My "New" CT-100 restoration

[old_tv_nut]

Nice! - but I hope you didn't buy that poncho <grin>

[WISCOJIM]

Originally Posted by dtvmcdonald

The good picture is with a 16 dB
amplifier with a 0.5 dB nf and the noisy one is direct.

I don't know if I missed it somewhere earlier, but could you give more information on the particular amplifier you are using? Brand, model, where to buy, sensitive to overload, pros & cons, etc.

Thanks.

.

[dtvmcdonald]

Its from Advance Receiver Research http://www.advancedreceiver.com/

It has to be a custom model. Its similar to P470VDG or P902VDG.
You just order a different center frequency. Price is $90 to $130.

Specs (at about ch 39)
NF 0.6 dB
Gain 18 dB
1 dB Comp dBm +12
1 dB BW 30 MHz
Type: GaAsFET
Power: 10-16 VDC

Mine's ordered for Ch. 44 so depending on how
it was tuned it is likely a little bit down in gain at Ch. 39.

For indoor use there are no cons except need for a wallwart. Outdoors,
it needs weather protection and power sent to it.

Doug

[WISCOJIM]

Thanks, Doug. I'll have to study those to see if they'll help me here.

.

[dtvmcdonald]

Originally Posted by WISCOJIM

Thanks, Doug. I'll have to study those to see if they'll help me here.

.

I should add it it really is better than the regular
ordinary broadband ones, even good ones. Ch. 44 is 68 miles from me over a hill, and without that preamp, its over the digital cliff. With it, 99.95%
reliable. (That's not with the bowtie ... it with a 20 element
single channel YAgi).

[dtvmcdonald]

Original circuit be damned ... I've ordered 20 200v Zeners for the focus.
So far, I have not changed the circuit functionality, as
I don't consider replacing a selenium with a silicon and series resistor
changed functionality. It really needs regulated focus.

[dtvmcdonald]

I installed 2800 volts worth of Zener regulating the focus this evening.
As an earlier restorer said, it works. Focus is perfect from
black to bloom.

[old_tv_nut]

Originally Posted by dtvmcdonald

I installed 2800 volts worth of Zener regulating the focus this evening.
As an earlier restorer said, it works. Focus is perfect from
black to bloom.

I'm curious - does it maintain focus if you vary the line voltage (so other things are varying while focus voltage is constant)?

[dtvmcdonald]

Originally Posted by old_tv_nut

I'm curious - does it maintain focus if you vary the line voltage (so other things are varying while focus voltage is constant)?

I didn't test. I will. I suspect it will, since the 19.5 kV is also
regulated and the convergence electrode runs off that. The small difference in screen voltages likely won't make any difference. My Variac can take the rather
large power need.

[dtvmcdonald]

I tested focus versus line voltage.

Yes, focus varies. +-5 volts is "OK" but needs adjustment.
+-10 volts is not OK and the height changes wildly
as the line voltage is changed. However color and overall
picture quality stay good.

[Electronic M]

If you have problems with AC mains voltage fluctuations then I would recommend looking in to obtaining a saturable reactor 120V isolation transformer. Those supply regulated 120V AC output as long as the input voltage is between ~85-155V.

[dtvmcdonald]

I used to have one of those transformers which I used on
my photo enlarger lamps. It weighed like 30 pounds but still
probably didn't have the power for a CT-100.

However, where I live the voltage is rock solid at about 120,
and I set the adjustable the B+ resistors for that voltage.

[dtvmcdonald]

One more niggling problem: horizontal ringing.

That is, ringing in the horizontal sweep waveform. I see it
in a scope waveform with a probe dangling near the yoke.
It looks to be about 175 kHz. This is very clear.

I see it as waviness in a diagonal line pattern across the screen,
barely visible. I see it as bright bars in a black background, barely
visible. I see it as scrunching up or spreading out in the bars
as a scene pans across the screen. This actually is clearer
than on the diagonal bars.

I've tried finding this problem in the archives here and
find two suggestions: replace the damper tube and the
270 pF capacitor in the yoke. I have no mica caps in a high enough
voltage ... would two 130 pF 6 kV ceramics in parallel do? I
don't know if our stock has the 6AU4GT.

Question: are there other things to check, especially ones
I can just "test" to see if they are good?

[dtvmcdonald]

I looked at the 270 pF capacitor in the yoke .... its the
biggest dogbone ceramic capacitor I have ever seen!

[dtvmcdonald]

I replaced the damper and it fixed the problem say 90%, which is
plenty good enough.

It turns out that the "shunt" pot in the tube tester was dirty
and it was giving wrong results for diodes.