The inevitable callback on RCA 17PT8072

[DavGoodlin]

I got this KCS109 set years ago for $10 and never had (still don't have) the Schematic If you have it, please PM me. I need to do it right this time.

Nov 16 2012 051.JPGNov 16 2012 052.JPG

I even have two Sams 392 and THIS folder is missing from both!!!
It must be RCA's only late 50s set with series filaments and doubler power supply with two seleniums.

A friend of mine turned a small room into a bar and I told him I had the perfect TV It worked OK for 3 years but is now showing the signs that I did not finish the job, barely starting it. The electrolytics must be replaced. The major symptom is the poor AGC and sync as it plays longer.

Since I had no schemo, I bluffed it by testing all with my EICO 950 and replacing only the obviously bad caps. Someone had repaired the power supply, changing the 3-section cap and subbing in silicon rectifiers, I had to trust and could not check this for OEM parts accuracy. The set did work, kind of, so I left it alone and just worked on the PC board, eliminating the bumblebees. I need the schematic to do the rest. It should be a hit this Christmas because it embarrassed me last year, rolling and tearing.

[Phil Nelson]

In addition to bumblebees, I think I see some "maroon drop" caps in there. They are brownish or maroon and look like modern orange drops except for the color.

Those are also candidates for replacement. My RCA CTC-7 had terrible sweep problems and other issues before I replaced them. It magically made a fine picture after all of the maroon drops had gone bye-bye.

Ditto, even more so, for any remaining paper caps. If the values are printed on the cases, you don't need a schematic to replace those. If you want this TV to work reliably for an audience, I would not even waste time testing those.

Phil Nelson

[DavGoodlin]

That is the way I used to work on vintage TV for years. I gave up on Radio Shack and once all the parts jobbers closed (except Moyers, 2 hours away), I assumed the sources for caps over 200 volts were gone. I did not know about places like Justradios so I tested all caps, replacing what was bad. I kept many late 60s RCA and Zenith chassis around for the orange and maroon drops.
Sorry to hear the maroons are suspect, they used to be a safe bet. If I ever get time to recap my CTC11, I will be felling pretty good!

I still need the schematic.

ETF does not have a KCS-109 in resources. ET circuit digests do not even cover it and my extensive RCA library is missing it as well.

[DavGoodlin]

Getting back on this project, I replaced all the Electrolytic and Paper caps.
I noticed a tube socket missing as I got a closer look..

KCS109A pcb 1 When I saw what makes the KCS109A special, a missing 4CB6 IF tube, it confirmed this set was sold in a strong-signal area, as a personal label on it had a Philadelphia address, one neighborhood away from the transmitters


KCS109A pcb 2 Has anyone seen this? I got the manufacturers' service manual and this "printed" is labeled "C257".

C257 "printed" instead of a 4CB6 like the KCS109C and D

[Electronic M]

Looks like they used a couple of adjacent parallel traces to form a capacitor to bridge the IF signal across the deleted tube...A very interesting approach!

It was not uncommon for manufacturers in the day on cheaper 'stripped down' models of sets (particularly stereo consoles) to delete sockets from PCBs and bypass signals and other electrical paths across the old socket pins....Though usually it involved soldered on wires or components, and or PCB traces strategically broken at the factory, rather than a alternate PCB foil pattern...

[old_coot88]

Originally Posted by Electronic M

Looks like they used a couple of adjacent parallel traces to form a capacitor to bridge the IF signal across the deleted tube...A very interesting approach!

Heh. Quite a 'gimmick'.

[init4fun]

Originally Posted by old_coot88

Heh. Quite a 'gimmick'.

Yes Sir , I am familiar with the "gimmick" capacitor from my tube FM radio work . It would usually appear as two wires lightly twisted together but of course not electrically connected together . An easy way to get a couple of picofarads without an actual component and somewhat adjustable by twisting tighter or loosening the twists . A neat way to accomplish a desired result

[Phil Nelson]

Not to go too far off topic, but speaking of unusual traces . . . when recapping my CTC-4, I noticed these two squiggly guys:



??

Maybe that's commonplace in PC boards, but I don't work with them often.

Phil Nelson

[kvflyer]

Originally Posted by Phil Nelson

Not to go too far off topic, but speaking of unusual traces . . . when recapping my CTC-4, I noticed these two squiggly guys:



??

Maybe that's commonplace in PC boards, but I don't work with them often.

Phil Nelson

I suspect it is an inductor?

[bandersen]

Could be a delay line

[Phil Nelson]

The luminance delay line is a thick cable-like component mounted above the chassis. The schematic shows two components (L62, L64), described as "filament choke," between the filaments of the IF tubes. I haven't tried to sort things on the foil side to determine whether these squiggles are indeed the chokes. Filament choke is a new concept to me -- maybe intended to filter AC ripple to keep it out of the video signal?

Phil Nelson

[dieseljeep]

I had a simular model that had a transformer power supply and a 5U4 rectifier. I lost interest in it, when I discovered, that someone hacked up the PC board, in some kind of a repair attempt.
Some of those RCA's were pretty impressive, buy miserable to repair.

[old_coot88]

Originally Posted by Phil Nelson

...The schematic shows two components (L62, L64), described as "filament choke," between the filaments of the IF tubes. I haven't tried to sort things on the foil side to determine whether these squiggles are indeed the chokes. Filament choke is a new concept to me -- maybe intended to filter AC ripple to keep it out of the video signal?

They're RF chokes intended to avert spurious coupling problems (ie., feedback) via the heaters.
Never heard it explained that way, but presumed that's what they were for.

[David Roper]

Delay line was my first guess too, then I realized the tight coil-like back-n-forth would probably have an inductance. Give 'em props for ingenuity.

[DavGoodlin]

Often, series heater sets have bypass caps at and small chokes between filaments on the tuner and IF tubes