Another RCA CT-100 discovery B8001599

[dtvmcdonald]

Originally Posted by etype2

Sounds like you have it nailed, post a few current screenshots.

No need to post more screen shots. I've posted plenty. The current ones
would be identical to the last ones.

That includes my avatar, which was taken on an earlier CRT. The original
file is bluish.

[Penthode]

Sorry for the silence. Other projects have been going and I wanted to think through some of the issues. New photos will follow soon.

If you recall the speaker was badly rusted. I took it apart cleaned it and reassembled it. It works fine except that it appears to have lost some of it's sensitiveness. I tested by substitution another speaker and suspect the problem is the pemanent magnet is not as strong as it was. I am looking for a local substitute.

The AGC overload latching problem: Recall with the RF modulator fed straight to the CT100, the RF/IF amplifiers would tend to latch in overload. The CT100 performed better following the suggestion to attenuate the RF signal. Studying the circuit, I have pondered the value and placement of the 4.7Mohm resistor. With six stages of ampligication, the video level at the 2nd detector is approximately 7 volts p-p. There is a lot of gain and the 4.7Mohm resistor was intended to positively add to the AGC line to no doubt improve S/N performance. My contention is that the major noise contributor would be the RF stage. So I wonder why the resistor is situated prior to 1R132 (560kohm)? Certainly moving it to the point indicated by the arrow will delay application of the AGC voltage until after the the IF stages have been applied sufficient negative bias to prevent overload.

2R176 (4.7Mohm) should be raised to at least 10Mohm or more if it is moved to prevent front end overload. Alternatively, removing the 4.7Mohm resistor altogether demonstrated that with full signal, the agc cicuit appeared to work better with no overload latching. I want to compare S/N in each situation.

I suppose this is less of an issue now that the analog broadcasts are no longer around. It will be a purely academic investigation.

The HV power supply wilting: when attempting to advance the brightness control results in peak white pictures blooming and going out of focus. I tried monitoring both Brightness and Focus voltages as the average brightness level rises. Both the nominal 5000volts focus and 19000volts second anode take a dive. This suggests the flyback supply is not up to the job of maintaing a constant voltage as the beam current demand increases. Is this normal or is there a fault which affects efficiency?

Other than that, I see quite a bit of pincushioning of the image.I would attribute this tithe geometry of the 15GP22: The display panel inside the tube is very flat hence the scanning radius will induce pincushioning. Not a fault. I only mention this as an observation.

[etype2]

Couple of thoughts.
I think modern signal feeds into your CT100 might cause overload compared to the old analog days of 1954 and perhaps the set was designed in this way.

Would like to see a photo of the pincushingion effect for comparison purposes.

[Tim]

The blooming and out of focus with high brightness happens on my set as well. I have heard it mentioned before by others and I noticed it on one of the sets at the ETF museum also, so it is not unique to your set.