Predicta 17" HV question

[Phil Nelson]

Thanks, we may be getting somewhere. I traced the negative voltage from the 6DQ6 grid, along the path you described, and it all looks reasonable until I get to the junction of the 100-ohm resistor and pin 3, so possibly something is funky there -- a bad solder joint, cracked trace, or bad socket . . . . I'll investigate tomorrow when I'm feeling fresher.

I recently got the Philco service manual, which is better than Sams in some ways. For instance, it has a wonderful diagram of the main circuit board, with a list of what all of the perimeter connecting wires are connected to -- something I had laboriously drawn by hand in the past.

I scanned the Philco schematic (in two parts) in case anyone might find that helpful:

http://antiqueradio.org/art/Philco10L4xSchematic1.jpg
http://antiqueradio.org/art/Philco10L4xSchematic2.jpg

An advantage of using the Philco docs is that the part numbers (and other IDs) match those printed on the circuit board.

Regards,

Phil Nelson

[Phil Nelson]

I checked the voltage divider components, 6DR7 socket, and relevant connections. All seem fine. While looking at some other voltages, the height started to bounce irregularly, and eventually I realized that the 6DR7 tube (a new one!) was failing in front of my eyes.

With a fresh 6DR7 in place, I'm getting some bias voltage on pin 3, although still weak, only about -7 volts. Currently, I have -43V on the 6DG6 grid (pin 5), -17V at the junction of divider resistors R58/R59, and -7 volts on pin 3 of the 6DR7. The picture is very nice apart from the height problem, only with a very slight bright foldover at top or bottom depending on how you adjust vertical hold, but that doesn't seem remarkable considering how compressed it is.

I temporarily bridged different resistors across the 1.5-meg (R57) to see what would happen with greater bias on the 6DR7 pin 3. Even when that voltage increased to around -17V, there was no significant change in height or the operation of the height and v lin controls.

I disconnected the vertical linearity pot for rechecking and it measures nearly 800 ohms max and operates smoothly. The schematic calls for a 1.5K pot (disregarding stops). I got rid of the old series resistor and installed a 750-ohm/2W resistor, making its total resistance close to 1.5K. It doesn't have a stop, but I'll just avoid turning it all the way to minimum, and meanwhile it should operate within the needed range.

My 1077B Analyst does provide a vertical grid drive signal, so tomorrow I can try wa2ise's suggestion about lifting C42 and injecting the signal there.

I contacted Moyers and they have the vertical output transformer in stock if I need one.

Phil Nelson