Another RCA CT-100 discovery B8001599

[Penthode]

Funny I had to take apart and reassemble the video and sound IF strips on the main chassis and so far there has been no problem there. I took the tuner apart to clean it but did not examine it closely.

Thanks for the offer. I trust the coupling I can fix but it is good to know where to turn if the original unit turns out badly.

This is a real dog to align as it has so many stages.

[Penthode]

This evening I found the problem. It was a pretty obvious fault once I started poking around. A decoupling cap was open. I found the fault when I started moving the leads. The 1000pf capacitor was a chassis mount with the capacitor screwed into the chassis.

I could power the tuner portion on the bench while I took measurements. I have replaced the faulty cap with a silver mica cap and the response now matches the RCA notes. And there was a lot more gain!

It earlier worried me that as I changed the signal level the response shape changed. Now I can wind the input level up and down and the response remains the same!

Below are some photos and you can see from my previous post the response matches the published RCA notes exactly.

[Penthode]

Further examination of the tuner revealed a second 1000pF capacitor physically separated and open. This bypasses to ground the 6AF4 local oscillator plate return: if it is open, it would likely reduce the oscillator amplitude a frequency marginally.

The third chassis mount 1000pF capacitor looked solid. I noticed the color was green as opposed to the amber faulty ones. Perhaps it was a different batch.

Anyhow, to summarize, it has been essential to run though the RF/IF alignment for any set as the problems would snowball. Besides affecting the luma response by introducing smearing and ringing in the picture, it would drastically affect the I and Q channel responses I plan to examine when I am done the front end.

Proper RF/IF alignment clearly differentiates between the construction costs between late forties RCA sets. Comparing the 721TS which has three IF stages, with an 8T243 or the 8T270 with four IF stages, a proper alignment reveal extra picture detail on later sets. You see the extra detail on the resolution wedge which may be resolved to the center of the pattern and the 721TS ther esolution wedge extending only 2/3 the way down. In this color set which has the the added complication of not only color but the separate chroma difference channel paths.

So this exercise I feel shall be justified by the results i will later find.

[Penthode]

Been plodding away at the alignment today. The pages so far is as far as I have gotten so far with successful results.

I am going to back over the entire process and document it. So far I have seen multiple restorations to get the CT100's running but I do not recall seeing anyone doing a full RF/IF alignment?

Has anyone been through this?

[Electronic M]

Originally Posted by Penthode

Been plodding away at the alignment today. The pages so far is as far as I have gotten so far with successful results.

I am going to back over the entire process and document it. So far I have seen multiple restorations to get the CT100's running but I do not recall seeing anyone doing a full RF/IF alignment?

Has anyone been through this?

Talk to Nick Williams miniman82....He did a full alignment on both his CT-100 and 21CT55 about a decade ago using a B&K 415 and a scope IIRC.

[dtvmcdonald]

I did a full adjustment of my CT-100. This includes what I seem to remember
as being 88 adjustments, including only one tuner position. Yes, I turned
every knob or pot or coil.

The IF alignment was quite straightforward. I didn't at the time have an accurate marker generator so I tried two methods: zero beating with the
second harmonic of my frequency synthesizer, which only goes to 40 MHz,
or simply picking up stray signal on my AirSpy SDR and setting at the
correct place. The final result matched the spec perfectly. I did not use the Fig. 22 (RCA manual) test head (just coax with hard wired parts with short leads) nor
the Fig 23 IF test block, rather just a 100 mHz scope with 1 megohm input connected by clipping a tiny alligator clip at the end of 4 feet of RG58 to an insulated wire (tack soldered on if necessary) at the appropriate point.

For the RF, I set the oscillator with the Airspy and just tweeked the response
per spec. For my Ch. 39 UHF strip I kludged it using the Airspy and
the harmonic of my sweep generator.

Its all just a lot of adjustments. I still don't quite understand Zenith's snarky
remark "50 adjustments and they all make it worse" as a complement (i.e. it was done right!) or simply saying that too many.

[etype2]

Originally Posted by Penthode

Been plodding away at the alignment today. The pages so far is as far as I have gotten so far with successful results.

I am going to back over the entire process and document it. So far I have seen multiple restorations to get the CT100's running but I do not recall seeing anyone doing a full RF/IF alignment?

Has anyone been through this?

My Westinghouse H840CK15, not a clone of the CT-100, received a full alignment. Here are a few notes and photos of the scope wave forms. The bird photo was just taken November 23, 2022. The restoration was over three years ago. I had to take screenshots to post the 79 MB resolution of about 198 MP of the 15GP22’s 585 K dots. Mike Doyle who is an inactive member here, did the chassis restoration. He worked at Cal Tech developing the Hubble telescope and later helped maintain the Hale telescope at Palomar and taught students at the University.

https://visions4netjournal.com/wp-co...5CE820782.jpeg

https://visions4netjournal.com/wp-co...0D3B487BA.jpeg

https://visions4netjournal.com/wp-co...8C571161C1.png

https://visions4netjournal.com/wp-co...5876908EC7.png

[Penthode]

I have run through the full video and sound IF alignment. I have achieved that has been published.

Note the IF response is flat to 4.2MHz! The chroma response is supposed to be full I to 1.5MHz and Q to 500kHz. The corresponding chroma channels will have a flat frequency response to complement the IF response.

Later sets used the "haystack" response with the color subarrier (42.17MHz) at 50% on the response slope opposite the to the video carrier. This means the chroma channel has a complementary sloping response to provide a flat double sideband +/- 500 kHz bandwidth chroma response.

I am going to go over the entire procedure once again to document and see if I can make more linear the nyquist slope.

[Penthode]

I am impressed with the H840CK15. It has 4 IF stages and appears to demodulate the chroma on the R-Y and B-Y axes. This means early on it appeared to be the sensible way forward.

However I question the written statement that the placement of the color subcarrier at the 50% point is on the curve depicted.

Although it is stated that the the color subcarrier (42.17MHz) is at the 50% point on the curve, it looks to me 42.17MHz is at the 100% point. Perhaps teh 50% is instead 41.70MHz?