Predicta 17" HV question

[Phil Nelson]

I'm working on a 17" tabletop Predicta, model H3412L, chassis 10L43. After recapping and replacing a number of off-tolerance resistors, the HV has disappeared. The set had some minor repair sometime in the last 30 years and it did make a picture of sorts when I got it.

I temporarily placed the service manual at this link:

http://antiqueradio.org/art/temp/Phi...Sams-466-1.pdf

All tubes test OK. B+ voltages are reasonable (except boost; see below.) I checked the resistances on all tube pins using the Sams chart and found nothing remarkable. All resistances look reasonable on the flyback transformer and related coils (L17, L18). All ground stakes on the board are making good contact.

At present, the voltages look bad on the 6DQ6A horizontal output tube. I measure around zero at the grid (pin 5) where it should be -45V. I also measure 296V on pin 4 where it should be around 175V. (The 400V boost voltage is also deficient, measuring around 235V where it should be 400V, but perhaps that's no surprise if the HOT isn't working.)

Below is a piece of the schematic showing the HV section. As you can see, I have replaced everything around the HOT except C51 and R69. (Items with a red check have been replaced.)

http://antiqueradio.org/art/temp/Phi...rSchematic.jpg

The waveform on pin 5 of the HOT is not what you'd expect (W14). It looks more like the W13 waveform observable on the upstream side of the K7 couplate:



Given all that, I'm inclined to suspect the K7 couplate that provides input to the HOT at pin 5.

Any other ideas? I'm not eager to pull the main PC board again to replace the K7 couplate if that's a wild goose chase.

Thanks!

Phil Nelson