HV module for use with 7JP4?

[Tom Albrecht]

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