Restoring a RCA 21CT55

[miniman82]

That could be your problem, original calls for 3.4 meg and you installed 4.4 meg. Compare the grid voltages on the 6BK4 with what the schematic shows, my guess is it’s a tad high. (Measurement should be taken with brightness at minimum)

Here’s how the regulator works, in case you were unaware:

Plate receives anode voltage, cathode is returned to B+, grid is tied to a boost source usually. If boost rises anode voltage must also have risen, since they are in the same circuit. This higher boost voltage causes the regulator to draw more current, since the grid got pushed closer to the cathode (the tube is normally biased off, with the cathode over the grid). With the regulator drawing current the anode voltage will get dragged down taking boost along with it, in turn taking the grid of the regulator lower and turning it more off.

In this way the regulator tube places a constant load on the anode, but if it’s biased a little too hot to start off with the result will be consistently low HV because the regulator is dragging it down.

The correct HV setting for this chassis is 25kv, I suggest you take Ben’s advice and figure out why it’s low before the flyback protests too much. You also need to know what the horizontal output tube is drawing for current, if it goes much higher than about 215ma, you’re in the danger zone.

We already witnessed another member destroy an irreplaceable CTC-2B flyback by not heeding sound advice, please don’t be the second.

[etype2]

Originally Posted by miniman82

That could be your problem, original calls for 3.4 meg and you installed 4.4 meg. Compare the grid voltages on the 6BK4 with what the schematic shows, my guess is it’s a tad high. (Measurement should be taken with brightness at minimum)

Here’s how the regulator works, in case you were unaware:

Plate receives anode voltage, cathode is returned to B+, grid is tied to a boost source usually. If boost rises anode voltage must also have risen, since they are in the same circuit. This higher boost voltage causes the regulator to draw more current, since the grid got pushed closer to the cathode (the tube is normally biased off, with the cathode over the grid). With the regulator drawing current the anode voltage will get dragged down taking boost along with it, in turn taking the grid of the regulator lower and turning it more off.

In this way the regulator tube places a constant load on the anode, but if it’s biased a little too hot to start off with the result will be consistently low HV because the regulator is dragging it down.

The correct HV setting for this chassis is 25kv, I suggest you take Ben’s advice and figure out why it’s low before the flyback protests too much. You also need to know what the horizontal output tube is drawing for current, if it goes much higher than about 215ma, you’re in the danger zone.

We already witnessed another member destroy an irreplaceable CTC-2B flyback by not heeding sound advice, please don’t be the second.

Thank you very much for this information.

[benman94]

I would like to point out that the Sams Photofact doesn't even show a 3.4 meg resistor in the regulator circuit. I'll pull my factory service literature tomorrow to look for running changes, but I'm not anticipating anything like that. Your set needs to be looked at by a competent tech, period.

[miniman82]

Originally Posted by benman94

I would like to point out that the Sams Photofact doesn't even show a 3.4 meg resistor in the regulator circuit. I'll pull my factory service literature tomorrow to look for running changes, but I'm not anticipating anything like that. Your set needs to be looked at by a competent tech, period.

I don’t see one in Sams or the white book (service clinic) covering this set, but interestingly Sams list the regulator as a 6BD4 same as the CT-100. White book calls it a 6BK4, wonder if this chassis has the same wiring goof the CT-100 has under the regulator socket?

[etype2]

Originally Posted by miniman82

I don’t see one in Sams or the white book (service clinic) covering this set, but interestingly Sams list the regulator as a 6BD4 same as the CT-100. White book calls it a 6BK4, wonder if this chassis has the same wiring goof the CT-100 has under the regulator socket?

Can you tell me more about “wonder if this chassis has the same wiring goof the CT-100 has under the regulator socket?[/QUOTE]“ to bring me up to speed.

[benman94]

Originally Posted by etype2

Can you tell me more about “wonder if this chassis has the same wiring goof the CT-100 has under the regulator socket?“ to bring me up to speed.

See this link:

http://www.earlytelevision.org/Deksn..._6BD4-6BK4.htm

In short, pin 8 on the 6BD4 socket in the CT-100 is used as a tie point. It is not used as a tie point in the 21-CT-55, so you can use any 6BD4A, 6BK4, or 6EL4A you choose.

[etype2]

I mentioned your recent comments to my friend Mike who has done 99% of the work on both the 21CT55 and the CTC-7. I myself can only do the basics. Mikes comments:

“The absolute high voltage rating for the 21AXP22 is 27,500 Volts and the design values are between 20,000 and 25,000 volts. I set yours for 21KV which produced good focus. And the Grid 3 volts (focus) volts will rise linearly with the high voltage since that voltage is derived from the tripler set of 3A2 tubes. And THAT grid has a maximum rating of 6600 volts which you do not want to exceed. And YES, the set will produce 30KV with the regulator pulled. I tested it that way when I was verifying the regulator functions. It does NOT mean that you run the CRT at that voltage!!! The regulator tube is, by definition, a SHUNT REGULATOR circuit. What that means is that it is designed to SHUNT the UNUSED high voltage to GROUND as a function of its design. There are no issues with capacitors as far as I could tell here. Are you having problems with the picture? You can look up the PDF file for the 21AXP22 if you want to learn more about its design values. There is a FUSE in the circuit to protect the flyback from overload in the event that it occurs.”

I will ask Mike to join in on the conversation as he is a VK member. He just had long needed surgery, so it might not be to soon.

Edit: I checked both the RCA and Sylvania data sheets and confirm the voltage figures cited by Mike. However, RCA states “25KV Max.”

[benman94]

This discussion is going to devolve even further if left open. I'm preemptively shutting this down for the time being. If another moderator sees reason to open back up have at it.

At this point, I think everybody has said their piece about the risks, real or "perceived", to Mr. Wozniak's flyback. There is little, if anything, to add to the discussion.

[Eric H]

I have deleted everything from this last day in an attempt to get this thread back on track.

If it can go forward in a more civil manner it will stay open.

[etype2]

Recap of the restoration of a RCA 21CT55 without having to read the entire 10 pages of this thread.

The set was fully restored last January, 2017. It took two years and we thank all the VK members who helped and especially Mike Doyle, also a VK member. In July we were doing a video of the 21CT55 and it was running continuously for about 4 hours. We were done and ready to shut the set down. I left the set running alone for no longer then three minutes for a bathroom break and after returning, found the horizontal collapsed about one third of the screen, badly converged color, and out of focus. No unusual sounds or odors present. I know, first rule, never leave a vintage set unattended. WHAT THE HECK HAPPENED! WHAT LUCK!

After much testing, we thought the flyback failed. The set was only producing 10KV. That was a good sign. More testing with the HV probe and we pulled the regulator tube and found the HV measured 20KV. I can’t remember what the previous voltage was. That led us to suspect a resistor in the HV regulator circuit. We tested it and found it “open”. The suspect resistor was “buried” under the damper tube. This was done at my home and with lack of proper equipment and expertise, off the chassis went once again to Mike’s test bench/shop.

Mike found the open resistor “buried” under the damper tube. He then addressed the color problem I complained about, weak green output. I blamed it on the somewhat tired 21AXP22. It still test “good” but on the low side at cutoff. It tuned out that a simple 1/16 turn CCW of a slug in the L44 Quadrature Transformer made all the difference. The green returned strong and looked good on Mike’s jigged CRT. The resistor was an original (made by International Rectifier) special film type, 3.4 Megohm 3 watt. Mike replaced it with QTY 2 (in series) OHMITE 2.2 Megohm 2 Watt film type which now makes the new resistor a 4 watt unit which is a 25% upgrade for power dissipation.

Mike let the chassis “cook” (exaggeration) several hours a day for a week. During this time, he monitors the chassis for any anomalies, checks voltages, etc. The HV is 30 KV with the regulator tube pulled. All looked good and the chassis was delivered back to me. We reset the gray scale and did a full setup. We could not get the convergence perfect. Settled for good convergence in the center, top, bottom and left side. The bottom convergence is just as important as the center. Broadcasts usually show text at the bottom. The 21AXP22 looks like a new CRT now. We are happy!

Operational observations: The set comes up with good convergence and seems to reach full brightness in 30 seconds. The image enlarges to a point that after about 25 minutes the annoying Macrovision streaks on the “Gentlemen Prefer Blondes” DVD disappear at the top of the screen.

The cabinet is in good shape but we will refinish the top which has blistering of the old varnish. We still have to polish the brass trim around the CRT safety screen.

Here, a few more screenshots from the 1953 Technicolor movie, “Gentlemen Prefer Blondes”. I’m very pleased with the results.

This link will open an image carousel and full resolution images of the thumbnails can be viewed.

https://visions4netjournal.com/vinta...-carousel-3964

This link will show images from OZ and THE RED SHOES.

https://visions4netjournal.com/vinta...-carousel-3856

If anyone has a copy of the “Service Clinic” booklet for the 21CT55, I’d appreciate it. I checked the ETF and internet. Nothing found. I would like to address the concerns expressed about the HV by several VK members. The resistor failure concerns me but then again, it’s a 62 year old set. Things fail. We can’t replace every single part.

Thanks.

[benman94]

Regarding the correct 25 kV ultor voltage, see this quote from the RCA booklet entitled "Technical Features of the RCA Victor Model 21-CT-55 Color Television Receiver":

The RCA Victor Model 21-CT-55 color television receiver is a direct-view console which reproduces high-quality television pictures in full color from a standard color television signal, and automatically, with no adjustment or attachments, high-definition black-and-white television pictures from a standard black-and-white television signal.

The picture displayed on the screen of a 21-inch tricolor kinescope, the 21AXP22, has an area of approximately 255 square inches (2 1/2 times the CT-100). The 21AXP22 employs electrostatic focusing and electromagnetic convergence. Ultor voltage is 25,000 volts. In conjunction with the 21-inch tricolor kinescope, the convergence and purity arrangements provide ease of adjustment, and increased accuracy and stability of color registration.

One of the customer convenience features in this receiver is that convergence and focus are no longer customer adjustments from the side of the cabinet but are internal and are preset by the service technician.

Additional features include: a removable top panel to facilitate servicing and adjustment, and the location of most of the functional controls at the receiver front. These control are located under the tilt-down control cover or behind the removable wood panel directly below the control cover. Dual loudspeakers are provided for sound reproduction.

(Emphasis added by yours truly.)

I'm still failing to see what is ambiguous about the RCA specified ultor voltage, but to be honest I really don't care any more... I tried to help, I only hope you correct the issue before your flyback goes up in smoke.

Anyway, here is the factory service data you asked for "1954 No. T13", published November of 1954: http://docdro.id/HBjbl9P

You'll note that the only 3.3 MOhm resistor is in the grid circuit of the triode section of the 6AN8 that serves as the R-Y amp and phase splitter. I'm not sure what resistor Mike replaced in the 6BD4A circuit with two 2.2 MOhm resistors in series. Apparently he's smarter than RCA's own engineers. I guess we should all defer to some random guy with a soldering iron instead of the factory service literature...

[Findm-Keepm]

Originally Posted by benman94

Regarding the correct 25 kV ultor voltage, see this quote from the RCA booklet entitled "Technical Features of the RCA Victor Model 21-CT-55 Color Television Receiver":



(Emphasis added by yours truly.)

I'm still failing to see what is ambiguous about the RCA specified ultor voltage, but to be honest I really don't care any more... I tried to help, I only hope you correct the issue before your flyback goes up in smoke.

Anyway, here is the factory service data you asked for "1954 No. T13", published November of 1954: http://docdro.id/HBjbl9P

You'll note that the only 3.3 MOhm resistor is in the grid circuit of the triode section of the 6AN8 that serves as the R-Y amp and phase splitter. I'm not sure what resistor Mike replaced in the 6BD4A circuit with two 2.2 MOhm resistors in series. Apparently he's smarter than RCA's own engineers. I guess we should all defer to some random guy with a soldering iron instead of the factory service literature...

From 1954......hmm... do you have the 13 Goldenrods (service Bulletins) issued against the 21CT55? (I don't either...) The last one was issued in 1972....and probably a parts stoppage report. Many of these Goldenrods were safety issues - without them, the original service literature is a snapshot in time.

By 1978 (when we got our first RCA service library, from RCA, 28 binders, TV only...), some 600+ Goldenrods had been issued by RCA for televisions. Some were minor part number changes, but in the case of ALL early color (CTC17 and earlier), RCA issued Goldenrods to limit the 2nd anode voltage in all sets to a value that reflected the amount of shielding around the regulator tube.

I'm not Chicken Little, crying about the X-rays - I'm merely pointing out that the original service lit has been revised many, many times and some original statements and values rescinded.

RCAman, a frequent poster, may still have an RCA library with the Goldenrods. Ours went the way of another shop when we closed up in 2008. They tossed all the tube stuff - I rescued what they still had - CTC72 and up, unfortunately.

Nick/Miniman82 - same as any NAVAIR pub change - remove the old stuff and put in the new, up-to-date stuff. Old values/procedures and part numbers no longer apply...

[miniman82]

Gonna have to disagree with the pub analogy, in this case I know better than to put unnecessary electrical strain on 60 year old parts. Besides, we’ve both been around long enough to see a pub or two with bullsh!t procedures in it.

[etype2]

Originally Posted by Findm-Keepm

From 1954......hmm... do you have the 13 Goldenrods (service Bulletins) issued against the 21CT55? (I don't either...) The last one was issued in 1972....and probably a parts stoppage report. Many of these Goldenrods were safety issues - without them, the original service literature is a snapshot in time.

By 1978 (when we got our first RCA service library, from RCA, 28 binders, TV only...), some 600+ Goldenrods had been issued by RCA for televisions. Some were minor part number changes, but in the case of ALL early color (CTC17 and earlier), RCA issued Goldenrods to limit the 2nd anode voltage in all sets to a value that reflected the amount of shielding around the regulator tube.

I'm not Chicken Little, crying about the X-rays - I'm merely pointing out that the original service lit has been revised many, many times and some original statements and values rescinded.

RCAman, a frequent poster, may still have an RCA library with the Goldenrods. Ours went the way of another shop when we closed up in 2008. They tossed all the tube stuff - I rescued what they still had - CTC72 and up, unfortunately.

Nick/Miniman82 - same as any NAVAIR pub change - remove the old stuff and put in the new, up-to-date stuff. Old values/procedures and part numbers no longer apply...

Interesting. So RCA issued “Adendums” / Goldenrods to update service data. I wonder if Mike has those for the 21CT55?

[Electronic M]

Thanks for posting the RCA info. It will be nice to be able to reference in addition to the sam's folder.