View Full Version : Is this gas?


SwizzyMan
10-06-2017, 09:11 PM
Found this while testing the 21amp4 on my newly acquired Rca 21d641. I firmly believe it is just electron bombardment on the glass due to the fact that it moves on the glass when switched from cutoff to emission test. And to add to that the cutoff isn't acting weird and isn't off the charts high. Maybe only 1/4 scale at the most. The tube tests great after cooking for an hour and this glow only appears in one place, but I just noticed this and need some other input to confirm my theory. I do believe it is not gas but electron bombardment on the glass. First picture is testing on cutoff and second is on emission and the results.

Kevin Kuehn
10-06-2017, 09:59 PM
I believe it's too focused for gas. Also more purple than blue.

benman94
10-06-2017, 10:27 PM
Gas can be blue, purple, pink etc, depending on the species of gas present, the voltage, etc

I do not believe that to be gas, it doesn't seem to appear to be between two electrodes...

Findm-Keepm
10-07-2017, 12:08 AM
If you had access to a BD-10, it would surely tell you, and tell you where the leak was, if any. Channel Master gave one to all distributors of CRTs to check duds - a neck and bell/neck junction test was necessary to identify gassy duds. For the longest time, Channel Master wouldn't accept a gassy dud.

Cain Electronics, where I worked in the 80s had one (actually, still has one..., although they are now PriestCain Electronics...) - the only gassy CRTs I remember seeing were old 21" B/W CRTs. We had one guy, Mike, that was trained on the test by Channel Master, but the training seemed to be aimed at where the BD-10 had to be pointed at on the tube - the gassy ones would glow purple and "vein" out from the leak - move away far enough, and the glow would disappear.. I never received training, just watched Mike testing the duds, and watched the customer's face pale when they realized they weren't getting the dud value credit. :sigh:

Maybe someone can get one, and have it as a loaner. Used ones are all over eBay. I'm curious if Nick has one at the museum..?

SwizzyMan
10-07-2017, 11:43 AM
Ok now I know it's fine. But now I have another possible issue. When checking voltages on the power transformer for the plate supply I get lower than expected voltages. On pin 6 of the rectifier tube I get 256 volts and on pin 4 I get 195 volts. The schematic calls for a plate supply voltage of 280 volts. Could my power transformer be bad or are these results ok?

benman94
10-07-2017, 12:23 PM
A weak rectifier, low line voltage, etc can all bring down B+. If pin six should have 280 volts on it, then it's about 10 percent low which is not something to worry about. If pin four should also have 280 volts on it then you're in trouble. Start by changing the rectifier and posting a schematic.

SwizzyMan
10-07-2017, 12:26 PM
A weak rectifier, low line voltage, etc can all bring down B+. If pin six should have 280 volts on it, then it's about 10 percent low which is not something to worry about. If pin four should also have 280 volts on it then you're in trouble. Start by changing the rectifier and posting a schematic.

These voltages were taken with the rectifier unplugged. I was testing the transformer itself to see if it was good. Line voltage is about 123v. EDIT: Pin 4 now has 207 volts.

benman94
10-07-2017, 12:36 PM
What is the specified secondary voltage of the transformer? 280 volts center tapped or 280 volts between each side of the center tap?

SwizzyMan
10-07-2017, 12:45 PM
What is the specified secondary voltage of the transformer? 280 volts center tapped or 280 volts between each side of the center tap?

Between each side. Center tap goes to ground.

teevee
10-07-2017, 01:55 PM
Unequal voltages on each side of the center tap usually indicates shorted turns on one half of the HV secondary. Usually the voltages are within a few %. I suspect you will find the transformer heats up (no load). Also, looking at the primary current can be a clue.

SwizzyMan
10-07-2017, 02:15 PM
Well now it seems like it is magically fixed. Both sides show 298v at 123v a.c. Must have been an intermittent connection.

Electronic M
10-07-2017, 06:40 PM
Sometimes transformers that sit gather humidity/moisture inside and do not work correctly until that moisture bakes out.

Kevin Kuehn
10-08-2017, 12:21 AM
Or sometimes the power switch has some funky corrosion that cleans itself after several flippies. Don't be surprised if it comes back, and it could too be intermittently shorted turns.