1963 Zenith 29JC20 restoration
 

I picked up this Zenith 29JC20 last week at the Richmond Antique Radio and TV museum estate sale. My hope is to restore the set and use it daily. Here are some pictures of it before I get started. It's PVA has started turning green. I may attempt the cataract removal process if it affects the picture much. The cabinet is a little rough but not too bad. The service tag (PSI?? weight??) lists a bunch of service in the late 60's. The Picture tube is a 21FJP22A rebuild dated late 1973. Looks like its got the same tie-strap around the cloverleaf that I read about on other 29JC20s. Looks like some recent replacements of a few capacitors with orange-drops and only two small electrolytics. First I will check the picture tube with my slightly flakey CR70. Then I plan to shot-gun the old caps, the electrolytics, replace all the off value resistors, check the coils, and replace the bad tubes before I power it up. I am curious how this narrow band chroma set with RE phosphors looks compared to my 21CT55 with its 21AXP22A (my avatar/signature).

My very first concern with that set would be the integrity of the plastic/(nylon?) coil form of the H efficiency coil, which tends to crystalize over time and fall apart. If that happens, the requisite 'dip' in cathode current of the H output tube is lost, allowing the current to soar 'waay over spec.

Very nice.. These Zeniths seem to be popping up lately.. I wonder how the CRT is..

I'm going to be getting one in a few weeks, so this should be an interesting thread.

I'm guessing that the tag *actually* belongs on a fire extinguisher. :scratch2:

Picture will be brighter for sure, but will you enjoy the colors as much as your wonderful 21CT55? Be interesting to see side-by-side photos when you are finished restoring the Zenith.

jr

Zenith sets do tend to have rather sharp monochrome detail, focus, and contrast...It can be so good that one can still think the picture looks good with moderate tint and gray scale miss-adjustment....Granted later roundys and rectangular Zeniths are what I'm familiar with. The only RCA based set that could hold a candle to it is my CTC-4, but the 4 don't get good color at the same fine tune point as monochrome, and the sound is best at a third spot so it could be an alignment fluke.

At the moment mine is still intact. Is there some way to visibly see that it is crystalized? On other sets where the ferrite cores have been tight, I have let them soak in a little WD40 which seemed to ease their movement. Any suggestions for this coil?

I used a hair dryer to loosen them, then some gentle prodding with the twiddle stick is usually all it takes.

:banana: Apparently the CRT emissions are good! :thmbsp: I coaxed my CR70 into working. My CR70 was tripping its power switch/breaker when I had this CRT plugged in and all set up. I found that turning the filament range setting all the way down, switching it on, and then bringing up the filament adjust to the 6.3V kept the switch from cutting out. Guess I should re-cap the CR70? Anyway it showed good emissions on all three guns, even with 5V on the filaments.

I am guessing that the more narrow chroma bandwidth will just mean a lower color resolution than the wideband 21CT55. It interesting that the current crop of 4K displays are using this same high-res luminance + lower-res color to lower the bandwidth to allow HDMI 2 to drive 4K:
http://hdguru.com/hdmi-2-0-what-you-need-to-know/

I think its based on the human eye seeing B/W in higher resolution than color.

Dave

I will definitely give that a try. Gentleness counts on these old sets.

Dave

From the photo, it looks like the coil form is phenolic. If so, you're in luck. The whitish, semi-clear plastic forms are the ones that crystalize.
If the three blue adjustment coils on the convergance board also have phenolic forms, you're doubly in luck.

That's exactly right. That's why earlier hi-resolution chroma was fudged to simply 'paint' color in the BW image.

Yes the convergence board coils also look the same. From a couple of tags on the set I think it was made in 1963. Maybe Zenith changed to phenolic coil forms by then.

It is the other way around. The phenolic forms are the old type, and Zenith moved away from them to the notorious plastic forms that they had from the mid 60's to the late 70's.

I tested the coils today and I think they are all ok. There is some wax on the bottom of the HV cage but the flyback otherwise looks and tests good. I'm not sure how concerned to be about the amount of leaked wax. The flyback shows 16 rings on my Sencore LC75. The 3A3 plastic socket cup is broken but I think it might be fixable. The cloverleaf has split and looking back at the service tag it lists a 43-473 which is the part number for the cloverleaf. So this set has had two of them and I guess both broke. I wonder if separating the cloverleaf into the three sections at the place where the plastic springy area is and then using an actual metal spring between the section could work.

I spent most of the day checking the resistor values and replacing the ones I thought were too far off. The Zenith solder cups makes it easy to lift one end to check values. I replaced all the resistors that were beyond or close to their tolerance band allowance. That ended up being 40 resistors. I like to replace resistors with higher wattage replacements, like 1W where there were 1/2 W. Next are the old non-ceramic capacitors and then the electrolytics.

Out of curiosity, how many resistors were bad and by how much (worst case)?

I also have one of these sets, so I'll be watching.

I notice two holes punched in the tuner bezel... Does this have an optional UHF Tuner? If so, I've never seen them arranged side-by-side on a Zenith roundie... Usually it's diagonal.

Worst case resistors were (Sams numbers) 470K ohm R184 was 950K ohm, 470 R82 was 1084, 22 R39 was 36.1, 22M R129, R130, R131 were 30.3M, 30.2M, and 32.5M. The rest were over, at, or just under their tolerance above their design value. I wanted all of the resistors to be within their tolerance, figuring they specified that tolerance for a good reason. I did find one bad factory solder joint on R158 that made it read open when checking between leads of the parts it connected to.

Yes this set has a UHF tuner which mounts to the right of the VHF tuner (when facing the front of the set).

Dave

Today I replaced the caps that were not mica, ceramic, or electrolytic. The Sams photos show bumble-bee (black beauties?) where my set had mainly Sprague orange-drops and brown-drops plus a couple of other tubular caps. I think the cleaner looking orange-drops were recent replacements as most of the orange-drops had a dirty film on them. Most of the orange and brown drops had leakage of around 1-15 uA at their rated voltage and some measured outside their uF tolerance, .0068 C46 was .0087, .15 C6 was .206, .056 C129 was .0647, and a couple caused my Sencore LC75 to not read properly even when out of circuit. I replaced them all with in tolerance valued capacitors, not wanting to trust 50 year old caps.

I started the first of the three multi-cap electrolytic can capacitors and found that the section labeling (square, triangle...) on my capacitor can does not match the Sams, but the chassis was wired as the Sams shows. So my capacitor values do not match those shown on the schematic or the parts list. I don't think this is a Sams mistake, for the following reason. The Sams schematic shows C3C as a 4 uF 150V cap with a blank label marking. The Sams shows that section wired as the cathode bypass cap of the V7 AGC tube with 35V across it. My set does have the V7 AGC tube wired to the blank labeled section, but in my can that section is a 40uF 25V cap. Which puts a 25V capacitor where Sams shows 35V. Here is what the Sams schematic and parts lists as the sections and what my can has:
Section Sams My set
A (square) 50uF 450V 20uF 450V
B (triangle) 40 uF 25V 4 uF 150V
C (blank) 4 uF 150V 40 uF 25V
D (1/2 round) 20 uF 450V 50 uF 450V


Ok, what am I missing here? Is it just a Sams error? But if so, then why does my set have a 40 uF 25V section connected to the V7 AGC cathode which Sams shows at 35V, unless that Sams measurement is wrong....

Also, Sams shows the 50 uF on the screen of the audio output tube, but the 20 uF on the power input to the vertical output transformer. Wouldn't it make more sense if the vertical output transformer power had the 50 uF and the audio output had the 20 uF? Should I just wire it as original?

Dave


Dave

I bought a Zenith Color TV Service Manual VOLUME ONE (1973) in hopes it might cover the 29JC20 and have a schematic I can compare the Sams to.

I was able to fix one of those broken plastic cups with some flex zap (a CA glue that has modifiers in it to make it less brittle.

Its still not as strong as an unbroken one but its ok, just have to be gentle with it.

That Fly looks fine.

That's a relief about the flyback. I already put some Loctite plastic bonding epoxy on the broken HV cup. As long as I never pull the rectifier out I think it will hold...

Now there is a part that would be a prime candidate for 3D printed reproductions. It's small and the originals were plastic anyway.

Yeah a 3D printer would be very handy to make one of these. I wonder if the 3D printing materials would hold up to high voltage?

Well now. I am a field tech for a company that sells and services copiers and network printers. It just so happens that we are going to start selling 3D printers and they will be sending us one for the showroom, as well as sending someone to train us to service them. Don't know when, but this might be a doable project. I will post later when we get it later this year. We have to learn to print something, might as well be a vintage tv part.

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

If you don't let it print hollow, it should...If not you can always thicken the walls in a cad program, and try again. Those convergence coil frames that like to crack are also prime candidates.

I've noticed all three of these I've seen pop recently, the clover leaf assy are broken..

I spent an evening checking the values and wiring for the parts I replaced. I'm glad I did, as I found one wrong value resistor, 1.5K instead of 15K, and two other resistors each with one lead to the wrong solder cup. All the caps values and wiring were ok.

I got the Zenith Color TV Service Manual by R. L. Goodman but it did not have a schematic for the 29JC20. I did have the 27KC20 which has most of the same circuitry for the electrolytic can I was concerned about. The 27KC20 schematic, as well as some information from the Oct 62 Radio Electronics page 71 showing the vertical supply cap as 20 uF, confirmed the Sams values for the four sections of the capacitor. My set was wired as the Sams schematic and parts list section markings, but my cap has the sections with different markings. Here are pictures of the Sams parts list markings of the sections and a picture of my can. You can also see that the 22-3632 can part number matches the parts list Zenith part number. I think my set was wired wrong from the factory. Its now wired as the Sams schematic shows.

Next I'm gonna check the tube socket resistance measurements to see if I messed anything else up.

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

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.

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.

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 :thmbsp:. 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.

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

I would not be surprised if the original 6BK4 was good. I've seen them work when they had much more x-ray blackening. X-ray blackening is usually a function of run time so I'd guess you have a low hours set. :thmbsp:

When I first tested the set, the fine tuning did not have much effect. I removed the tuner side cover and saw that a plastic gear engages a fine tuning screw on each channel coil insert. The plastic gear had split down its length. A goggle search for 14 tooth 1/4" diameter with a 1/8" inside diameter 3/8" long gear shaft turned up everything but what I needed. I could epoxy it, but that would probably fill up the gear teeth which might cause more splits. If I could find the right inside diameter c-clip/cir-clip I might be able to put it around one end of the gear and fit it into the tuner, but there is not much room in there. I noticed that only about half the length of the gear had dark grease marks on it from engaging the tuner screws. So I'm going to experiment with using some heat shrink tubing to squeeze the gear at the end that does not seem to engage the tuner screws. Here are some pictures of the split gear, the gear with two layers of heat shrink on one end, and with it mounted back on its shaft. The shaft has teeth that engage the inside of the plastic gear. The shaft is driven by the fine tuning knob and the mechanism moves the gear to engage it into the tuner screw teeth. The gear now fits snuggly on its shaft and works surprisingly well, for now...

Dave

I made an attempt modify the broken cloverleaf to get the convergence coils closer to the right positions. I cut the cloverleaf coil support section apart and put springs between them. The put it back together, but its really too floppy and needs more work. I wanted to make an initial attempt at getting some kind of picture. I quickly set purity and convergence. The tuner now sort of works. Here are pictures of the cloverleaf and the first pictures using channel 3 RF. When the set is first turned on I get some color but it quickly fades away after a few minutes. There is also quite a bit of ringing in the video when there a large image contrast changes. I will take a look at it on my scope. Probably will need to do an IF alignment, or maybe its just a wiring mistake I made.

Dave

I've fixed mucho many of those FT gears that 'd split. Just clean the shaft and gear thoroughly of all grease. Then apply epoxy sparingly to only the splined area of the shaft using something like a toothpick, and slide the gear back on. Never had one fail.

Re. the cloverleaf, back in the day there was an aftermarket item for that, consisting of a bungee sized to fit snugly around the outside perimeter. What you're doing is just as good, probably better.