Predicta 17" HV question
 

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:

http://antiqueradio.org/art/temp/Phi...T_Pin5_W14.jpg

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

what does W13 look like?, and what does Pin 6 of the 6CG7 show for voltage and wave form?

Phil,
I would check to verify that the 6DQ6 cathode (pin 8) is making contact to ground, by measuring to the tube pin itself. The readings you're getting suggest the cathode might be 'floating'.

Carefully check for bad connections on the PC board at the 6CG7 and 6DQ6 tube socket pins.

Thanks, I will check those things. Waveform W13 looked like this:

http://antiqueradio.org/art/temp/W13.jpg

I think the scope was not locking well on the signal, but you get the idea. Much the same as what appears at pin 5 of the HOT.

Phil Nelson

Pin 6 measures 188V (schematic says 195V). Here's the waveform:

http://antiqueradio.org/art/temp/Pin_6_6CG7.jpg

Phil Nelson

I would rebuild the couplate. I do have a fly if needed.

with the screen voltage that high (no current draw) and no negative on the control grid, the HOT should be screaming to get out of the TV. I would have to guess the HOT is dead or as mentioned earlier, the cathode is not on ground. I would be careful if the tube is dead as a new one may overcurrent violently without drive.

Before rebuilding the couplate, i would verify that the cathode>ground connection is good on the HO tube. If still no go, sub with a known good tube (a 'BQ6 would do for the test).
Once the tube is conducting, the G1 drive `might` assume the correct waveform.

The 6DQ6 is making good ground contact on pin 8. I wrapped a thin wire around that pin and reinserted the tube. My ohmmeter shows it's connecting to ground. I grounded that little wire to the chassis and briefly powered up the set, and got the same old symptom.

I borrowed another 6DQ6 from my Philco Miss America and briefly tried it out. Same symptom: no HV.

Before attacking the couplate, I'll take out the chassis and re-clean the holes in the 6DQ6 and 6CG7 sockets and verify that they are connecting to the right spots.

If I do rebuild the couplate, what about voltage rating for those little caps? I have 500V dipped mica caps in the right values. I have some 1KV and 2KV rated caps, but they are ceramic and much larger.

I'm tempted to try replacing the couplate from above, since every time you pull and reinstall that board, you increase the chance of creating new problems.

Phil Nelson

do you have a analyst? would be a good set to try it out on, although it sure seems like it must be the couplate. I had ground issues, but it was the damper tube filament IIRC.

I do have a 1077B TV Analyst. Do you mean using it to provide a horizontal grid drive signal? I haven't tried using it that way. Would you pull the 6CG7 and connect the signal lead to the 6DQ6 HOT grid?

Phil Nelson

There is also a plate drive on that unit that you can connect to the plate cap of the HO Tube which can confirm the fly and other circuits downstream of it. without the need to worry about properly connecting the grid test signal.

I pulled the 6CG7 multivibrator tube and connected the TV Analyst's grid drive to the HOT grid, and got a bright raster.

The Analyst makes a nice clean signal:

http://antiqueradio.org/art/temp/BKT...riveSignal.jpg

A label on the PC board points to the HOT grid:

http://antiqueradio.org/art/temp/Hor...tGridLabel.jpg

The HV is back, yay!

http://antiqueradio.org/art/temp/RasterBack.jpg

The raster is nice and bright with full horizontal deflection. The vertical locks, but the maximum height is what you see in the photo.

Perhaps replacing the K7 couplate will fix the HV problem, after all. Not sure what to think about the height problem. When I played it before recapping, the height was lacking somewhat, but not as much as now. I would have expected recapping to improve the height, if anything. With my luck, the vertical integrator (K4 couplate -- ugh) is also flaky.

Phil Nelson

The height might not be a "real" problem. The vertical circuit relies on the +400V boost voltage, which might not be where it needs to be using a substituted horizontal drive signal.

Check the boost voltage before ripping into the vertical circuit. It is quite possible that you will not get proper boost until you fix the HOT drive problem.