I've been blessed!

[old_tv_nut]

A friend took apart one of the bad pots, and here's what he found. The carbon element (deposited on paper/phenolic?) is burned and has lifted from the (phenolic?) substrate.

[N9ZQA]

I wonder if that couldn't be glued back down and then bridged with some sort of conductive paint - rear window defogger coating, maybe?

[old_tv_nut]

The guy who took the pics tried something called "liquid wire", but foudn it's not so conductive - the burned segment should be 25 ohms, but he ended up with a few hundred ohms. The total is supposed to be 100 ohms. I suspect this low resistance and the high currents involved (compared to a video or audio signal) are pushing the carbon pot technology beyond its limits. I suspect high contact resistance could develop and then there would be local heating. Don't know why they didn't use wirewound pots for this function. The circuit I came up with using two pots has a wirewound 25 ohm in it, mainly because that's what I could find. There is no problem with coarseness of adjustment for this application.

[jshorva65]

I'm also looking for info on how to fabricate the correct mounting brackets for retrofitting a glass 21FBP22 (actually a delaminated 21FJP22) in place of an (aired) metal 21AXP22 to restore a Sylvania model 31C304M, hopefully without hacking up the shroud used with the 21AXP22, thus making the retrofit reversible in case a fix for leaker 21AXP22's becomes available.

Another question I hope someone here can help answer is whether anyone has data on feasibility of making an adaptor for use in bench testing an early 21" color chassis with a test rig.

I have the GTE/Sylvania CK-3000 test rig and know someone with a TeleMatic rig. For both units, the earliest color chassis supported directly by its adaptor complement was the RCA CTC-7. If a correct set of Yoke Program, Convergence Ballast, and Yoke Cable characteristics could be determined, then the use of test rigs would help simplify these early sets' restorations greatly. It would seem that the correct yoke wiring harness could be as easy to create as making an adaptor to mate the leads of the existing "roundie" yoke harness to the correct plug for the older chassis. That would leave only the correct Yoke Program and Convergence Ballast values and pinouts to be determined, since the CRT cable should be directly compatible with all 21" color tube applications.

[John Folsom]

Wayne, Surplus Sales of Nebraska
http://www.surplussales.com/Potentio...Shafted-1.html
has 100 ohm 2 W wirewould pots. I think you could disassemble the pot, modify it to add a fixed 25 ohm tap, and reassemble. A little tricky, but it could be done.

[old_tv_nut]

John, thanks for the link. Right now, filing for future reference, Need to think how I would mount them in place of the existing ones that are dual-consentric.

[Findm-Keepm]

Before going the Surplus Sales of Nebraska route, I'd give Play Things of the Past a try - they are a LOT less expensive, and he has tons of pots.

Play Things of the Past Pots

He had the pot for an old Philco TV I helped restore. Just be aware that the Clarostat RTV series are huge 2-1/2" round pots. Dual ganged pots they are, but huge.

Anyone have an old Clarostat catalog with their A47 series stuff? Seems they would have made replacements for all the RCA stuff - they covered (gasp!) Muntz! And yes, they made dual element and tapped stuff too.

IRC and CTS also made kits that had various elements, shafts and housings so that custom pots could be "built" - perhaps someone might have one of their old catalogs or an Allied Radio catalog with their potentiometer line listed in it.

What are the orginal RCA part numbers for the pots? Drawing numbers? Maybe RCA superceded them with a new number -they loved doing that!

Cheers,

[old_tv_nut]

Thanks for the info and ideas. I had some eye surgery recently, and have to stay away from lifting the chassis in and out while I heal. So, it's a time to make plans, but out of action for a while.

[ChrisW6ATV]

Originally Posted by jshorva65

I have the GTE/Sylvania CK-3000 test rig and know someone with a TeleMatic rig. For both units, the earliest color chassis supported directly by its adaptor complement was the RCA CTC-7. If a correct set of Yoke Program, Convergence Ballast, and Yoke Cable characteristics could be determined, then the use of test rigs would help simplify these early sets' restorations greatly. It would seem that the correct yoke wiring harness could be as easy to create as making an adaptor to mate the leads of the existing "roundie" yoke harness to the correct plug for the older chassis. That would leave only the correct Yoke Program and Convergence Ballast values and pinouts to be determined, since the CRT cable should be directly compatible with all 21" color tube applications.

Maybe you could get ideas for the correct adapter needs by comparing schematics of the CTC-7 (and -9, -11 etc.) with the older RCAs to see what major differences there might be. With very early color sets, there may not be too many differences from one brand to another, except the connector types.

[Bill R]

Would it be possible to pull the yokes and convergance assembly with the chassis and hang them out of the way for a test on the CK3000. Just connect the picture tube socket and the high voltage lead. I have done this on a newer set I didnt have the programmer or yoke adaptor for.

Bill R

[ChrisW6ATV]

If you left the deflection and convergence yokes from the TV connected and hanging loose, but plugged the test jig's CRT and high-voltage connectors in, all you would get on the test jig is a bright spot in the middle of its screen if the TV chassis was at least partly working. That would tell you the chassis had high voltage, but almost no other help.

[jshorva65]

Originally Posted by ChrisW6ATV

If you left the deflection and convergence yokes from the TV connected and hanging loose, but plugged the test jig's CRT and high-voltage connectors in, all you would get on the test jig is a bright spot in the middle of its screen if the TV chassis was at least partly working. That would tell you the chassis had high voltage, but almost no other help.

It would also damage the CRT in your CK3000 by overheating and burning the phosphor in the center of the screen. Operating any CRT without deflection is a BAD idea. Even prolonged operation of a set which had full horizontal sweep but no vertical sweep would quickly burn a line across the face of the CRT, thus ruining the tube. Deflection is required not just to produce meaningful test results, but also to distribute the energy dissipation due to electron bombardment evenly over the entire screen surface to prevent damage to the phosphor from overheating.

[colorfixer]

A wirewound can pot create stray inductance in an AC circuit. This is probably why they used a carbon pot.

[ChrisW6ATV]

Originally Posted by jshorva65

It would also damage the CRT in your CK3000 by overheating and burning the phosphor in the center of the screen. Operating any CRT without deflection is a BAD idea.

Yes, I should have pointed that out as well. I don't know what I was thinking. DO NOT just hook up your chassis that way.

[jshorva65]

The most important consideration (aside from the obvious ones such as compatibility with 70-degree color chassis of any kind in the first place) in using a test rig with a particular model of 70-degree color chassis similar to one which it supports directly is matching the impedances of the respective yoke windings in the rig to those of the set's own yoke within a reasonable tolerance. For both windings, +/-10% is preferred, while +/-15% is about the maximum tolerable degree of mismatch. Most 70-degree color yokes will typically have similar impedances, within +/-20%, as an educated guess. Uncontrollable vertical "rolling" seems to be the most frequently occurring symptom when operating test rigs with 70-degree color chassis when there is a moderate mismatch of convergence ballasts. Of course, a severe mismatch could easily damage the rig or the chassis, or both.

The importance of avoiding CRT "burn in" on the rig, but especially the prevention of damage to the picture tube in the set under test, cannot be overstated. Burn-in is damage which not even a CRT rebuild can correct. Just like a snapped neck, the only cure for a badly burned screen is a replacement tube. Both of my "daily use" 5AXP4 tubes for B/W bench testing, my "spare" 5AXP4 tube, and my 8XP4 and 8YP4 tubes all show signs of having been operated without deflection at least once before I acquired them. When using these tubes, I insert them into the yoke of the chassis under test or disconnect the set's own yoke and connect the extension cables of an appropriate B/W "substitution box" unit (basically, the B/W equivalents of the CK3000, using either a 5" round or 8" rectangular test CRT) to all of the appropriate connections on the chassis under test. The tubes in these units have pre-existing "dead spots" in the center of their screens from prior misuse, but are still capable of producing a picture adequate for troubleshooting purposes. Until I can locate undamaged examples of them (probably would have to be NOS, since ALL "used" tubes seem to have been MISused at least once), my existing tubes will just have to suffice.

When performing grayscale setup of a color chassis, during which the Service switch will disable vertical sweep, it is very important to preset all three G2/Screen controls fully CCW and set CRT Bias and Brightness to completely extinguish the raster before placing the switch in the Service position. Failure to do this could permanently damage the set's own CRT in a matter of mere seconds. The MAXIMUM brightness of the Service line should be just barely visible in a dimly-lit room. Grayscale setup procedures outlined in textbooks on color TV servicing and/or service literature vary somewhat for different textbook publishers and/or set models. Some will specify that the Service line should be barely-visible, while others will state that the G2/Screen controls should first be set to the point where the line just appears and then backed off to the point at which the line is just extinguished.