Philco 50-T702 7 Inch TV
 

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.

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-