Predicta 17" HV question

[Don Lindsly]

Some things to try:

Bridge the 100 mfd cathode bypass.
Rotate v lin while measuring cathode, pin 9. It should go to 0 at extreme end
Bridge electrolytic on the 275 volt source
Recheck cap values that were replaced. C 40 will have most effect on height.
Height and lin pots and the .0015 (C43) were common failure points.
The small pix accounts for the low neg voltage on pin 7 since the source is the output plate.
Make sure pin 7 measures 68 k to ground.

[Phil Nelson]

Thanks for the ideas. Here's what I found.

Bridge the 100 mfd cathode bypass.

Tried, no change.

Rotate v lin while measuring cathode, pin 9. It should go to 0 at extreme end.

If you mean going to zero volts, it never goes below about 20 volts. Resistance of the v lin pot varies from about 10 to 770 ohms. I wonder if this is a replacement? The manual calls for a 1500-ohm pot with a stop at 180 ohms. This looks like a (roughly) 750-ohm pot and someone added a 950-ohm/1W resistor in series to make a max of about 1700 ohms. So, with the resistor the pot is varying between about 960-1700 ohms. Shorting that resistor didn't change the height noticeably.

Bridge electrolytic on the 275 volt source.

Tried, no change. The 275V source measures 273V.

Recheck cap values that were replaced. C 40 will have most effect on height.

Rechecked, all caps look OK. I replaced C43 (.0015/1KV) by wiring two .0033/630V in series. In Sams, C44 is listed as .033, with .022 as an alternate value. I temporarily subbed a .022; no change.

I noticed one difference: R55 (between pin 8 and vert hold pot) should be 33K and I installed 39K for some reason. (Perhaps 39K is what I found there.) Not sure how important that is. I measure 8.6V on that pin; Sams calls for 7.5V and the Philco manual calls for 10V.

Make sure pin 7 measures 68 k to ground.

It measures 67.5 K.

Regards,

Phil Nelson

[Don Lindsly]

Phil:

Agree that nothing seems out of range. 33K vs 39K will only affect vert hold range. It locks OK.

The vertical output transformer could have some shorted primary. Check resistance with a DC meter, not a digital one. Some digital meters are fooled by high inductances because they use an AC voltage for resistance.

Make sure the 100K 1W is not burned. It may be necessary to sub a VOT for test. You can try driving the output stage with about 20 VAC, 60Hz to see if it will sweep at all. It will be distorted, but it will give an indication.

[Phil Nelson]

I hauled out my trusty old analog Triplett meter. The primary measures lower, if anything -- 400 ohms, give or take a couple. This is measuring from pin 1 of the VOT to test point 33 (275V source) in Sams.

The 100K/1W resistor (R60) is new and it measures 100K.

I just remembered that my BK 1077B Analyst can provide a vertical plate drive signal. (I need to use that thing more often!) Tomorrow, I'll pull the VOT and see what happens with that signal.

Regards,

Phil Nelson

[old_coot88]

Originally Posted by Phil Nelson

...I just remembered that my BK 1077B Analyst can provide a vertical plate drive signal.

Dang, i had forgotten that too, having last used an Analyst many aeons ago. I was fixing to suggest what Don L just suggested.. drive the output stage with AC.

[Phil Nelson]

OK, the vertical plate drive signal from the 1077B looks like this:



I removed the 6DR7 vertical output tube and injected the Analyst's vertical plate drive signal on pin 1. Here, the 1077B's Amplitude control is set about midway:



The height's about the same as before, with linearity very compressed toward the bottom.

Cranking up the Amplitude control nearly fills the screen:



Phil Nelson

[wa2ise]

Phil, can your Analyst generate a signal to inject on the grid of the vertical output tube? Reason I ask, is that you still could have shorted turns on the vertical output transformer, and turing up the Analyst output would cover it up, sort of speak. The sam's says "do not Measure" so you don't know what the amplitude of the plate waveform should be. But the grid drive is shown. But it's inverted compared to the plate's, as you'd expect. If your analyist can make a grid waveform, dial in the amplitude, and disconnect the coupling cap C42 from the vert osc circuit and inject it via that cap to the output triode grid there. And see if the height is good.

[DaveWM]

just a long shot, what about a defect in R53 (thermistor in yoke).

[Don Lindsly]

Test the bias divider:
1. Turn down brightness
2. Remove 6DR7
3. Measure grid, pin 3

It should be roughly half of the 6DQ6 grid bias or about 22 volts. That saves unsoldering some parts.

[DaveWM]

one more thing, if the lin pot has been hacked up, make sure the cathode bypass cap (100uf electro) is NOT being cut of of the circuit when the lin pot is turned to min, esp since that 950 fixed resistor is in there. Worst case you have a 950 unbypassed cathode resistor with the lin pot turned in such a way as to take the cap completely out of the circuit. IIRC there is something different between how the schmatic is drawn and how the lin pot and cap are actually done, but I may just be remembering that wrong. If it was me I would for grins just put a 500 ohm fixed resistor and the cap in there and see how that works (disconnecting the hacked up lin pot/resistor combo), as is sure seem like its a gain issue on that tube, assuming of course the grid bias and drive are correct.

[Phil Nelson]

With the 6DR7 removed, I measure about 0V on pin 3. (It starts out higher when the set is cold and then sinks to zero, never going negative.) I will slide out the chassis and inspect that voltage divider.

Phil Nelson

[old_coot88]

Wunner if there could be a carbon track in the tube socket itself lugging down pin 6.

[DaveWM]

don't try to use the scope to check DC voltages to sams when wave forms are present, the wave forms are avged out in a meter and the sams takes that into account.

[DaveWM]

hmmmm, I search on predicta K5 and found a construction article, a 150k resistor is used where the sams (and you) have a 68k now. Do you have the old K5 around to check the pin 3 and 4 for resistance.

[DaveWM]

just checked mine, 150k pin 7 to ground of the 6DR7