RCA 21" triage CTC20 CTC16x CTC12

[dieseljeep]

Originally Posted by DavGoodlin

Very interesting that RCA did what Philco and Magnavox did. Reverse engineer the new rectangular chassis for a surplus of round tubes.

Actually it was the cabinets that they had a surplus of. I worked on one that had a real fancy cabinet like the prior year CTC16X.
Curtis-Mathis had a cheapie knockoff of the CTC20, but used a 6KM6 horiz out instead.

[Yamamaya42]

R202 & R203, RCA is famous for burning those up in color sets, esp if there is leaky caps.

But the 2 470 ohm resistors in the vert yoke section really makes no sense,
the vert output transformer CAN'T put out that much power to be doing that, something must be really wrong
shorted yoke? horz- vert? You don't even need vert at this point, horz seems to be powered direct from +385v (3) , you could yank the vert output, (v9), disconnect the vert drive wires from the yoke, leaving the horz only, pop the top off the HV cage, and first see if you can draw a healthy arc off the top of the FBT, also at this point, if you measure any voltage at the vert yoke connect points, you know there is a problem.
And... if you CAN get a good healthy arc from the FBT and are feeling bold, unhook the CRT anode and close the cage and see if you can get any DC, stick it in a mason jar or something!
Yes, I have been watching a lot of shango66 videos!

[DavGoodlin]

Shango_66 gets right to the issue with great commentary. So far as I can tell, this TV was "run to fail" and later found when someone moved at last from a home that was also new in '66, and forever seemed new to the first ones to live there.

First the volume control froze, with the shaft worn out from a tool used to operate it. The escutcheon was all scratched up from this awful practice. Good thing the tuner knob and channel cup was OK, that tool would have annihilated a tuner shaft. The cord was arced on the plug/recept as well. Tubes as often the case after 50 years, were from all over, Radio Shack - Raytheon (Japanese) - GE - Sylvania - RCA (new logo) GE "engineering test samples" , an Amperex 6GH8A made in France O.o, but that "new" Colorbrite CRT had me going until I hooked up some tester-source DC to it and saw the purple genie of death in the guns.

After the CRT lost vacuum, even that did not stop someone from using it for sound. HV was hitting a brick wall, likely overloading the B+ via those resistors. No wax stalactites under the fly is very odd and I cant figure that out, being common for RCA. The HOT cap wire was scorched from the heat yet the 6LQ6 tested good.

I will get back at it this evening, and post some pictures of the schematic copy with bad parts circled. I have another yoke and will try that with the vertical wires disconnected. I have not measured 6LQ6 cathode current, that remains to give a clue. I will jig up the CTC16XL next. CTC12 is last and best hope for watching Rudlolf so Im going for speed, just like old days in a shop where you did easy stuff first to get it back in the customers home and get paid. Not always in that order

Those "tough dog" customer sets similar to this CTC20 you had to take several breaks and clear your head to get at what the issue really is. When you nailed it finally, after a week of shake-down tactics, owner and everyone was interested in the play by play even the road guy.

I appreciate the help, thinking these were old hat for me. I must have forgot some basics of RCA sweep circuits.

[Yamamaya42]

I am really starting to wonder what the RCA engineers were smoking when they made the CTC-20...
From what I have seen, almost all leading up to it has had the FBT power taken from a shared point on the vert output transformer, and thus, if the yoke was not plugged in, HV was shut off as a safety feature. ( this is a guess by observation ).
But there seems to be no such thing in the 20, what were the thinking? :O

[DavGoodlin]

The mystery burned resistor in the cathode circuit of the regulator tube 6BK4 is R141 - 1000 ohm half watt. I replaced it with a 1 watt film resistor and like the 680 ohm 15 watt, does not run hot now. Not sure how R141 got burned in half. The tube is a good 6EL4A.

[Yamamaya42]

Well, if the dummies were running it with a bum CRT, who knows what kind of feedback it created, R141 is the path for the shunt regulator to try and clamp down the HV, if that let go, who knows what may have happened, especially if the HV rect let go, and flooded AC all over the place, it could have even got into the LV power supply, if they were really not watching things.

[DavGoodlin]

Here is the first of three RCA 21" color. The model is GH but the tag was half missing. The Sams lists possible models as GH608, GH618, GH626 or GH630, ending with a W for walnut finish. I have many extra knobs, least of the problems.

RCA Victor CTC 20 model GH6XXw.jpg

[DavGoodlin]

Originally Posted by Yamamaya42

Well, if the dummies were running it with a bum CRT, who knows what kind of feedback it created, R141 is the path for the shunt regulator to try and clamp down the HV, if that let go, who knows what may have happened, especially if the HV rect let go, and flooded AC all over the place, it could have even got into the LV power supply, if they were really not watching things.

The 2.7 ohm resistor in the HV cup was a half watt and looked original, schematic and Sams picture shows it as 1 watt. It drifted to 4.2 ohms so the 3A3C was not bad as would have seemed. I replaced it with a 1 watt

[Electronic M]

Originally Posted by DavGoodlin

Here is the first of three RCA 21" color. The model is GH but the tag was half missing. The Sams lists possible models as GH608, GH618, GH626 or GH630, ending with a W for walnut finish. I have many extra knobs, least of the problems.

Attachment 201869

That's a nice CTC20. Makes me miss my old one....If you get sick of it and it shows up at an ETF meet you could sell it to me fairly easily.

[Yamamaya42]

that looks almost exactly like my CTC-16XL :O

[DavGoodlin]

Originally Posted by Electronic M

That's a nice CTC20. Makes me miss my old one....If you get sick of it and it shows up at an ETF meet you could sell it to me fairly easily.

The one I saw in VoTech made an impression on me too, with the last-gen tube 2-PWB chassis no less. Two variations that look just like CTC16 AND CTC16x sets make a '20 necessary to round out any RCA collection.

If you have any tube-color (non-WID) by Motorola, its an even trade

[DavGoodlin]

Until I take my last swing at it tonight, an update. The Fly is good and I got HV last night. The cathode current on 6LQ6 is 126ma. yet it was 195 initially. HV is low, 22 kV and control is at max. Boost at 1040v is right on.

The grid voltage on HV reg tube 6BK4 is affected by two 1.5 Meg 1-watt carbons, R139 & R140. These tested 1.44 and 1.55 meg respectively but the schematic note on both the RCA and Sams notes these as a "matched pair". This is a regulator problem because the only voltage out of whack is 6BK4-G1 which goes from 365 to 375 over the HV control range. Weak/worn 6LQ6's can cause low HV,, yet voltages on it are spot-on, even G3 which is not grounded but instead feeds some h-blanking network Oh well whatever it takes to make it work, right?
CTC20 schematic.jpg

The CTC20 apparently needed its own yoke, instead of the CTC12 yoke and convergence parts Im using on the jig for all those 15s and 16s I worked on for others. Even the jig is showing a full raster, all colors present and a blue H-K short in its gun seems to have healed itself. The HV cage and shields go back on so I can flip the chassis on its side to put signal and test using Sencore VA-48.

As much as we note RCA's shortcomings on design, shops all made good money because so many were sold and looked pretty darn good so everyone wanted them fixed at some point. Even RCA factory service had a shop to service all those C-M inspired service agreements.

[DavGoodlin]

Originally Posted by Yamamaya42

I am really starting to wonder what the RCA engineers were smoking when they made the CTC-20...
From what I have seen, almost all leading up to it has had the FBT power taken from a shared point on the vert output transformer, and thus, if the yoke was not plugged in, HV was shut off as a safety feature. ( this is a guess by observation ).
But there seems to be no such thing in the 20, what were the thinking? :O

True for the rectangular sets using a yoke plug. None of the 21" round sets used an interlock in the B+ as those did, all were connected with the familiar Red and Blu for Horz and for vertical, Yellow and Yellow with a Black-stripe all individual leads.

Sams did not show (dotted line) that the CTC20C has the Yel/Blk lead grounded! I lit the 6DW4 damper tube up when mis-wired the yoke leads initially

[Yamamaya42]

Originally Posted by DavGoodlin

True for the rectangular sets using a yoke plug. None of the 21" round sets used an interlock in the B+ as those did, all were connected with the familiar Red and Blu for Horz and for vertical, Yellow and Yellow with a Black-stripe all individual leads.

Sams did not show (dotted line) that the CTC20C has the Yel/Blk lead grounded! I lit the 6DW4 damper tube up when mis-wired the yoke leads initially

Out of curiosity I have looked at almost all RCA CTC schematics, (sams), and noticed that almost all but the 20, power the horz from the vert output transformer, ( GRN tap ) , often from the far side of the centering pot, at first I thought this was a bit strange, cause it would feed vert pulses into the damper circuit and so on, then I realized they are using this as some sort of safety feature, and that most likely any pulses would get filtered out by the coils the have before the damper anyway, but what they are doing is, vertical centering (10 ohms) + vertical yoke winding (23 oms ish ) allows horz to energize, but remove it, and HV/ HORZ cuts out, to prevent screen burn.

[Yamamaya42]

Originally Posted by DavGoodlin

Until I take my last swing at it tonight, an update. The Fly is good and I got HV last night. The cathode current on 6LQ6 is 126ma. yet it was 195 initially. HV is low, 22 kV and control is at max. Boost at 1040v is right on.

The grid voltage on HV reg tube 6BK4 is affected by two 1.5 Meg 1-watt carbons, R139 & R140. These tested 1.44 and 1.55 meg respectively but the schematic note on both the RCA and Sams notes these as a "matched pair". This is a regulator problem because the only voltage out of whack is 6BK4-G1 which goes from 365 to 375 over the HV control range. Weak/worn 6LQ6's can cause low HV,, yet voltages on it are spot-on, even G3 which is not grounded but instead feeds some h-blanking network Oh well whatever it takes to make it work, right?
Attachment 201876

The CTC20 apparently needed its own yoke, instead of the CTC12 yoke and convergence parts Im using on the jig for all those 15s and 16s I worked on for others. Even the jig is showing a full raster, all colors present and a blue H-K short in its gun seems to have healed itself. The HV cage and shields go back on so I can flip the chassis on its side to put signal and test using Sencore VA-48.

As much as we note RCA's shortcomings on design, shops all made good money because so many were sold and looked pretty darn good so everyone wanted them fixed at some point. Even RCA factory service had a shop to service all those C-M inspired service agreements.

Odd I thought they were all interchangeable.
But if you look closely at the Sams info, is says for the CTC-20, 70 degree, Horz 12mh Vert 38mh, same data for the 16/xl
but the 15 is Horz 12.4mh Vert 40mh, so I guess one from a 16 should work, but not a 15? I have no data about the CTC-12