I've got a Hallicrafters 505 (same chassis as T-54) which is fully restored and works very nicely... for about 5 minutes when you turn it on. Then the HV slowly drops over the next 5 minutes until the screen goes completely dark.

In the past, I've isolated the problem to the HV RF coil, and others have reported similar problems in some (but not all) other Hallicrafters T-54 and 505 sets. Seems to be similar to problems reported for 7" Philco sets. I've baked and recoated the coil, which had a good effect for a few weeks, but it seems they reabsorb moisture or whatever the problem is, and return to their previous bad state.

Another forum member found an extra HV coil from a Motorola (which doesn't show this kind of problem), but the circuit is sufficiently different that I can't get the new coil to oscillate. The Motorola circuit uses feedback from ring around the 1B3 to drive a 25L6 oscillator; the Hallicrafters uses a feedback winding on the HV coil with a 6C4 oscillator. I tried adding a feedback coil to the Motorola coil, but things seem to be simply too different for it to work. I tried both phases of the feedback coil, and with and without the series resonant circuit used in the Hallicrafters design. But it never worked.

So I'm interested in other options:

- Anyone know of an available solid state 5 kV module? I know others have used them in various electrostatic sets with success.

- Anyone have a suggestion on how to get a Motorola coil to work in a Hallicrafters?

- Anyone have a coil which is closer to what Hallicrafters used? I'm working on a Teletone TV149 right now for someone, and its coil seems quite similar to that of the Hallicrafters. Any spares out there? Any junker Hallicrafters chassis (hopefully with a coil that doesn't have the same sickness)?

All suggestions welcome!

Tom

Emco makes a HV module that can be used to replace the defective Philco HV supply but they are bloody expensive!

http://www.emcohighvoltage.com/

Tom, here's the page with the information on the replacement module.

http://www.myvintagetv.com/philco_hv.htm

Have you ever talked to John Folsom about possibly replicating this with his super-duper coil winding machine? Knowing nothing about coils, I don't know whether it would be easy or hard . . . .

Phil Nelson

Surplus Sales of Nebraska has some.

We used to cannibalize Conrac monitors for the HV supply module - and it was variable! Something like 9-17 KV with 24 volt input, and about 1.5" x 3" x 6" in size. Output was to a female plug attached to HV wire.

Cheers,

We used to cannibalize Conrac monitors for the HV supply module - and it was variable! Something like 9-17 KV with 24 volt input, and about 1.5" x 3" x 6" in size. Output was to a female plug attached to HV wire.



Back in 1979 I used to work for some toilet of a company that made boxes to convert video to hard copy on Xray film. Customers were hospitals and clinics that had CAT scanners, and they were used to using Xray film for image storage, so our box had a video monitor inside, and a lens and Xray film holder to expose. The monitors were ones intended for security camera work, or pay extra and get our box with a Conrac monitor. The QQA was multisync (long before VGA!), and used those HV modules. We swapped the CRT for a smaller but flat 5 incher. We considered using different HV modules, but the chief engineer said the excessive HV will just make for sharper spot size, and as it's inside our box, Xrays from the CRT was not an issue. Not enough to fog the film after going thru the lens it seems.

The boss there turned out to be the 2nd worst manager of my life, knocked from 1st place by some idiot boss I had at Bell Labs. :thumbsdn:

wa2ise, I remember seeing such a machine in the hospital that I worked at! Basically, you stuck a big piece of sheet film into a lower drawer that contained a crt. You were essentially making a 1:1 contact print directly onto the film. You watched the action on a live monitor that sat atop the exposure unit.

The Doc saw me reading through the manual in the break room, and commented with something like "oh yeah, that thing was a real waste of money."

Not sure if the machine used x-ray film or not, but at the time I was using x-ray film in my view cameras because I could get it from the hospital for cheap, so it certainly was sensitive to light well into the visible spectrum.

Maybe you could use a photocopier HV supply like this?

http://tesladownunder.com/HVsupplies.htm#HV%20supplies%20from%20a%20photocop ier

You might try this from Chuck A.

http://www.myvintagetv.com/philco_hv.htm

If someone out there has a Hallicrafters HV coil they would be willing to donate, I could take a look at it and see if my machine ca rewind it. I would have to unwind the coil to tet the turns counts for the windings. If the pi sections of the HV secondary are too thin, I might not be able to reproduce it. But I need a sample to play with to know.

Give me a PM or email [email protected] if you have a donor coil.

Don't have one to donate, but I believe this is the critter we're talking about.

Phil

http://antiqueradio.org/art/halli50513.jpg

I would bet that transformer is salvageable.

I fixed one from a 10" set and it operates at about 10kv, it just up and quit on me in the middle of a recap, the ohms were fine so I figured it was just leaking between the windings.

What I did to fix it:

1. Removed it from the chassis and washed it several times in a tub of hot soapy water, I used something called Ascot Purple Cleaner but 409 or similar would probably work as well.

The reason for failure was obvious, tons of brown murky goo came out of it, probably Cigarette Tar and dust since the set was missing the HV cover and had probably run uncovered for a long time.

2. Rinsed it thoroughly several times until it stopped foaming when gently blow dried with compressed air (be very careful not to blow it apart!)

3. Dried it in the Oven at 180 for about 30 mins.

4. Let it sit overnight in a dry warm place to get all the moisture out, then sprayed it generously with some of this---> Insulating Epoxy Coating (http://www.aervoe.com/paints_coatings/Insulating-Epoxy-Coating.html) I let it run like water to get inside the windings, repeated that two or three times to get a good thick coating, let it dry and gave it a try, it worked good as new!

That looks very similar to what's in the '49 Sylvania.

I haven't had trouble with mine yet, but the one in our family had two of those die with an open secondary. It was impossible to find where.

The secondary windings have a wire that goes from the top of one stack to the bottom of the other. In handling it or washing it one should take care not to break one of these.

John

I have access to a bunch of older laser printers at the Helpdesk where I work. One of them might have an HV power supply of the appropriate voltage. We're just going to recycle them anyways. PM me if interested.

Lots of good suggestions here -- thanks!

A little history on my coil: After I first restored my Hallicrafters 505, I noticed the HV fade problem. At that time, I found that baking the coil fixed it -- termporarily. After a week or two, it would reabsorb moisture and the problem came back.

To try to give it a more permanent fix, I washed it, baked it for a long time, and then coated with multiple coats of varnish. This kept it alive for a while, but after a month or so it was back to the same. This didn't really surprise me too much, since all polymer coatings (varnish, epoxy, etc.) are permeable to water. None of these kinds of coatings really seal out water.

After reading various related discussions here and on ARF, I've been trying to boost the drive circuit a bit and see if that can solve the problem. It already is doing pretty well right now -- I can run it for about an hour before the picture starts to bloom. However, it is very tricky to tell whether the improvement is due to my work, or just the fact that running it repeatedly over a number of days has the effect of baking the moisture out. So not clear how it will be a week or a month from now.

I also picked up a nice little muffin fan at Radio Shack which I will mount in the HV cage, since it is clear that air flow helps. With the lid on the HV cage, it fades faster than with the lid off.

What surprises me is that even after HV has faded, the coil is barely warm to the touch. It's certainly not as though the thing is really heating up.

I'll try a little more work on the circuitry, and also the fan and see where things stand. I'll also look into picking up one of the various HV modules mentioned above in case that is needed.

Tom

It'll be interesting to see if the fan helps.

The 6C4 seems a little whimpy to me for this application. Did you try another one?

John

I hope I'm not drifting too far off topic. I'm posting some info on the Sylvania RF HV supply, it's possible problems and solutions. Maybe there's some similarity to the Hallicrafter.

When you adjust the HV they have you tune to the high capacity side of the HV curve. It seems to help compensate for whatever changes happen as it warms up. I haven't been able to come up with any convincing ideas about why this works.

They started with a 6SN7 and switch to a 6Y6. I do know that trying various 6Y6's is important. Ones that test the same perform quite differently.

John

I once had a Meck 7" TV. (well, I think it was a Meck, it has been quite a while since I owned it). Its HV was stable, but just would not go above 4KV no matter what I tried. Full recap, of course. Like the Hallicrafters, it used a 6C4. I built a little board with two 7 pin sockets wired in parallel which plugged into the 6C4 socket. When 2 6C4s ran in parallel, the HV was a robust 5KV. While this is not the same as the fading HV problem, it might be worth a try, and is simple to implement.

Excellent suggestions. The Sylvania circuit is indeed very similar to that of the Hallicrafters. I'm certainly not averse to making an adaptor and using a bigger tube, if that's what it takes to use the original coil. If I can do the conversion without having to hack holes in things, etc., it's worth a try.

It'll be a few days before I can get back to the Hallicrafters, but I'll let you know how it turns out.

I think I tried several different 6C4s when I worked on this in the past, but I'll try it again to make sure that the fix isn't as simple as swapping the 6C4.

Tom

An update on how this problem has been solved: Changing the 6C4 had no effect (I was pretty sure I tried that in the past). Somewhat to my surprise, increasing the value of one of the bypass capacitor on the B+ feeding the HV supply helped a little. So did increasing the B+ supply voltage to the HV module. However, none of these steps was sufficient.

Somewhat to my surprise, adding a small muffin fan inside the HV box, and raising its lid 1/4" with some spacers for better ventilation to the outside world, has proven to be a definitive fix. After doing this, I was able to lower the B+ feeding the HV box to its original value. I left the bigger bypass cap in place, not sure whether it is really needed now or not.

Adding the muffin fan was done in a reversible manner, so although it's not a very "historically correct" fix, it's not as though any permanent change has been made.

So now the HV seems to be working just fine on this set.

It sure does have a finicky RF/tuner section. Another project to figure out one of these days...

Tom

adding a small muffin fan inside the HV box, and raising its lid 1/4" with some spacers for better ventilation to the outside world, has proven to be a definitive fix.
Interesting. Perhaps the problem would not have arisen if they'd used perforated metal for the HV cage, as in some TVs. With comparatively low HV, I'm guessing the radiation hazard is not huge in the first place.

Phil

The RF HV supply generates a considerable amount of RFI (radio frequency interference). You may note that with the lid off the HV enclosure, RFI shows up in the picture as a herringbone pattern. This is why all the 7" sets had their HV supplies in sealed metal enclosures. Sets with a flyback type HV supply generate less RFI, so can be enclosed in a perforated enclosure.

Well, Im glad you found a fix, IMHO it would have been a shame to put a SS module in this iconic set. To me, the symptoms you describe had the earmarks of capacitor failure, not inductor. Im sure you have checked these but IMO might be worth checking again. One possible culprit might be C71, the 4.7pf between plate and grid of the oscillator. This cap will heat up on its own due to the combination of high frequency and high voltage, and could start leaking if not fresh and rated at least 1kV. A leak here would bring down the HV in short order.

Finally, I once built a 500kV Tesla coil, so I know about this stuff..:)

Hi John,

I agree that the main purpose of the shielding is to keep herringbone interference out of the video. Interestingly, on this set, with the lid completely off (leaving the four sides intact) there is only a tinge of herringbone visible. With the lid on, but attached by spacers, there is no herringbone interference.

On other sets, with a one-piece shield, herringbone interference is quite severe with the shield not in place.

Hi polyphase,

I have not recently checked the 4.5 pF feedback capacitor, but nonetheless I think I've been able to show pretty definitively that the problem is the coil itself. For example, baking the coil and drying it out worked very well as a temporary cure for a number of days. When this was done, the capacitor was not baked. Others have also attributed the problem to the coil, not just for this Hallicrafters model, but also for some early Philcos with similar behavior.

Tom

The Hallicrafters have a well known history of having this coil issue. I would be very surprised if the 4.7pf cap was the cause, but a quick squirt of freeze spray would answer that question. The Philco 7" sets have a similar fading symptom, though the coil in that set operates at around 17Khz, and is layer wound, not a universal wound multi-pi construction like the RF HV coils, which operate at 100s of Khz.

I had an exact duplicate coil would for the Philco set, and much to my disappointment, it exhibited the same fading problem as the original coil. My mistake may have been in having the coil potted in varnish (enamel?), though I will never know for certain.