HV module for use with 7JP4?

[polyphase]

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..

[Tom Albrecht]

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