

Kevin

Wow. Thought I had seen just about everything service-wise, but this is the very first 'Lazarus flyback'. :D
Regarding that DMM not reading cathode current, you probly need an analog meter there. Digital meters tend to balk or go nuts on anything with complex or high level waveforms.

if its working fine why not leave it be and keep the new as a spare?, dont kick over a rock just to see what crawls out..

I have a fly in an old maggie that works fine and make yours look mint by comparison.

Yah, who knows what really happened with that flyback. I know that I saw a big blue arc and a bunch of new melted wax, and I know that the TV works well now. The rest is all guesswork.

I dug out my little old dime-store analog meter and it pegged the needle in the 250ma scale, so I guess the current is higher than the 100-140ma given in the schematic (assuming that this 25-year old Radio Shack special is accurate). Not sure where to go with that, since the grid voltages on the HOT look normal, as noted earlier.

http://antiqueradio.org/art/temp/Phi...MSchematic.jpg

I'm a little peeved with Philco for using cheap sockets on the HOT and damper. Their terminals are above the chassis.

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

When I began unsoldering the cathode lead, the terminal instantly snapped off. Altoid tins are made of better material. The pin socket is like a bent strip of tinfoil with holes -- not very practical to repair. I have wonderful new sockets in the parts chest, but they're a normal style with terminals on the bottom. The original sockets are wedged into tabbed holes in the chassis, so replacing a socket with a standard type is not a plug-n-play job. Meanwhile, the TV plays fine with a cathode lead soldered directly to the HOT pin, so I'm tempted to pretend that I never saw those oddball sockets.

Leaving the old fly in place is sounding better. Instead of looking for more trouble, I'll play the set for another long spell today and see how it holds up.

Regards,

Phil Nelson

yikes 240ma :thumbsdn:

you may want to try that with something else, Thats more than I get on most of the color sets that operate at 25kv.

A couple things I frequently do is add a cathode fuse (although that may or may not protect depending on how quickly it blows) and I add a B+ fuse right at the output from the diodes (if SS that is). The idea being it may save something in case of a runaway B+. I know its overkill but since parts are hard to come by, why not. the HOT cathode fuse makes for an easy check of currrent. Did you bypass the meter leads with a .1uf cap when reading the current?


but all that aside, if its operating and not too warm, then I would just leave it alone.

There aren't many "something else" choices around here. I have a 50-year old Triplett multimeter, a flea market item that seemed to work when I briefly tried it with voltage and resistance. If I can scrounge a pair of probes that fit, maybe it's worth a try.

I didn't know you're supposed to bypass the meter leads with a .1 uf cap <blush>. Do you mean connect the cap between the cathode and ground, and then touch the meter leads to the cap leads?

Phil Nelson

It's ugly and crude, but it worked. However, it would bother me too. If you can get to those sockets easily, I see what looks like the metal of the pin and terminal metal in the rectangular slot halfway between the actual pin socket hole and where the terminal sticks out. Using a smaller soldering iron tip, you might be able to melt some solder to one side of the metal I see exposed in that rectangular slot, and if you can, connect that cathode lead to that point. Slightly less ugly. :yes:

Or as you said, just pretend it's not ugly. :thmbsp: And I'd just tag and leave the new flyback on the shelf, so you can break it out if the old one does die. Not borrow trouble.

Oh, you could test your rat shack meter with a resistor and a source of DC (ohms law) to see what 120ma looks like on it. Though it's not DC on a cathode lead on a HOT. As mentioned above, use the 0.1uf bypass cap on that cathode, close to the tube.

Back in the 70's I was in college (Syracuse University class of 78) and in a EE lab we built and played with el cheapo kit Radio Shack meters. One of the exercises was to show how the AC measurements were done by the meter, and that its circuit assumed sine waves only. Give it a square wave and the indication would be wrong. Found it recently, but the plastic became all crumbly, on the plastic knob shaft. Useless except as a souvenir of college days. With a sticker from a box of Raisin Bran at the time with the Sun logo saying that it belongs to WA2ISE. :D

As for vertical linearity, you probably already replaced all the paper caps, so check the resistors. Dirt on the circuit board might cause some leakage.

Fixing the socket terminal was a good idea. I got a wire through the side of the socket, wrapped it around the little holed piece on the inside and the old stub on the outside, and soldered. Left a pigtail until I'm done messing around in that area. It won't survive caveman yanking, but the original terminal wasn't that sturdy, either :)

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

With a .1 uf cap in place, I measured 118ma cathode current with the Triplett meter. That's the middle of the 100ma-140ma range given in the schematic. I'm going to quit stressing over this, since the voltages on the HOT and general performance are also normal.

The vertical (drive?) line definitely becomes more visible if you turn the fine tuner CW to the point where focus becomes too sharp and you start to see ghosting contrast lines on the sides of objects. If you back off on the finer tuner, it basically goes away. I'm crossing that off the problem list, too.

Re the linearity, I previously replaced three resistors on the sweep board (R65, R69, R70) and that improved it somewhat. I just replaced three more (R66, R67, R68) and it's about the same. Please don't tell me it's the vertical integrator K1; I have built replacement integrators before, but they're kind of a pain.

Thanks,

Phil Nelson

Looks good. I'd probably just solder the other end of that pigtail to the terminal of the other tube socket (gently!), maybe creating a loop with the old wire to hold the pigtail to the terminal when you solder.

As far as vertical linearity goes, did you replace the vert out tube's cathode bypass cap? I had one go bad, and IIRC with it bad I couldn't get reasonable linearity.

So how's it holding up? :scratch2: Hoping its :thmbsp:

Maybe this model Philco is picky about Vertical tubes like our Barber Pole models and you have to go through a bunch of 'em to find some that perform well enough to correct that linearity issue.

It's holding up so well that I have gone on to the next project. Miss America plays all day long to entertain me in the shop until we decide where to put the set.

The linearity is just bad enough to be annoying. I have replaced practically every resistor in the vertical section, even the two on pins 9-10-11 of the yoke socket.

Swapping tubes is a good idea. I tried subbing the 12AU7 but don't have a 6CM6 (vertical out) on hand. I went on a treasure hunt and found many odd & wonderful tubes -- why do I have two dozen 6BH6s? -- but no 6CM6 or equivalent. May as well order one.

I was a little disappointed in the bass response and even thought of hotwiring a new tone control. Then I put the back cover on and, duh, the bass sounds fine.

Phil Nelson

I could swear I have a 6CM6 in the garage, I'll go look.

I wish I had a picture of the Zenith 16C21 I trashpicked as a teenager 31 years ago - it had an almost opaque coating of sepia cigarette "tar" almost 1/8" thick - took a whole roll of paper towels to wipe it off. That and replacing the 1K3 and it was as good as new - best low-VHF TV DX set I ever had!