Hello all,

I also hang out on the antiqueradios forum but thought I'd post a thread here and let you guys jump in too! I very recently acquired a really nice 1950 Philco 50-T702 7 inch TV in the wood cabinet. This set uses the 7JP4 CRT with electrostatic deflection. When I bought it the TV had been recapped but the individual who had done the work made several errors which resulted in a nice bright raster, but no signal going through the set. After fixing all the goofs it's playing quite nicely now all in all, with one exception: vertical linearity is not good.

To the point: the upper 1/4 of the screen is rather compressed and the image becomes increasingly elongated as you progress down to the bottom of the screen. I did replace the ceramic capacitors to the deflection plates with mylars which generally should help linearity, but it's still off. I'm thinking next step is to go thru and check / replace all of the resistors in the vertical sweep, but meantime does anyone else have any thoughts on where to proceed with this?

I know with these electrostatic sets linearity is frequently problematic, but i'm fussy and like to get stuff as good as they can "be got"!!

Thanks,
-George-

Good to see that you made it! :)

(By the way, he bought the set from me.....but I wasn't the re-capper ;) )

IIRC increasing the capacitance (IIRC by about double) of the high voltage vertical coupling caps is a cure for vertical linearity issues in electrostatic sets. If the vertical stage ceramics you replaced are good and of the right value, try tacking them in parallel with the mylars and see if linearity improves any.

IIRC increasing the capacitance (IIRC by about double) of the high voltage vertical coupling caps is a cure for vertical linearity issues in electrostatic sets. If the vertical stage ceramics you replaced are good and of the right value, try tacking them in parallel with the mylars and see if linearity improves any.

Well I tried increasing the capacitance as noted above to .01mF and no difference. The linearity is just horrible, either way. Heres a picture of a test pattern to give you an idea..........

-George-

Since the coupling caps didn't help, the still-squashed raster shows the vert sawtooth having a slow rise-time. I would consult the schematic to see what R or C values might be adjusted to give a faster rise time without changing the sweep frequency.

I had linearity troubles with both my electrostatic sets. The problem in both cases was not one thing. When you increased the coupling cap values to 0.01 did you make
a QUANTITATIVE test of linearity ... make a screen tracing one way and compare to
the other? If it improves, leave it with higher cap. I went all the way to 0.02 in my Pilot TV-37, which has the same value centering resistors. That done, there were still other problems with off-value parts. With all fixed, its got perfect linearity.

Use your scope. Luckily I have scope probes with 750meg and 2GOhm resistances that
work great at 60 Hz. At 15kHZ I need to use a capacitive divider. Check every spot
in the circuit. It could also be problems with the resistive focus chain. Note the
uusual place the vert amp B+ comes from.

Well this morning I had a bit of time to start looking things over, and I did find that someone had eliminated resistor R96 (150K) which runs from the height control to ground with a wire - in other words going from the pot straight to ground. Replacing that resistor improved things slightly, however there's more work to do. I'm noting that when the set first comes on cold linearity is fairly decent but gets worse over some minutes as components warm up.

So firstly I'm going to go thru every resistor in the vertical section and replace anything more than 10% off. If that doesn't cure it, next I'll leet it warm up and go shoot each resistor with freeze spray to see if I can nail down any offenders and see where it goes from there...............

-George-

George, as I read this thread and didn't see if you had trying substituting tubes to shoot this trouble. If you have other tubes, you might try substituting whatever tubes are in the vertical oscillator and output section. Who knows? As you know, even if a tube tests good, the real "tube tester" is the circuit in which the tube is used.

Just a thought...

Thats a good thought. The vert amp is a 7F7 which is also used for the sync separator. If I don't have an extra one and I doubt if I do, I can swap these and see if theres a difference. Same for the vertical oscillator which is a 12AU7. Its also used for the video detector and also I think for the horizontal oscillator/amp. Worth a shot!
-George-

What comes to mind is that the problem seems to be "temperature" dependent. Yes, discrete components can be temperature sensitive, but tubes can also. For instance, output tubes (50L6GT,25L6GTcomes to mind) can exhibit grid emission when they get hot.

OK, so last evening I went thru and replaced the 7F7 tubes as well as the 12AU7 tubes. No difference. So I'm back to focusing on the resistors. Over the next day or two I'll be going thru and checking all of them and replacing any more than 10% out of spec, and if that doesn't find the issue, I'll go in with the freeze spray next.

I am, of course, presuming and assuming that Philco would never have put a circuit into production that passed such poor linearity....... !!!!

-George-

Actually, since the problem's already shown to be temperature-related, I would do the freez-spray first, before shotgunning. That'd give you better clarity as to how/why the drifty part affected linearity. There might even be more than one component temp-sensitive.

Yeah, I think thats the game plan. Gonna initially focus my attention to the vertical amp first and work back. This set appears to have had quite a bit of use in its day, so I'm sure there other stuff going on too :D

-George-

I did a lot of simulation on this circuit.
EVERYTHING is vitally dependent on the vertical transformer. The amp
does not integrate.

Scope the oscillator FIRST, get it making a sawtooth. Don't just randomly
try new parts (except, if you have one, a genuine Philco exact replacement
transformer.)

OK Dudes,
I think I'm onto it: everything (cold) checked out A-OK on my ohmmeter, so I powered it up with my test pattern fed into the set: came up pretty damned nice linearity and within about 8-10 minutes it was squashing out. So on a hunch, I went after R97 and R98 which feed to the input side of the vertical amplifier (7F7 tube) which affect grid 1 and cathode 1 respectively and sprayed both with freeze. Within 5 or 6 seconds the vertical linearity returned to normal and within a few minutes or heating back up again it began to gradually squash down again. My guts telling me it is the 2.2 meg resistor feeding grid 1.... but they are both kinda buried deep in a tough spot, so if I'm going in, I'm gettin' them both.

Will swap them out tomorrow sometime and report back, but i'm feeling fairly confident here......... Stay tuned!
-George-

Well guys I think I'm there. I found that the root of the vertical linearity issue was a mic capacitor (C73) which couple the first and second sections of the 7F7 vertical amplifier. After the set would warm up, and even with the new resistors there was still some flattening at the top, so when i freeze sprayed that cap the picture came right back up. This is a 100pF capacitor. Soldering in a replacement did the trick. All and all, Im really pleased with the linearity. After getting the chassis back into the cabinet I'll make final adjustments of the height and width, i typically set it 10% oversweep to compensate for voltage variations.

The image is actually a lot sharper than you see in my picture, but I had to set the picture like this to get a good shot of the test pattern. Its quite nice to watch, actually...... After around an hour I do notice the picture gets a bit darker which is typical of this set, but I have plenty of adjustment room on the brightness so i can just tweak it up a bit and I'm fine.

-George-

You might want to check ham-fests for a line isolation type Voltage Regulating Transformer (2 of my 3 came from one day at a ham-fest). I run my regular use sets on them. Those transformers put out a constant 117V or 120V (depends on vintage) line voltage as long as the input is somewhere between 80-160V. They are great at squelching line transients too...Ever had a fridge of the central air system kick on and cause a TV to 'bloom'?...A VRT will end that problem. Some neighbor sharing our pole transformer runs something that, when on, causes that blooming effect several times a minute so a VRT is essential here.

Good to see you tracked down the vertical linearity issue.
The picture is much better.:yes:
Ed

Yeah another reminder to keep your mind open to suspecting mica caps when doing our work. They do fail in increasing numbers now. Remember when you virtually never saw one fail???
George

Update for your guys: all done and all buttoned up. I encountered a slightly "rare" issue with the vertical sweep which delayed finishing this. The brand new 100pF capacitor I installed which corrected the linearity problem suddenly began to fail! After about 10 minutes the vertical height would begin jiggling erratically downward off and on. Replacing it with another new capacitor corrected the issue and after a good 8 hours of play time I think she's reliable and ready to close up. Here is what it looks like now!

-George-

Change all the Micas in the verticle and horizontal sections. These 7jp4 crts if bumped the deflection places can bend.

Yeah another reminder to keep your mind open to suspecting mica caps when doing our work. They do fail in increasing numbers now. Remember when you virtually never saw one fail???
George

Yup they do fail and they all failed in all of my 7 inch electrostatic sets. Mega decades old Micas do fail, age and maybe other components like caps or resistors being out of tolorence helped those Micas fail so next time there is a verticle or horizontal problem and can't seem to find out why even if a slight problem, suspect those little devils. Each set of mine had small verticle problems that could not be adjusted out nor could I find what was going on and I was even told don't touch the Micas they never go bad. So now any set that has Micas in the horiz and vert circuit are now on the list to be changed with all the others as standard. However you will be told here on videokarma do not change any of these Micas in the IF or RF sections unless you are absolutely sure they are bad and I agree 100% . Sometimes things won't be the same or it may need a complete alignment .

Fer sure. I e reached the point whenever I experience issues in the sweep sections I go for the mica caps. I suspect voltage levels have a lot to do with it.

BTW: you may remember me, I am the guy that bought your 1946 FADA (RCA 630 clone) a few years back. I eventually replaced the 10BP4 on it with a NOS I came across but we used that sucker in our master bedroom regularly till last year when I scored a mint DuMont RA103 Chatham (doghouse)!

George

OH ok yes I remember you are from pa but that set had a 12lp4 and you changed it to the 12kpa without the ion trap. I see you finally got on here since when you had tried you had problems becoming a member because of something to do with your email. Well ok welcome to videokarma.

Very nice George.

Say George would you do us a favor? Could you post some nice pictures of the knobs from your TV? There might be a little confusion as to what they look like. I've placed an ad in the Classifieds here at VK and there was some discussion as to what are the right knobs. Also I'll have a good picture to post of what I'm actually looking for.

Much appreciated.

OH ok yes I remember you are from pa but that set had a 12lp4 and you changed it to the 12kpa without the ion trap. I see you finally got on here since when you had tried you had problems becoming a member because of something to do with your email. Well ok welcome to videokarma.

Hee hee! Yes. Turns out the fellow I acquired this 50T-702 is the site administrator and he figured out the technical problem and got it fixed so I could join! :banana:

And yes, you're right. It was the 12" CRT and not the 10" and I upgraded to 12KP4 which was a HUGE improvement! That was a really very nice set (the 630's are very well engineered and executed), although I think the DuMont's are a touch even better.

-George-

Say George would you do us a favor? Could you post some nice pictures of the knobs from your TV? There might be a little confusion as to what they look like. I've placed an ad in the Classifieds here at VK and there was some discussion as to what are the right knobs. Also I'll have a good picture to post of what I'm actually looking for.

Much appreciated.

I am actually out in Ohio until late Sunday when we return home to PA. When I get back I'll take some pix of the knobs for this set and post them on this thread. So my apologies and I ask all be be patient! :yes:

George

I am actually out in Ohio until late Sunday when we return home to PA. When I get back I'll take some pix of the knobs for this set and post them on this thread. So my apologies and I ask all be be patient! :yes:

George

No apologies required. When you get to it. Much appreciated.

Hello All,

Per your request, I have posted pictures of the Philco 50T-702 original knobs on another separate thread here so everyone looking to see them will easily find them!

-GEorge-

Thought I would post an update regarding the apparent design flaw in the 50T-701 & 702 chassies which causes the HV to drop off after the set has been playing and gets hot. In my set, I have noticed that the picture will gradually get darker and after about 2 hrs I will notice the picture size bloom slightly, indicating that the HV is dropping off. Although the set is still watchable, I decided to think about preventing the condition from occurring without replacing the entire HV section.

I installed a 3 inch cooling fan to the back cover of the HV cage by drilling a 3" hole in the rear cover and mounting the fan on the cover directly across from the transformer which is apparently the offending component. The fan is so wired that it turns on when you turn on the set and off when you shut it off. After more than two hours, there is no - zippo - visible drop off in HV. There is absolutely no dimming of the picture, or any blooming of the picture, nor any loss of focus or sharpness. The airflow is pushed into the HV cage where it keeps the transformer and other components cool, and then flow up and out through the ventilation holes at the top of the cage, whereupon it reflects off the inside top of the cabinet and back down again so it helps keep all of the chassis top components cool.

The fan i used is a generic 3" (3100rpm) 117 VAC fan I picked up at Radio Shack. The fan is audible, but its not bad nad not annoying or for that matter much noticeable. The big benefit is that the TV is now a nice, reliable steady daily player. This seems to be a very viable solution to a well known problem!

-George-

Nicely done!

Nice that it worked, but I really suggest avoiding cutting a hole in the original back and HV box :sigh:

Well that would have been great but unfortunately there was no space to mount the fan without doing so. The alternative was to eliminate the rear cover altogether which I felt was a worse option. That all said the set plays well and you can actually watch a whole two hour program on it and that's with the original HV in it. (Assuming of course eyestrain over watching a big, luxurious 7 inch screen doesn't s in first) !!

You could have made a new back from some masonite and preserved the original. I'm not trying to give you a hard time just some friendly advice. Many collectors prefer originality.