21" RCA B&W from 1955

[radiotvnut]

A friend gave me this 21" RCA table model B&W set that is housed in a metal cabinet and the CRT, a 21AMP4A, appears to have an intermittent K-G1 short that won't blow out (it's a shame because the emission tests "good").

Actually, I had a melt-down because the 4 CRT testers that I have easy access to all appear to have issues. The first tester I used was a mid '70s RCA and it showed a hard K-G1 short, and "remove shorts" did nothing. In fact, it appeared that this aspect of the tester was dead (before now, I've never used the "rejuvenate" or "remove shorts" function of this tester).

The second tester I tried was a B&K 466 with a flaky meter. I could align the pointer with the screw on the meter Lense, turn the tester on to set the heater voltage, and then the meter pointer would land in the incorrect place when I turned off the tester. Finally, I got the meter to behave, but who knows how long that will last. That tester also showed the same K-G1 short and the instructions say to let the tube cool for 5-10 minutes, before attempting to remove a short. That did nothing and I determined that the short only exists when the tube is hot. So, I quickly hit the "remove shorts" button before the heater cooled and it flashed in the neck, but it didn't remove the short. The short seems to be intermittent when tapping on the neck, but it does not go away for any length of time.

The third tester that I tried was a B&K 465. It indicated no short (possibly a fault with the tester) and I could not properly set cut-off with that tester (meter stays about mid-scale, no matter the knob setting).

The last tester that I tried was a Sencore CR70 and the last time I used that one, it would not register emission, but would rejuvenate a tube. Today, the heater would not light and I suspect the problem is with the adapter cable.

One day, I need to sit down and go through all of these old CRT testers. The ones that can easily be fixed will stay in service and the "dogs" will get thrown out. However, today was not the day for that because as mad as I was (all I need to do is test a CRT, something that should take 5 minutes, and got four freakin' CRT testers that all appear to have issues....I just want something that works when I need it to work), I would have smashed all 4 of them and then regretted it later.

Anyway, I'm going to check with a friend to see if he has a 21AMP4A tube, or a substitute. If he doesn't, I'm probably going to scrap the set because buying a tube plus the high shipping cost probably would not be worth it.

[ARC Tech-109]

I've had luck with both my 466 and 467 hitting the K/G1 remove shorts multiple times and not blowing the tube, I've modified the 466 to use the 467 style plugs and both show the same readings after the remove shorts hits. The 467 keeps the heaters on during the remove shorts so I don't think it's going to kill the tube. If you have a good high voltage power supply and 10uf 450v cap you can basically emulate the removal by heating up the tube and throwing the charged cap across the cathode and grid by hand to bust the short open. I've done this with the little Sony 5" direct heat Trinitron tubes only I used a 1uf cap given their delicacy.

[Notimetolooz]

You should spend the same type of effort testing and restoring the CRT testers as you would a TV. Vintage electronics is vintage electronics, the TV and testers are made of similar stuff.

Check the manuals for possible checks of the testers. You could also look at the schematics and come up it some ways of confirming operations. Most CRT testers have fairly simple circuits. Basically testing a CRT like a triode.

You could also connect the 21" CRT to a working TV to double check the operation.
Try the testers, but not the "clear short" function, on a known good CRT.

" The fault, dear Brutus, is not in our stars, but in ourselves, that we are underlings. "

[Electronic M]

Originally Posted by ARC Tech-109

I've had luck with both my 466 and 467 hitting the K/G1 remove shorts multiple times and not blowing the tube, I've modified the 466 to use the 467 style plugs and both show the same readings after the remove shorts hits. The 467 keeps the heaters on during the remove shorts so I don't think it's going to kill the tube. If you have a good high voltage power supply and 10uf 450v cap you can basically emulate the removal by heating up the tube and throwing the charged cap across the cathode and grid by hand to bust the short open. I've done this with the little Sony 5" direct heat Trinitron tubes only I used a 1uf cap given their delicacy.

You do know B&K offered a factory made adapter that allows 467 adapters to fit the 466, right? My 466 has one. The adapter also has a knob for selection of 'normal-special'...Never had a tube that needed special IIRC. Universal adapters for these aren't hard to make...I had a dud scope tube with a 10BP4 style base with all (14?) pins populated that I used to make a universal adapter. Any CRT I don't have an adapter for, especially ones I can't find an adapter for (like my 15GP22s and Protelgram CRTs) I just look up the pinout and whip that out to test them.

[radiotvnut]

Originally Posted by Electronic M

You do know B&K offered a factory made adapter that allows 467 adapters to fit the 466, right? My 466 has one. The adapter also has a knob for selection of 'normal-special'...Never had a tube that needed special IIRC. Universal adapters for these aren't hard to make...I had a dud scope tube with a 10BP4 style base with all (14?) pins populated that I used to make a universal adapter. Any CRT I don't have an adapter for, especially ones I can't find an adapter for (like my 15GP22s and Protelgram CRTs) I just look up the pinout and whip that out to test them.

Yep, I believe SP66 was the part number for the 466 adapter and SP65 was the adapter for the 465 and these will allow the newer style adapters that go with the 467, 470, 480, and 490 to be used with the older 465 and 466 models. Somewhere, I have an SP65 adapter to go with my 465 (when I get it working). Those adapter boxes don't seem to be very common.

[radiotvnut]

I opened up the RCA WT333B tester and could not find anything obviously wrong. So, I simulated leakage by placing a 270 ohm resistor between the K and G1 terminals on the socket and, as expected, a short was shown on the meter. Then, when I moved the function switch to "remove shorts" and pressed the button, the red lamp that indicates current flow illuminated. I then connected the tester to the tube and I still showed K-G1 leakage that varied between a hard short to midrange to no short at all, and this could be stimulated by tapping on the neck. In "shorts removal" mode, I was unable to clear the short and no light came on. Just for the heck of it, I moved the switch to "clean/balance" and hit the button, which caused the light to blink (telling me that it arced between K and G1. That had no effect on the leakage/short, but the emission and cutoff read very good and the tube passes a life test. It's just a shame that I can't clear that leakage/short between K and G1.

[Notimetolooz]

Why did you chose to use a 270 ohm to simulate the problem?

There is no standard used by the CRT tester brands on how much leakage is too much. They probably use different voltages to measure leakage for instance.

Other than the testers how did you determine that the tube had a problem?

Any leakage could be due to a problem in the tube base and not necessarily internal in the tube. Perhaps the set was caught in the rain at some point and has a deposit left in the socket.
It is a bit difficult to believe that the tube would show good cutoff if it has a K-G1 short.

I noticed that the picture of the inside back of the set shows no ion trap, that tube should have one.

[leadlike]

I have the same model RCA and it too had a bad crt with extremely low emission. I never found a 21amp4a during the couple of years it ran on a brightener, but I was able to put in a nearly-new 21cbp4 without much fuss- I simply removed all of the magnetic focusing hardware. I don’t remember this tv ever really giving issues as a daily watcher, other than the board that has the vertical output section on it had some bad cooked traces, corrected with some point to point jumpers.

Edit: looks like the ETF has a couple of 21amp4s.

[dieseljeep]

Originally Posted by Notimetolooz

Why did you chose to use a 270 ohm to simulate the problem?

There is no standard used by the CRT tester brands on how much leakage is too much. They probably use different voltages to measure leakage for instance.

Other than the testers how did you determine that the tube had a problem?

Any leakage could be due to a problem in the tube base and not necessarily internal in the tube. Perhaps the set was caught in the rain at some point and has a deposit left in the socket.
It is a bit difficult to believe that the tube would show good cutoff if it has a K-G1 short.

I noticed that the picture of the inside back of the set shows no ion trap, that tube should have one.

It looks like a newer issue CRT that doesn't use a ion trap!

[dieseljeep]

Originally Posted by leadlike

I have the same model RCA and it too had a bad crt with extremely low emission. I never found a 21amp4a during the couple of years it ran on a brightener, but I was able to put in a nearly-new 21cbp4 without much fuss- I simply removed all of the magnetic focusing hardware. I don’t remember this tv ever really giving issues as a daily watcher, other than the board that has the vertical output section on it had some bad cooked traces, corrected with some point to point jumpers.

Edit: looks like the ETF has a couple of 21amp4s.

The main problem I had with the vert sweep board was bad capacitors.
The set came out in mid-1955 as a '56 model! The KCS96 was identical to the KCS97, circuit-wise, but a different layout.

[jr_tech]

Originally Posted by dieseljeep

It looks like a newer issue CRT that doesn't use a ion trap!

21AMP4B : Aluminized, no ion trap, according to my 1964 GE tube manual.

jr

[mr_rye89]

That one looks similar to my KCS94, made the same year I guess. I also put a newer jug in mine that didn't need the ion trap. An upgrade IMO.

[dcl0]

In case you need it, I uploaded all the literature and service info ( & good quality scan of the schematic) I had for the RCA WT-333B to the BAMA web site.

[dieseljeep]

Originally Posted by mr_rye89

That one looks similar to my KCS94, made the same year I guess. I also put a newer jug in mine that didn't need the ion trap. An upgrade IMO.

Wasn't that the first transformer less series-string, voltage doubler chassis?
The UHF models had a range control, local-distance adjustment as a user control.
IIRC, the set was referred as the "Special" model.

[mr_rye89]

It might be the first doubler set but I don't know. And VHF only.....