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.

Thanks, I'll reserve judgment on the vertical until I replace the K7 couplate.

I discovered that there's (barely) enough room to sneak a soldering iron under the board and unsolder the couplate leads without pulling the board. Tomorrow, I'll make the replacement and give it a whirl.

Regards,

Phil Nelson

If you have to deal with tough bugs in both deflection circuits, and if they are both on the same (tough to remove) board then it might be a good idea to mod it so all the wires going to the board are numbered quick connects to facilitate checks during trouble shooting.

on mine I was able to get at a lot of the caps with a soldering GUN with the tip bent 90 degrees. The problems I had with the predictas were the bent wires (not through hole plated) so its easy to mess up the traces if you are not very careful. I used a lot of solder wick (coated with flux) to get everything cleaned out. Also low power on the dual heat gun. I dont recall if there was enough opening near the couplates for the bent gun tip trick.

I always thought the output from the plate of the horz osc was a sine wave, that was modified by a RC network (the couplate in this case) that made it look like the correct Hor output tube grid drive. Guess what I am getting at is to make sure you are getting the correct wave from from the horz osc. of course if the couplate is goofed up maybe that is not possible. It sure seems like the couplate has to be the problem if all the voltages are correct at the horz osc tube.

Well, I removed the couplate last night and I had to cut it up to remove it, since the access is very tight and a couple of the leads were bent. It's hard enough to remove one of those long, multiple-lead jobbies all in one piece when you have normal access.

As I recall, voltages on the 6CG7 were normal except the negative voltage on its grid was deficient or absent, similar to the HOT. Yesterday I did check that 6CG7 socket carefully, confirming that the resistance is normal on each pin and that every pin actually connects to the right spots. I also pushed things around and didn't see any evidence of an intermittent pin or broken trace.

I'm not confident that I can install a new couplate in this cramped space without messing up a trace underneath. Whether the TV will need further work at this stage is unknown. Adding quick disconnects on all of the board leads seems worth a try. Then I can swap the board in and out faster and less destructively. This local place has small Molex connectors:

http://shop.vetcosurplus.com/catalog...oducts_id=6845

I already connected three of the six ground stakes with a bus. If I complete the bus to connect all six of them, then I can use one Molex to connect the bus to a ground outside the board and perhaps reduce problems with bad board grounds.

Some of the leads from the board will need lengthening. I'm going to leave the existing wrapped/soldered connections alone and cut each lead an inch or so from the board to install the disconnect.

This will take a while. My hand-drawn diagram lists about three dozen leads, plus the six ground connections. After drawing that diagram, I got a copy of the Philco service manual, which has a nice diagram labeling everything on the board. I'll use their numbering scheme this time around.

Phil Nelson

I like the idea of leaving the WW alone and cutting about 1" away, then attaching the connectors, doing 1 at a time and if possible some way to color code or number. If I ever work on another predicta that requires any kind of tough dog work (like a busted socket) then that is how I would go. I like sermos connectors (aka anderson power poles) because I have used them in the past for RC airplane use (my other hobby).

Thanks for the connector suggestion :yes: I'm about to dive into my first predicta and have been searching around for connectors. I had no idea what they were called though and was coming up empty.

The Molex connectors are bigger than I expected, once I got one installed:

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

I bought 10 of those today, all the store had in stock. Maybe I'll go back there (or a hobby store) tomorrow and look for power pole connectors. I built an RC plane some years ago and do remember those connectors being smaller.

The Philco manual has a diagram that labels the connection points and tells what they connect to. This snippet shows WW connectors L38 and L46, etc., as well as ground connector G16:

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

I'm a little concerned about creating a mess of wires & connectors hanging over the board. The factory installation tucks the leads along and under the board edges. Things may get weird by the time you have three dozen connectors up above, leaving enough slack to unplug & plug things without accidentally snapping leads off the WW pins.

Edit: The powerpole connectors look about the same size as the Molex, 1-2/3 inch long when complete. They also appear to be the crimp-on type, which I like less than soldered connectors.

http://www.mouser.com/catalog/specsh...heet-42848.pdf

Phil Nelson

Well, I came home with a sack of powerpole connectors, which are even thicker than the Molex type.

http://antiqueradio.org/art/temp/Pow...Connectors.jpg

The parts store had nothing smaller. The only alternative was spade-type connectors, which are just too danged ugly.

I'm starting to wonder if this is a bad idea . . . .

Phil Nelson

maybe stagger cutting so they dont line up, I can see how its going to get kinda messy with a lot of connectors, considering the space limits. You need to do a lot of planning before making the 1st cut.

Yah, I'll stagger them for sure. For some of the WW pins, I may remove the old lead and install a new one made of more flexible multi-stranded wire. That will be the short pigtail to which the other long end connects.

Maybe I'll sit down and draw a diagram, as a way of thinking through the whole layout wire by wire.

Phil Nelson

yep. maybe even just tape them in place to see how its all going to work out. This looks like one of those deals where you really want it all figured out before starting. Of course if it does work out, I am sure there will be a lot of predicta owners that would follow your approach.

Would something like Molex female terminals pressed right onto the ends of each board pin work?

I just tried out a Molex female terminal, and the answer is yes. The size is right and it fits pretty securely.

The Molex terminal doesn't go down the full length of the pin, but I compared it to the factory video lead, which plugs onto the board the same way, and that connector only covers about half the pin, too. The video connector is simply a female terminal about the same size with a bit of shrink wrap around it.

The question is whether you would want to trust all three dozen leads to that sort of junction. Philco used that type for the video lead, so it's not completely loony. On the other hand, this board is dodgy under the best of circumstances.

You'd need to extend a number of the leads a bit, to accomodate this kind of connector. That would be less work than installing two-piece connectors all around, though.

If you did use push-on connectors all around, there would be some risk of accidentally dislodging one when replacing a tube, etc.

Phil Nelson

Ugh. The powerpole connectors look too big. I laid out several next to the terminals where they'll connect:

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

Picture each one of those with a 1-2 inch pigtail to the board and a longer lead trailing above or alongside. Seems like a real mess to me.

By loosening the ground stakes and dismounting the rear pots and range switch, I was able to lift the board a bit, perhaps enough to install the couplate. If that works, I'll reconnect the grounds and see what happens.

Phil Nelson

yep that would get messy oh well maybe someday somebody will find a micro connector that is no larger than the wire itself. Too bad, it would be nice to be able to easily separate the pcb for easy access.

Well, the Molex female terminals are about as small as anything can get and still allow you to solder onto it. A few of the board's WW pins would be easy, since the old leads have enough slack to simply attach a Molex terminal on the end. A number of leads are tight and would need lengthening. There are at least three WW pins with two wires attached.

Meanwhile, the new K7 couplate works fine, but the height is still deficient.

http://antiqueradio.org/art/temp/K7Replaced.jpg

The 400V boost voltage measures 400V on the money. On the 6DR7 vertical tube, the voltage at the oscillator grid is low (-.03V rather than -3.5V) and the plate is somewhat high (155V rather than 115V).

As with the horizontal section, caps & resistors around that tube have already been replaced, which leaves couplates K4 and K5, I guess.

Phil Nelson

I've been puttering around with the height problem, still seeking a solution. Height is 1/3 of the screen at most, and linearity is poor.

I wonder if the vertical output transformer could be funky?

I have subbed known-good 6DR7 vertical tubes, replaced the K4 and K5 vertical couplates, checked or replaced Rs and Cs around that tube. The pots for hold, height, and linearity are working and have the right resistance. The 400V boost voltage is normal. Here's a piece of the Sams schematic:

http://antiqueradio.org/art/PhilcoH3...lSchematic.jpg

Voltages are close except that pin 7 (grid) is only -0.5 volts rather than -3.5. Resistance of the vertical windings on the yoke looks normal.

On the vertical output transformer, the secondary measures 2.6 ohms (expected 2.5) but the primary measures 410 ohms where it should be 425 (or 430 in the Philco schematic).

Could a few shorted turns in the output transformer account for the lack of height? I feel like I've eliminated just about everything else. This problem existed before I did any restoration.

Phil Nelson
Phil's Old Radios
http://antiqueradio.org/index.html

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.

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

Tried, no change.

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.

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

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.

It measures 67.5 K.

Regards,

Phil Nelson

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.

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

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

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

http://antiqueradio.org/art/BK1077BV...riveSignal.jpg

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:

http://antiqueradio.org/art/PhilcoH3...tPlateSub1.jpg

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

Cranking up the Amplitude control nearly fills the screen:

http://antiqueradio.org/art/PhilcoH3...tPlateSub2.jpg

Phil Nelson

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.