Scored #5 on list: Pilot TV-37

[dtvmcdonald]

Deleted: crossed in the mail

[dtvmcdonald]

I looked under the bottom. Its been worked on fairly recently.
There are four brown dipped caps and one orange dipped,
radials. If I recall correctly the orange ones are usually good
and the brown ones iffy. Is that correct?

The electrolytics appear all either original or quite old. I'm not going to
restuff the two above-chassis ones as there is plenty of
room under the chassis, at least not at first.

The general condition is best described as very very good, except for
the screws in the wood and one the wood edges which will need repaint.
The masonite looks like new.

Doug McDonald

[dtvmcdonald]

Having seen that the set had been worked on in what looks like late 60's
to mid 70s, I decided to try it on a variac and a 100 watt (quite too low wattage)
dim bulb. Turning it up over an hour and a half, at 45 volts (on the TV itself)
I saw heater glows. At 70 volts I saw, at max brightness, a nice bright
vertical line on the CRT . At this time I hooked up
a signal. Nothing. At 75 volts, still nothing. 80 volts, as I tuned around
(this is a continuous tuning set) Ch. 3 where my DTV box outputs,
I heard a very very dim very distorted sound. I suddenly realized that
this is an intercarrier set and that there is no AGC: "Contrast"
is a manual RF-IF gain control! I had it at minimum. Turning
it to max and tuning I got normal sound, albeit at several spots
on the dial . This is very good. Maybe at 80 volts you
don't even expect horizontal. In any case, there is enough hum
to worry that the caps are not all one could wish, so the next step
is replacing all the electrolytics and then, a few at a time, the papers.
This will take a while as I have to order them and the Zener clamp for
the CRT filament.

Doug McDonald

[dtvmcdonald]

I hope somebody sees this!

I checked the resistor values tonight. A vast number are high by 20-50%.

Question is ... should I replace ALL of these? Or should I
just change ones that look to be timing rather than as the rather common
voltage dividers? One obvious thing to note is that since many are attached
to the same terminals as capacitors, it would be easier to replace those ones at the same time. What is usually done?

Doug McDonald

[bandersen]

If it's high by over 20%, I'll typically replace it - especially if I'm replacing a capacitor on the same terminal. If the original tollerance is 10-20% and it's 20% high or less, I'll leave it alone.

[dtvmcdonald]

So that's what I propose, change high resistors if very off
or attached to changed capacitors, or if it is in a voltage divider or
timing circuit. This leaves out lots of resistors in the
RF-IF and heater chains.

But I noticed a bizarre circuit twist. The HV supply supplies
the CRT anode. That sucks some current. There is a voltage
divider chain that provides the DC voltage bias
for the deflection plates, which presumably take no DC current,
and the focus, which likely doesn't either. The focus control wiper
is bypassed to ground. But the divider chain continues on
to lower voltage .. but not ground. It terminates (for DC) in
the plate of the first vertical sweep output tube! The
total resistor chain is about 9 megs.

If the first vertical amp tube were to lose transconductance
a lot, the plate voltage would drift up to about 2 kV! I do
of course note that the plate resistors in the deflection circuits
are all in the megohm range, as this is electrostatic deflection
which should draw no current. Its not high power like in magnetic
deflection/flyback HV generation sets.

What would happen to a 12SN7 with 2 kV on the plate?
(And to its socket?)

Also ... if the bypass on the focus put wiper is too low the
huge vertical sweep AC voltage will get onto the focus
electrode. The plate resistor (for AC) of the sweep tube
is about 3 megohm, depending on the focus control setting.

Do people agree that this is bizarre? Why did they not simply
add a resistor to ground?

Doug McDonald

[dtvmcdonald]

Progress!

I redid all the electrolytic caps and the .1 uF across the AC line.
The latter was a paper that looks like a mica ... and it was already split in
half. This made restuffing it easy. On this set getting the
two chassis mount cans out is not easy so I just disconnected them
and put the new ones under the chassis. They fit easily.

A picture ... in sync, without touching the horizontal
or vertical holds. Of course, there has to be big problems ... in this
case, only 1/8 of the picture shows, its so off center.
And the HV is only 1.5 kV rather than 2.5 kV (I luckily have
two HV probes, a 30 and a 50 kV, both scope probes so I have
to use my scope.)

Next step: all paper caps and out of tolerance resistors in the HV
and sweep output tube areas.

[kvflyer]

Sounds good and I anxiously await the results.

[dtvmcdonald]

Today I recapped and partially reresistored the horizontal section,
the video output, and the DC restorer. This is a
delicate job due to some close fitting huge capacitors.

When I turned it on it still worked but the picture
was weak and noisy. But even with the contrast (actually
RF-IF gain) all the way up the sync still worked. 45 minutes
of probing later I found the problem: somehow the
RF terminals had broken wires: one lead from the balun
was broken, and the other lead from the TV to the terminal
strip was broken, thus only probably 0.2pF of capacitive coupling.
Fixing this resulted in a stable and quite nice picture.

The vertical centering is all the way to one side and
the linearity is bad. The vertical circuit is odd: its
a hybrid of the Sams and Riders schematics, and
'I'm pretty sure that is the problem, perhaps both problems.

Doug McDonald

[bandersen]

Originally Posted by dtvmcdonald

... It terminates (for DC) in
the plate of the first vertical sweep output tube![/B] The
total resistor chain is about 9 megs.

If the first vertical amp tube were to lose transconductance
a lot, the plate voltage would drift up to about 2 kV! I do
of course note that the plate resistors in the deflection circuits
are all in the megohm range, as this is electrostatic deflection
which should draw no current. Its not high power like in magnetic
deflection/flyback HV generation sets.

What would happen to a 12SN7 with 2 kV on the plate?
(And to its socket?)...

That's a typical design for electrostatic sets. The AC line voltage doubler doesn't produce a large enough B+ voltage to drive the CRT deflection plates so it's derived form the HV divider chain. Arcing in the vertical tube or socket does sometimes occur.

[dtvmcdonald]

Well, the recap, etc, is done. It works.

I found the vertical centering problem: a magnetic field. I had always had it
on the side ... when I put it bottom down it was the horizontal that was out
of whack (and its control has plenty of range).

But it still needs a repair. If the RF-IF gain control is set very low,
so that less than 1.5 or 2 volts comes out of
the video detector the sync system works fine, but the contrast is too dim.
If I get 3 or 4 volts the contrast is fine but there is a terrible hum in the
video. Higher and the sync clips.

What would cause this hum? It clearly comes from the RF and IF
circuits. I checked the power supplies and they are OK. I tried
adding 100 uF to each of the two 40 uF caps and it didn't change anything.

What could it be? Heater-cathode short? Plain 60Hz pickup?

Its not hum riding on the signal, its hum modulating the signal. The
modulation appears not only at the detector output but at the
RF output of the final IF amplifier.

I have not recapped or re-resistored the RF-IF part.

Any suggestions?
Does anybody know what the peak-to peak video should
be at the output of the detector in normal operation?
None of the data I have for this set give any scope traces.

Doug McDonald

[dtvmcdonald]

The hum problem was two bad 6AU6s in the IF chain. There still was hum
in the vertical sweep from inadequate filtering in the B- supply. The
original "100 uF" filter cap measured 160 uF, the new one, right at 100 uF,
so I paralleled it with another 100uF, and it is much improved.

Also ... this set appears to be a slight hybrid of the Rider's (1948) and
Sam's (1949) designs. In particular, the vertical sweep output feedback is
Rider's while the rest is mostly Sam's. Since the (dogbone ceramic) capacitors in this
circuit were very low (.001 uF rather than .0015) I tried the Sam's circuit
and it works much better. The previous owner replaced the 3000 kV .005 uF paper
vertical sweep with .0033 uF ceramics, which is too low. .005 uF is too low too,
so I'm ordering up 4 .01 uF 6 kV ceramics to get .02 uF. These are not cheap.

Other than the vertical sweep problems, it now generates a very nice picture.
I'm going to try an alignment using my proposed non-sweep method:
using Comcast digital as a source, a spectrum analyzer to look at the
output, and a nice synthesizer as marker generator. It may not need alignment
since it seems to work OK.

Doug McDonald

[Sandy G]

These l'il guys were touted as the CHEAPEST sets you could buy when they were new... Considering how common they still are 60-odd years later, they MUST have made a blue zillion of them ! I'd say that if they ran out of one part, they simply subbed whatever was available that would work-Even if it wasn't exactly "According to Hoyle" ie in the diagrams...And what WOULD work back in 1949, might not today because of component aging issues. Just my .02 worth...

[dtvmcdonald]

This job is almost done!

As to the vertical linearity problem, I've got it figured out.

My set was a hybrid of the Rider's (early) and Sam's (later)
schematics. It had the Rider's vertical linearity fix in it.

This is a feedback circuit with two 0.0015 uF capacitors and
a 1.2 meg resistor in the vertical. Rider's says to adjust the
resistor for best linearity. This didn't work in my set. I removed
the three parts and found out why: the capacitors were very low,
well below 0.001 uF. The idea is that this circuit has a time
constant that generates a nonlinearity in the vertical sweep feeding one of the two plates that cancels the nonlinearity caused by the too-low
value of the 0.005 uF coupling capacitors going to the CRT plates.
Of course this results in a one plate having a waveform different from the other.

I tried the Rider's feedback circuit, which is a simple 5.6 Meg resistor.
This made the nonlinearity worse ... but the two sweep signals
at the vertical output plates are now absolutely beautifully linear. So
I tried adding .005 3 kV caps to the coupling capacitors to the CRT,
adding up to 0.02 uF, and now its a perfectly linear sweep. I've ordered four
0.01 uF 6 kV caps ($4 each!) from Mouser and when they are installed I hopefully
will be finished.

If you work on one of these keep these facts in mind.

Doug McDonald

[dtvmcdonald]

Its done! (Well, I suppose the cabinet base could use some black paint).
It works nicely. I do note that at only 2500 volts the thing is bothered by the
earth's magnetic field, thus the tilted picture. The four biggest round
orange ceramic caps are the four .01 uF ones I used to get the vertical scan
to the picture tube. The electrolytics replacing the cans, which were not
restuffed, are at lower left. There are more of them than in the original
since I paralleled some since that's what I had. The IF frequency response
is good enough that if I look at the screen with a 10X magnifier I can
see the color information as moving bars in a color bar pattern, though not
as well as in my RCA 9T246. In fact they are visible in the non-Arness guy's face.

Also, I added one of those Surge Suppressors in the AC line. The zener gizmo
across the CRT nicely protected it, but some tubes, especially the 6AL5,
had bad transients. This helped, not perfect, but better. Also, the minus
supply warms up faster than the positive ... putting a small (-12v) inverse
voltage on the B+ power supply electrolytics for a few seconds. I presume that that's
harmless since it has to have been there originally, being determined by the
warm-up speed of the two rectifiers. A 1N4007 across the B+ could fix it.

Pictures attached
The "before" bottom has the black paper electrolytic insulator lying in front.

Doug McDonald