RCA 21CT55 #8802897 is Dead

[Electronic M]

Originally Posted by dtvmcdonald

Tom: The "one resistor" means one less. A couple of values in the matrix are different. I doubt you will be able to see a difference. Only the relative noise in the colors and the (small) spurious color line caused by the different demod axes
will be different.

The difference you will see will be determined almost
entirely by differences between the types of peaking coils used
to replace the nasty white ones that self-corrode. That makes huge differences.
I spent hours and hours getting the right coils to get the sharpest and least ringy color bars. I tried a couple of the ones specified as replacements in the manual, and they were disastrously wrong.

I just use the peaking coils off digikey that Nick Miniman82 recommended many years ago. Such as this (this is one of several from that list): https://www.digikey.com/en/products/...-223-rc/969835

I've heard those coils criticized by more than one person, but the people doing the criticism don't site modern off the shelf parts as being part of their better solution...I don't have time to hunt out of production borderline unobtainium, I also don't have the equipment or skills to rewind much less wind new multi-layer coils or properly characterize them or verify performance outside of eyeballing the screen (I also don't have money or time to pick that up)...So until someone comes up with a better list of current production off the shelf parts I can buy off mouser or digikey I'm going to stick with what I know works decently in both of my sets and very soon a set I'm restoring for a customer.

[dtvmcdonald]

Originally Posted by Electronic M

I just use the peaking coils off digikey that Nick Miniman82 recommended many years ago. Such as this (this is one of several from that list): https://www.digikey.com/en/products/...-223-rc/969835
.

Those or the API Delavan (Spelling?) look-alikes were what I ended up with
in all cases. Only one, the 6.8 mH one in the Q, needed a value much different than the spec. I also found them to be the best ones in my HF radio 7 to 11 pole bandpass filters.

[Electronic M]

Originally Posted by dtvmcdonald

Those or the API Delavan (Spelling?) look-alikes were what I ended up with
in all cases. Only one, the 6.8 mH one in the Q, needed a value much different than the spec. I also found them to be the best ones in my HF radio 7 to 11 pole bandpass filters.

Interesting. Is there any significant difference between the API made ones that makes them better in some circuits or is it the same either way? Do you remember what value or part you used for the 6.8mH Q coil?

[Penthode]

Originally Posted by Electronic M

I just use the peaking coils off digikey that Nick Miniman82 recommended many years ago. Such as this (this is one of several from that list): https://www.digikey.com/en/products/...-223-rc/969835

I've heard those coils criticized by more than one person, but the people doing the criticism don't site modern off the shelf parts as being part of their better solution...I don't have time to hunt out of production borderline unobtainium, I also don't have the equipment or skills to rewind much less wind new multi-layer coils or properly characterize them or verify performance outside of eyeballing the screen (I also don't have money or time to pick that up)...So until someone comes up with a better list of current production off the shelf parts I can buy off mouser or digikey I'm going to stick with what I know works decently in both of my sets and very soon a set I'm restoring for a customer.

Just to let you know how I wound my peaking coils, I used a 1megohm 1watt resistor to wind the coil on. Cut out with scissors two cardboard disc from old business cards and slipped them over the resistor to hold the windings in place. Roughly calculated the number of turns to achieve needed inductance from an on line calculator (I Googled). Bought off Amazon appropriate gauge wire small spool. Made a spool holder from a coat hanger wire C clamped to a table. Fit the end of the resistor I was using to wind the coil in my variable speed power drill. Slowly (5 turns a second) ran the drill. (There was typically around 600 turns for a 1 mH peaking coils so winding that did not take too long).

When wound, I would apply the nail lacquer I bought at Walmart to hold the windings in place. I would then take the coil and measure the inductance using a series resistor, signal generator and oscilloscope to determine the 0.707 point when the inductive reactance equals the resistance. I would tend to be a bit generous with overwinding so that I could remove turns to hit my target inductance.

I did this for almost all of the 8 or so open peaking coils in my CT100. The coils in the I and Q chroma path were fairly critical in order to get a nice flat 1MHz response for the I channel and 500kHz for the Q channel. I frequency swept the luminance and chrominance channels and they matched closely the RCA published curves. And the net result was a an extremely nice sharp picture, free from ringing.

Looking at my bunch wound coils, it would be difficult to distinguish from a new commercial replacement. And my coils looked a heck of a lot better than the original ugly white blobs.

[Kirk]

I have used the Digi Key replacements for the failed white coils- the only difference is that I put them in Series to add up to the desired value using a L-C-R meter. I believe it is the difference in DC resistance that causes the ringing- not the coil itself. By increasing the DC resistance it minimized the undesirable ringing (you can see it on the scope) to be indistinguishable.

Great job winding your own!

Kirk

[Electronic M]

Originally Posted by Kirk

I have used the Digi Key replacements for the failed white coils- the only difference is that I put them in Series to add up to the desired value using a L-C-R meter. I believe it is the difference in DC resistance that causes the ringing- not the coil itself. By increasing the DC resistance it minimized the undesirable ringing (you can see it on the scope) to be indistinguishable.

Great job winding your own!

Kirk

Did you add series resistors or get a large assortment of coils and test them in series with each other until DC resistance and inductance matched target values?