1963 Zenith 29JC20 restoration

[Zenith6S321]

I finished replacing all of the electrolytics. I removed all of the connections from the old electrolytics and mounted the new ones on the underside of the chassis. For now I have wired the caps as per the Sams schematic. The Zenith manual I ordered should be here in a day or two and I will compare the schematics.

I also checked the diodes and found a problem with the three section convergence diode. Two of the diodes read like a silicon diode. The third is shorted. Can I just use three 1n4007s for these? I think I read to use Schottky diodes but I don't know what current or PIV should be. A suggested part number would be great if anyone has had success with it.

Dave

[zeno]

Originally Posted by Zenith6S321

I finished replacing all of the electrolytics. I removed all of the connections from the old electrolytics and mounted the new ones on the underside of the chassis. For now I have wired the caps as per the Sams schematic. The Zenith manual I ordered should be here in a day or two and I will compare the schematics.

I also checked the diodes and found a problem with the three section convergence diode. Two of the diodes read like a silicon diode. The third is shorted. Can I just use three 1n4007s for these? I think I read to use Schottky diodes but I don't know what current or PIV should be. A suggested part number would be great if anyone has had success with it.

Dave

Should be a 103-### on the part. Cross it to NTE. You can get
the specs that way.

73 Zeno

[Zenith6S321]

Originally Posted by zeno

Should be a 103-### on the part. Cross it to NTE. You can get
the specs that way.

73 Zeno

Its a 212-25, the other diodes are 103-###. A google search for "NTE 212-25" did return a "212-25 NTE Equivalent NTE116 1A 600V rectifier,
NTE116 General Purpose Silicon Rectifier 1A 600prv"
I will try 1N4007s. Thanks.

Dave

[Zenith6S321]

One of the three convergence selenium diodes I thought was bad was just an incorrect in-circuit reading due to a coil. Once I isolated that diode it read ok. So for now I'm gonna leave it as is and come back to it later if I have convergence issues.

All the resistance readings checked ok, although I had to reverse polarity on the ohm meter once. I started cleaning the sticky goo off the wires to the CRT, yoke, tuner, and controls. Iso alcohol works well to take it off. After that, next is mounting the patched up HV rectifier socket and replace the bad tubes. Then I can try powering it up.

[Zenith6S321]

The HV socket really had a tight grip on the 3A3, so I bent the socket pins open a bit to loosen their grip in hopes pulling the tube will not break the socket plastic mounts. I had to grind some of the epoxy off of the outside patched up shattered HV socket mounts to get the socket to fit over them. I used new 6-32 machine screws short enough not to bottom out in the plastic. Here is a picture of the socket re-installed. I added some washer because the plastic around the mounting holes was cracking. The 3A3 and the 6BK4 are the original Zenith tubes with dates codes of 62-04. Even though they both tested ok I put in new tubes. I used a 6BK4C/6EL4A in place of the 6BK4.

[Zenith6S321]

I powered the set up on the bench without the HO tube and got good supply voltages and some noisy TV audio. I added a jumper to make it easy to check the cathode current on the HO and VO tubes. When I put the chassis back in the cabinet the chassis control shafts did not line up. The chassis control shafts were testing on the opening in the cabinet front and made turning them tough. I enlarged some holes so I could tilt the controls up enough to line up right.

I put the chassis back in the cabinet, reconnected everything and powered it up. Here are some pictures.

Yes we have a raster . Good HV, good raster, good CRT, but I could not get much audio or video. Turning the fine tuning did very little. Seems like a tuner problem.

I unplugged the tuner output and drove the IF with the Sencore VA62 45.75 modulated video and tone. I performed the Sams setup procedure and found the HO cathode current was 210 mA. Sams shows the current should be 190 mA. The horizontal efficiency coil was already adjusted at the dip. I could reduce the current by increasing the HV adjust, but it was 28KV to get it under 200 mA. My line voltage was at 118V, but I have measured it at 125V in the past.

[Zenith6S321]

I powered the set through my variac and tried different line voltages and read the HO current. Here is what I got.

At 125V the HO current was 225 mA. At 115V it was 200 mA. At 110V it was 190 mA. The picture didn't change much. I decided to add some dropping resistors in series with the line input. I used four 8 ohm 20W resistors in parallel and mounted them to a terminal strip and the line fuse on the top of the chassis in an open spot. That dropped my 118V down to 111V which gave me 190 mA HO cathode current. The raster fills the screen and I have 25.5kV high voltage. I'll settle for that.

Dave