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

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.