Another RCA CT-100 discovery B8001599

[Penthode]

Spent a bit of time on the set tonight. It is working well. Issues are fairly minor. Here is a rundown of the good vs bad points.

The Good:
1. Repeated the full IF sweep alignment. This was to ensure each preliminary stage of the IF alignment fell into place so that the final overall sweep alignment only required minor adjustments. Succeeded with only having to make minor tweaks to get the proper response. With the entire RF - IF alignment, chroma bandpass and demodulators complete and successful peaking coil replacements for the luma, I and Q channels, the picture is nice and sharp with no ringing.
2. Convergence very good. Better than I had expected with the electrostatic convergence.

The Bad:
1. Cannot get as good purity as I think it should. Could not get rid of a green contamination in the lower right raster. I removed the chassis to demagnetized as best I could with a degaussing coil to no avail. Was able to conceal it with compromises.
2. AGC problem. Although I fixed the open choke, the AGC still provides "bi-stable" operation. Set starts up okay. However, if you interrupt the signal briefly and reapply, the the AGC voltage drops to zero and the video overloads. By detuning with the fine tuning control and retuning the picture returns. The problem disappears if I remove the AGC delay 4.7Mohm resistor (between AGC line and B plus). Checked all components and wiring. (Recall I had to disassemble the IF chassis to remove the heavy rusting).
3. 15GP22 socket intermittent. The red dropped out this evening. Checked okay the voltages for the red gun at the tube socket. Flexed slightly the socket and the red returned. Suspect it is the Red Screen pin 4 connection which is iffy. I earlier cleaned a rebuilt the CRT socket on the set. The 20 pin plug on the tube it looks like a large chunk of the socket previously broke away and the previous set minder reconstructed it with an epoxy resin. Appears they used a mould for the resin and was well done. However it would be nice to put a good original socket on it. Does anyone know if there are any available?

I hooked up the DVD player and played the prerequisite Wizard of Oz...

[dtvmcdonald]

Quote: Spent a bit of time on the set tonight. It is working well. Issues are fairly minor. Here is a rundown of the good vs bad points.

comments interspersed:

The Good:
1.
2. Convergence very good. Better than I had expected with the electrostatic convergence.

IF you adjust adjustments 82-88 (of 88!) carefully you can get almost perfect convergence. These are the tilts and translations of the yoke.


The Bad:
1. Cannot get as good purity as I think it should.

My first CRT, now gassy, had great convergence. My current one has
a bit of red in two places that I just can't get rid of. Its really very minor,
even on a B&W picture.


2. AGC problem.

I had the exact same thing. Simple cure: since you are running on a
local modulator, signal is too strong. Install a pad right at the set terminals. I have a cheapie balun followed by a balanced 300 ohm pi-pad made from 1/2 watt resistors. That instantly cured this infuriating problem. Signal to noise is still essentially perfect. I recall that my uncle, in 1954, had the exact
same problem ... he lived 3/4 mile from the tower!
But it was already known, since his huge B&W set had the same pad necessary. So does my Hallicrafter's 7 inch set. Other sets work fine with the huge signal.

3. 15GP22 socket intermittent.

I had the same problem on a filament terminal. I got a replacement socket from the ETF but did not use it ... I just carefully bent the contact a tiny bit
in the original socket. The replacement would work, but its not the same constrution and would need some machine shop work to be reliable.

I hooked up the DVD player and played the prerequisite Wizard of Oz...

[Penthode]

Originally Posted by dtvmcdonald

2. AGC problem.

I had the exact same thing. Simple cure: since you are running on a
local modulator, signal is too strong. Install a pad right at the set terminals. I have a cheapie balun followed by a balanced 300 ohm pi-pad made from 1/2 watt resistors. That instantly cured this infuriating problem. Signal to noise is still essentially perfect. I recall that my uncle, in 1954, had the exact
same problem ...

That was it! I attenuated the RF by using a 6 way splitter and that fixed the problem. I remain curious why the AGC latches? Obviously the heavy overload which prevents the keyer from generating a negative voltage but is fine once recovered. I see that an additional 0.1 ufd following a 680kohm resistor feeds AGC beyond the IF line. This means upon the sudden application of higher than normal RF, the IF AGC line will react faster than the RF AGC because of the additional low pass filter. Meaning the RF amplifier may go into heavy overload and not recover.

Removing the 4.7 Mohm AGC delay resistor alleviates the problem at the expense of lowered S/N. (As if that is a problem nowadays). Perhaps lifting the 0.1 ufd would fix this?

I am not going to bother since simply attenuating the signal was the fix. Very appreciative of this tidbit of information.

And than to both of you for the 15GP22 socket tip.