1950 Admiral 16" Set; Recapped... No Picture!!

[Magnavox300]

Found a 1950 Admiral Set at my local dump the other day...It came in a nice cabinet, knobs and trim are all good!
It was sitting in front of the large dumpster and I just couldn't let it get crushed!
Brought it home, took out the chassis and CRT, easily came out in one piece, nice and convenient to work on!
CRT tested strong, so I thought I would proceed...
Recapped, and replaced bad resistors, but I am not getting the CRT to light up at all.
Tubes are all correct, good and strong, and I replaced what was needed.
The only thing is the 6BQ6 tube next to the HV cage was dead, and I didn't have a large coke bottle type to replace it with, but did have a smaller one... it's a little short, which I'm not sure is working properly... I had to use alligator clips to make up the height.
But it's still a 6BQ6, don't know if the size would make any difference.
Moved around the ion trap, no difference.

Is there a way to test for HV with a multimeter? I have a feeling
it's not getting the proper voltage.
Rechecked all my recap values and joints 3 times, everything looks good.
I believe there is a doorknob cap under the 1B3 tube?
Not sure how to check it, or if they ever go bad...
I appreciate any suggestions to help get this baby going again,
I'm sure I am close!!!!

[dieseljeep]

Originally Posted by Magnavox300

Found a 1950 Admiral Set at my local dump the other day...It came in a nice cabinet, knobs and trim are all good!
It was sitting in front of the large dumpster and I just couldn't let it get crushed!
Brought it home, took out the chassis and CRT, easily came out in one piece, nice and convenient to work on!
CRT tested strong, so I thought I would proceed...
Recapped, and replaced bad resistors, but I am not getting the CRT to light up at all.
Tubes are all correct, good and strong, and I replaced what was needed.
The only thing is the 6BQ6 tube next to the HV cage was dead, and I didn't have a large coke bottle type to replace it with, but did have a smaller one... it's a little short, which I'm not sure is working properly... I had to use alligator clips to make up the height.
But it's still a 6BQ6, don't know if the size would make any difference.
Moved around the ion trap, no difference.

Is there a way to test for HV with a multimeter? I have a feeling
it's not getting the proper voltage.
Rechecked all my recap values and joints 3 times, everything looks good.
I believe there is a doorknob cap under the 1B3 tube?
Not sure how to check it, or if they ever go bad...
I appreciate any suggestions to help get this baby going again,
I'm sure I am close!!!!

Are you sure that it takes a 6BQ6?
Check the tube location diagram as most used a 6CD6, a different tube.

[timmy]

Did you have the ion trap off because putting it on backwards it won't work , maybe reverse the trap and see if that works.

[Electronic M]

Unless you have a special HV probe designed to allow your DMM to measure up to at least 20KV don't try measuring your HV with a DMM.

Ways to measure approximate HV level without a DMM include taking an insulated screwdriver with a grounded blade and seeing how long of an ARC you can draw between the CRT bell and screwdriver. It should be in the 1/4"-1/2" range. You can check if the flyback is being driven properly by the output by holding a CFL light bulb or neon indicator lamp close to the flyback and the plate leads of the H output tube and HV rect while the set is warmed up and running....If the flyback is being driven properly the unconnected bulb will illuminate within an inch or 2 of those parts as a result of the RF harmonics of the sweep frequency being radiated.
If the first test fails but the second test passes the problem is either the HV rectifier, doorknob cap (they can fail, but about half the time are fine), resistors on the base of the rectifier or HV rect filament winding.
I check doorknob capacitors by disconnecting what ever end is easiest and placing the bottom of the rectifier (and cap if still attached to the rect) in a glass jar or dish so it can't arc to chassis, etc.

Definitely follow Dave's advice first before going into the weeds I've paid out above.

[Magnavox300]

Originally Posted by Electronic M

Unless you have a special HV probe designed to allow your DMM to measure up to at least 20KV don't try measuring your HV with a DMM.

Ways to measure approximate HV level without a DMM include taking an insulated screwdriver with a grounded blade and seeing how long of an ARC you can draw between the CRT bell and screwdriver. It should be in the 1/4"-1/2" range. You can check if the flyback is being driven properly by the output by holding a CFL light bulb or neon indicator lamp close to the flyback and the plate leads of the H output tube and HV rect while the set is warmed up and running....If the flyback is being driven properly the unconnected bulb will illuminate within an inch or 2 of those parts as a result of the RF harmonics of the sweep frequency being radiated.
If the first test fails but the second test passes the problem is either the HV rectifier, doorknob cap (they can fail, but about half the time are fine), resistors on the base of the rectifier or HV rect filament winding.
I check doorknob capacitors by disconnecting what ever end is easiest and placing the bottom of the rectifier (and cap if still attached to the rect) in a glass jar or dish so it can't arc to chassis, etc.

Definitely follow Dave's advice first before going into the weeds I've paid out above.

I checked the ion trap, put it in both ways, still nothing...
Checked the resistors under the 1B3, the 470K is good, but the 2.7 ohm is reading 5.7...Not sure if that would matter?
Removed the doorknob cap, it's 500PF at 20,000 volts.
Can I test it like it is, or will it fry my multimeter?
Wonder if I could bypass that cap and run the set to see if I get a picture, or is that crazy?

[JohnCT]

Originally Posted by Magnavox300

I checked the ion trap, put it in both ways, still nothing...
Checked the resistors under the 1B3, the 470K is good, but the 2.7 ohm is reading 5.7...Not sure if that would matter?
Removed the doorknob cap, it's 500PF at 20,000 volts.
Can I test it like it is, or will it fry my multimeter?
Wonder if I could bypass that cap and run the set to see if I get a picture, or is that crazy?

You can test the cap with a meter for value if your DMM reads capacitance. I don't know too much about doorknobs but they may read OK for value but start turning into resistors when you put a lot of voltage across them. Maybe someone here can address that. Because they're ceramic, I don't think they get leaky like paper caps do with voltage across them, but I'm not positive.

If the doorknob reads like a resistor on your ohmmeter, it's bad.

Of course, don't put any meter in the running circuit unless it's a HV probe.

John

[Magnavox300]

Originally Posted by dieseljeep

Are you sure that it takes a 6BQ6?
Check the tube location diagram as most used a 6CD6, a different tube.

Thanks, that would be the most obvious problem, since the tube that was originally in there was a 6CD6... but stamped in the chassis next to the tube socket it says “6CD6 or 6BQ6”. Are they not the same tube?
All I had was a 6BQ5 to replace it with.

[Magnavox300]

Originally Posted by Magnavox300

Thanks, that would be the most obvious problem, since the tube that was originally in there was a 6CD6... but stamped in the chassis next to the tube socket it says “6CD6 or 6BQ6”. Are they not the same tube?
All I had was a 6BQ5 to replace it with.

Sorry, meant 6BQ6...

[dieseljeep]

Originally Posted by Magnavox300

Thanks, that would be the most obvious problem, since the tube that was originally in there was a 6CD6... but stamped in the chassis next to the tube socket it says “6CD6 or 6BQ6”. Are they not the same tube?
All I had was a 6BQ5 to replace it with.

I looked up the tube data. The 6BQ6 is a totally different tube!
The 6CD6 has a totally different pin-out. The 6BG6 might work in a pinch.
All the side-mount 16" chassis used a 6CD6.

[old_coot88]

Originally Posted by dieseljeep

I looked up the tube data. The 6BQ6 is a totally different tube!
The 6CD6 has a totally different pin-out.

IIRC, plate connector cap is different diameter too. Like about 1/4" for the 'BQ6, 3/8" for the 'CD6.

[Notimetolooz]

Seems like the wrong tube is the problem.
If it still doesn't work you can check that the horizontal output is running, and therefore there should be HV, by bringing a compact fluorescent bulb near the horizontal output or flyback. If it lights up those are good.

[old_coot88]

You can eyeball the inside of the plate connector, to see whether it fits a small or large diameter cap. If large, it's not 6BQ6.

[Magnavox300]

Originally Posted by dieseljeep

I looked up the tube data. The 6BQ6 is a totally different tube!
The 6CD6 has a totally different pin-out. The 6BG6 might work in a pinch.
All the side-mount 16" chassis used a 6CD6.

Why would both tube numbers be stamped near that tube socket?

[Magnavox300]

Thanks for all the help...
I was wondering why the HV lead wire cap didn't fit on the top of any 6BQ6 tubes I had!
I'll have to just go ahead and order a 6BC6 tube...
Of course I have a million tubes, but not a 6BC6!!
I am assuming they stamped both numbers on the chassis next to the socket for different chassis circuits?
Will update once I get the new tube!
Thanks again!

[Electronic M]

Originally Posted by JohnCT

You can test the cap with a meter for value if your DMM reads capacitance. I don't know too much about doorknobs but they may read OK for value but start turning into resistors when you put a lot of voltage across them. Maybe someone here can address that. Because they're ceramic, I don't think they get leaky like paper caps do with voltage across them, but I'm not positive.

If the doorknob reads like a resistor on your ohmmeter, it's bad.

Of course, don't put any meter in the running circuit unless it's a HV probe.

John

Doorknobs tend to fail leaky or shorted so a DMM won't catch it unless it shorts badly....I had one that leaked intermittently sometimes the set would wakeup and work normally other times it had absolutely no HV.
The best way to test a doorknob when you suspect it is the cause of no HV is to disconnect it on one end and keep use a glass object to prevent the HV from arcing at the disconnect point. Bypassing the doorknob cap (by that I assume you mean putting a short across it) is about the same as putting a gun to your foot and pulling the trigger...
The resistor in the rectifier base that has doubled in value is VERY bad and should be changed...that resistor is designed to limit the rectifier filament voltage and it's value must be precise. Too low and the HV rect filament is overvolted and dies fast, too high and the HV rect filament does not heat up enough for HV to pass through the rect to the CRT.

Your incorrect H output tube is your biggest problem right now though...Get that tube right first and then worry about the rect and doorknob if the HV stir ain't working.