Scored #5 on list: Pilot TV-37

[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

[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