PaulOF
07-03-2008, 12:32 PM
Kind people,
I feel a bit guilty that I only seem to post here when I need advice, but not guilty enough to stop.
My 1951 zenith "entertainment center" (am-fm receiver, 16 inch porthole, and cobramatic) is coming along. The radio is working nicely after recapping and a bit of micro-surgery on one of the IF cans. I have a problem with the cobramatic that I am stuck on, but that is for another day.
Todays questions are about the tv. I'm largely inexperienced in working with video stuff. I've recapped it and at this point both B+ supplies are giving the right voltages, the B+ boost is right on target. Voltages and waveforms all look reasonable (well except for the sync input to the vertical oscillator. The "integrator" shorts the signal to ground instead of integrating. Fortunately, Sams' shows an alternate circuit that doesn't use the special "integerator" thingee, and as soon as I scrounge up a germanium diode I expect that to be working. Haven't tried silicon cause I suspect the forward voltage drop is too high.)
In particular, the audio looks like audio, the cathode of the crt looks like video, and I have sawtooths at about the right frequencies on the vertical and horizontal deflection coils. The problem is this: I bought one of those test crt's on ebay that I have been trying to use -- 5axp4 -- cause that big old metal tube with several square feet of 13000 volts scares me a tad. When I put the test crt in place, I get no raster, nothing, no hint of light even in a dark room.
According to its poop sheet, it needs no beam bender, and is autofocus.
The filament lights. Pin 10 has 360V (as it should). The cathode looks like video. Pin 2 has a series of -30V pulses at 60 cps which I assume are for vertical blanking. I don't have a high voltage probe (need to get one) but the anode lead will light a fluorescent sitting near it so there is some high voltage there at least. The doorknob holds a charge.
Now, I am looking for alternatives to the test CRT being bad.
One thing that concerns me is that the test thingee is auto focus but to put it through the yoke, I also have to put it through the big old focussing perm. magnet. Could that be screwing things up? Is there any easy way of testing the test crt? I don't even know if the original 16" death trap tube is good so putting it back in won't prove much, plus it does use an ion trap so trying to find a non-existant raster on it might be a challenge. The schematic does show a 470K resistor connecting the filament of the 1B3 to the anode lead. This actually ohms out at closer to 900K. It must be hidden inside the tube socket, and I am loath to try to disassemble that without knowing that I can get it back together. I am guessing that there is so little current involved that the actual value of the resistor is not pertinent but please tell me if I am wrong about that.
I would appreciate any thoughts you might have.
Paul
I feel a bit guilty that I only seem to post here when I need advice, but not guilty enough to stop.
My 1951 zenith "entertainment center" (am-fm receiver, 16 inch porthole, and cobramatic) is coming along. The radio is working nicely after recapping and a bit of micro-surgery on one of the IF cans. I have a problem with the cobramatic that I am stuck on, but that is for another day.
Todays questions are about the tv. I'm largely inexperienced in working with video stuff. I've recapped it and at this point both B+ supplies are giving the right voltages, the B+ boost is right on target. Voltages and waveforms all look reasonable (well except for the sync input to the vertical oscillator. The "integrator" shorts the signal to ground instead of integrating. Fortunately, Sams' shows an alternate circuit that doesn't use the special "integerator" thingee, and as soon as I scrounge up a germanium diode I expect that to be working. Haven't tried silicon cause I suspect the forward voltage drop is too high.)
In particular, the audio looks like audio, the cathode of the crt looks like video, and I have sawtooths at about the right frequencies on the vertical and horizontal deflection coils. The problem is this: I bought one of those test crt's on ebay that I have been trying to use -- 5axp4 -- cause that big old metal tube with several square feet of 13000 volts scares me a tad. When I put the test crt in place, I get no raster, nothing, no hint of light even in a dark room.
According to its poop sheet, it needs no beam bender, and is autofocus.
The filament lights. Pin 10 has 360V (as it should). The cathode looks like video. Pin 2 has a series of -30V pulses at 60 cps which I assume are for vertical blanking. I don't have a high voltage probe (need to get one) but the anode lead will light a fluorescent sitting near it so there is some high voltage there at least. The doorknob holds a charge.
Now, I am looking for alternatives to the test CRT being bad.
One thing that concerns me is that the test thingee is auto focus but to put it through the yoke, I also have to put it through the big old focussing perm. magnet. Could that be screwing things up? Is there any easy way of testing the test crt? I don't even know if the original 16" death trap tube is good so putting it back in won't prove much, plus it does use an ion trap so trying to find a non-existant raster on it might be a challenge. The schematic does show a 470K resistor connecting the filament of the 1B3 to the anode lead. This actually ohms out at closer to 900K. It must be hidden inside the tube socket, and I am loath to try to disassemble that without knowing that I can get it back together. I am guessing that there is so little current involved that the actual value of the resistor is not pertinent but please tell me if I am wrong about that.
I would appreciate any thoughts you might have.
Paul