Another RCA CT-100 discovery B8001599

[Penthode]

Spent much of yesterday determined to solve the AGC problem. AGC was wonky:picture would appear about a minute late and if you advanced the AGC control and returned it, you would enter picture overload and it would not recover unless you detuned.

Checked the waveforms (courtesy of the Sams notes) fror AGC keyer in and keyer plate and was correct.

I had changed the paper capacitors and checking resistors, the 4.7 Mohm resistor (used for agc delay) was open. After replacing the resistor, the AGC system slowly oscillated between overload and positive video.

As it is a closed loop system, tried checking tubes. After about four hours, it occurred to me to check the AGC line for wiring. Recall the IF section was badly rusted due the mouse inhabitants. I finally discovered a little choke feeding AGC voltage to the 2nd IF stage open! It was affected by the mouse urine and had earlier tested okay after I cleaned it and reinstalled it. The corroded leads broke of as I removed it a second time. So I performed an autopy on it and in the process found it was simply a winding of a single layer of AWG 44 wire. I took a 1 Mohm 1/2w resistor and made replacement holding the resistor in a variable speed Ryobi power drill.

Installed the choke and AGC worked properly and as it should.

There was a happy side benefit to all this: I checked again the IF response due to the added choke. Went through the full sweep alignment in an hour as outlined in the RCA instructions. Recall from my earlier postI had to make addition tweaks to stages to minimize a bump in the response around the video carrier? Wekl that annoying bump disappeared after the choke fix. The RCA instructions said you only need to slightly tweak T109, T110 and T111 (third fourth and fifth IF stage bifilar transformers) to get the proper response. Sure enough, the final oberall sweep was very close and as RCA prescribed, only those three transformers needed minor tweaks to get a good response!

I have included pictures of the chokes and the set as it is now performing.