If a 7jp4 lights partially, is it ok??

[rca2000]

I acquired this old 7jp4 yesterday, at an auction, where I also got a 7" admiral set, 4 boxes of tubes(MANY NOS, ) and a couple of boxes of RCA parts. (knobs, switches, chips, coils,a crt board, etc)

The Admiral will NOT work, it draws excesive current from the B+(caps, no doubt,) and I could not check the 7jp4 on it. My cr-70 is not working, either, BUT I did get a good filament from the tube on it.

So-- I have another 7" admiral set in the garage, I got about 5 years ago, right after 9-11, IIRC. Figuring there was not much to lose, I dragged it in the house after-hours, (for obvious reasons...)and started it up, without the rectifier tube. All filaments glowed including the 7jp4 in the set. SO...I just "touched" the 5y3 in its socket, while watching the screen--and I got SOME raster!! Plenty bright, but all distorted and compressed into a blur of an inch or two. This means that the tube in the set is pretty good.

Next-- I unplugged the set's tube, and plugged in the "new" one I got. VOILA!! I get a "partial" raster from this one, too. ALL compressed, jerky and bent, but PLENTY bright!! ALL of this took place in the kitchen, under the celing fan light. I had a more than bright enough raster to see in that light.

(Needless to say-- ALL of these tests were only a second or 2 long--as I was fearful of leaving on my set any longer--the very loud hum from the speaker was threatening to wake the girls..and I would have had some explaining to do....but I DID get some raster on both tubes..)

SO...does this mean that this CRT is good? Can a 7jp4 be "bad" even though on a set with problems it produced "some" raster? Will the tube still produce the same decent brightness in a set that is working right??

[jpdylon]

the only real way to tell if a 7jp4 is good is if it produces a good bright picture in a lit room. If you have to crank the brightness to see a picture or turn off the lights, your tube is most likely a gonner. There is an excellent thread I started on this:
How to tell if my 7jp4 is dead

[rca2000]

As I could still have a bad tube, and it be able to produce a pretty bright raster ( compressed to an inch or two and full of hum) in a well-lit room??(that is what I got from BOTH tubes. the one I got and the one in the set.)

Is that correct??

[bgadow]

I am not an expert on these tubes but I cannot see how the crt could be bad in that way. It seems unlikely to me that the elements could be damaged in such a way that it would produce a partial raster but not a full one. Sounds like that defect is just in the set. I think you did good!

[wa2ise]

Originally Posted by rca2000

I acquired this old 7jp4 yesterday

... house after-hours, (for obvious reasons...)and started it up, without the rectifier tube. All filaments glowed including the 7jp4 in the set. SO...I just "touched" the 5y3 in its socket, while watching the screen--and I got SOME raster!! Plenty bright, but all distorted and compressed into a blur of an inch or two. This means that the tube in the set is pretty good.

Next-- I unplugged the set's tube, and plugged in the "new" one I got. VOILA!! I get a "partial" raster from this one, too. ALL compressed, jerky and bent, but PLENTY bright!! ALL of this took place in the kitchen, under the celing fan light. I had a more than bright enough raster to see in that light.

SO...does this mean that this CRT is good? Can a 7jp4 be "bad" even though on a set with problems it produced "some" raster? Will the tube still produce the same decent brightness in a set that is working right??

Realize that when you restore the TV set chassis (recap and so on) and get the deflection to give you a normal sized picture, the "brightness" of the raster will be dimmer as the electrons will be spread over a larger area. IIRC, early TV set CRTs were dim compared to more modern ones anyway. That you'd have to draw the shades to watch the soap operas on a sunny day...

Both CRTs are probably good. And the fact that you did get a raster indicates that the flyback is highly likely good. And various other hard to locate parts likely are good too. Replace all the paper caps, and check high resistance resistors (above half a megohm tend to go bad to higher resistance) and the electroytics should be replaced too. I'd say that there's a good chance that a resoration should be successful.

[David Roper]

Flyback...?

[rca2000]

Originally Posted by wa2ise

IIRC, early TV set CRTs were dim compared to more modern ones anyway. That you'd have to draw the shades to watch the soap operas on a sunny day...

Both CRTs are probably good. And the fact that you did get a raster indicates that the flyback is highly likely good. And various other hard to locate parts likely are good too. Replace all the paper caps, and check high resistance resistors (above half a megohm tend to go bad to higher resistance) and the electroytics should be replaced too. I'd say that there's a good chance that a resoration should be successful.

Well.... my GE 810 does NOT have a dim raster at ALL!! IN fact..it has a GOOD, bright picture, that is PLENTY bright enough to watch in a well-lighted room. It is in the basement, and there is a 300 watt bulb nearby, when it is on, the picture is still plenty bright enough to watch. Of course, I realize that we are comparing "apples and oranges" here.... a 10" set with a flyback and magnetic deflection and such, compared with a 7" Electrostatic set with RF-type HV. Still...not all of the old round BW sets were that dim!! I expect that with a full raster it (the 7jp4) will be dimmer...but probably still plenty watchable.

BTW-- pictures coming of the 810....maybe tonight, (after hours..)!!

[wa2ise]

Originally Posted by David Roper

Flyback...?

Oops, this is a set with the high voltage from a 60Hz transformer, yes? Be real careful of those....

In any event, the very high voltage must be working to see a raster of any sort on the CRT face.