Another RCA CT-100 discovery B8001599

[Electronic M]

You mean the face mask or screen mask.

Shadow mask is an internal part of the CRT behind the phosphor screen used to prevent crosstalk between each phosphor and the 2 guns that shouldn't scan that color of phosphor.

[Penthode]

That is a funny slip on my part. I should have called it the mask or the CRT surround. I guess I had crts on my brain.

Thanks.

[etype2]

That is very nice! Wish your photos were better focused.

[Penthode]

Originally Posted by etype2

That is very nice! Wish your photos were better focused.

So do I! I find the posting of photos on the forum reduces resolution. I think the images may be transcoded from the 3MB original to 300kB. Perhaps the answer is to transcode to 300kB prior to posting?

Has anyone else come across this?

[etype2]

From my experience, I would agree resolution is reduced on this site. Your chassis photos look good. I upload photos from another server and post a link.

[Penthode]

It has been a couple of months since I worked on the CT100. Too many diversions made me put this aside for a couple of months. With the onset of winter, I have the chassis back on the bench and have begun the full IF alignment.

The sound IF discriminator alignment was very straightforward. I have just completed the final IF stage sweep alignment and the traps required a minor tweak. The response matches the service data exactly. I am documenting the entire process so there will be more to come on this.

I was proceeding to do the full IF sweep alignment when I made an interesting discovery. The procedure requires applying a -6.0 volts to the IF line and to connect to chassis ground the junction of 2R133 and 2R134. First I see no 2R133 in the v117 (6AN8) plate circuit so assume it was a typo: I see only a 2C133 connected to 2R134.

I am looking forward to examining the chroma and luma channel responses and posting what I see here.

[Penthode]

Run into a bit of a snag with the IF alignment.

Lined up the Audio IF and the last (6th) IF stage with no problem. The first IF stage (6U8) located on the tuner I cannot achieve the RCA published response curve. The coupling transformer between the 1st and 2nd IF stage must provide a wide bandpass. I have stagger tuned the primary and secondary in an attempt to secure the best response. the sweep alignment is supposed to include a 6MHz bandpass with a litle dip in the middle. the best I can provide is a hump in the middle with the low frequency end at 41.25MHz elevated above 45.75MHz.

Using the prescribed input head to couple to the mixer diode and from the testpoint through the detector block through T1, the signal is fed to the 6U8 in a common grid amplifier configuration which then is coupled to the grid of the 6U8 pentode section. The output is coupled to the tuner IF output via transformer T2. The detector block is connected to point T2 at the plate of the penthode section of the 6U8 on the diagram. The response curve is not affected by stray capacitance or moving the cables. It is a good consistent trace. I swapped the 6U8 (with NOS 6U8A's) and there is no change.

I am using for the sweep generator an HP8601a and the marker is it's companion HP8600a. The HP equipment is very precise and perfect for television sweep alignment. The swwep alignment of the detector stage matches exactly the picture in the notes.

With the response I achived so far with this coupling, the poor response adversely affects the subsequent sweep alignment from the mixer to the second IF stage as prescribed. Hence I have gone back to investigate further.

My plan next is to pull the tuner top and have a look at the transformer and components around it. The great thing is I may not even have to disconnect any connections as the complete tuner chassis holding the RF amplifier, mixer, oscillator and 1st IF stage is easily removed from the tuner box by removing a few screws.

Curious note: the rest of the curves depict low frequency to the left and high frequency to the right. You would normally get a reversal when the signal is hetrodyned or mixed with a local oscillator with frequency about the desired incoming. In this case no hetrodyning is done so why do the notes show a reversal? I checked Sams notes and they copied the RCA diagram exactly.

Whether you are familiar or not with this tuner, I would appreciate your comments.

[etype2]

Proper alignment will be a big improvement. I’m sorry, but our Westy has a different tuner, but was disassembled and cleaned. Sorry not much more constructive to say.

[Electronic M]

I haven't done enough TV alignments to offer constructive advice, but...

If you reach a point where you decide you need another CT-100 tuner for parts or to experiment with I somehow ended up with 3 spare CT-100 tuners in my parts stash...

[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?

[etype2]

Excellent work. I’ll go back and check my notes.

[old_tv_nut]

The CTC7 and later chassis used the X-Z demod with output matrix/amp/DC restorer stages. Any direct R-Y/B-Y parent would not apply. Plus, it was more stable with tube drift due to the final DC restoration.