1950 Zenith G2350RZ1 "Porthole" Television
Here are some photos of my new porthole set. I have not had time to get into restoring the set yet but when I do I will post some photos.
Feb. 15, 2004: As you can see, the original finish is intact (unlike on the H2250R) but it is not perfect. It did clean up very nicely with some Kramer's Best Antique Improver. I will have to repaint the escutcheon and finding the right shade of maroon is going to be tuff. I did bring the set up on a variac and could get a horizontal line meaning that something is wrong with the vertical sweep circuits. The metal 12UP4 picture tube tests a little weak on my B&K 440 but I think it will do. I do have a spare 12LP4 glass tube I could put in it (with a few modifications) if I need to.
Apr. 27, 2004: I pulled the chassis out of the cabinet this past weekend and put it on the bench. The chassis was filthy. I took the CRT off of the chassis (after discharging it) and pulled all of tubes to clean the chassis. I took the chassis outside to brush off most of the dust with a paint brush then went over it with a mixture of hot water and Simple Green cleaner on a scouring pad. That allowed me to get the grime off. I couldn't get into some of the tighter places with this method but it still looks much better than it did before.
I had previously brought the set up on a variac and was able to get a raster on the screen. There was only a horizontal line so the vertical sweep is out. I didn't leave the set on for very long for fear of burning a nice line in the phosphor of the screen. When I pulled the chassis out when I first brought the set home I hadn't noticed that the picture control switch, the one for changing between a 3x4 picture and a big round picture, was attached to the escutcheon. The H2250R wasn't that way. It had the switch attached to a bracket on the chassis. This set does have the bracket and the service manual shows it as being attached to this bracket. Anyway, when I pulled the chassis out I also pulled this switch apart. I tested the set without fixing this switch. I got to thinking that maybe I wasn't getting any vertical sweep because of this switch so I repaired it. I put the CRT and all of the tubes back in the chassis, plugged it in and turned it on. No dice. Still a horizontal line.
I do plan to perform a complete recap of the set but I think just for fun I will troubleshoot the vertical section to see if I can find the problem. BTW, I purchased a Zenith service manual for this set from ebay. Or at least I thought it covered this set when I bought it. What I bought covers sets that use an auto-trasformer in the power supply and series tube filaments. The tube compliment is exactly the same as what I have except for higher filament voltage equivalents of the tubes in some places. I dont' see any differences in any other components or the circuit, except for the filaments. I do have a schematic from the Sams Preliminary Data (Set 91A, Folder 13) for this chassis (23G24).
Here are some photos of the chassis before I cleaned it:
April 30, 2004: I found the cause of the lack of vertical sweep to be a couple of bad capacitors (what d'ya know!) in the vertical oscillator circuit. I can now get a full raster on the screen. However, the raster is very dim. I have put the CRT on my CRT tester and I think it may be testing weaker than it had before. I have even upped the filament voltage from 6.3 to 8.4 and let it sit for about 24 hours with minimal results. I think for the meantime I may put a half-wave rectifier and cap across the filament supply to to supply a higher DC voltage to the CRT filament.
I also had noticed that the fine tuning control is missing. The shaft for the front panel knob is present but the shaft that goes into the tuner and varies the capacitance across the oscillator coil is missing. I'll have to see if I can fabricate a new one.
May 16, 2004: I have been working on recapping the chassis. There really aren't a whole lot of paper capacitors in this set. There are about 25 paper caps but the most difficult thing is finding places to mount the new electrolytics. I have one electrolytic can completely replaced using a terminal strip with the mounting tab soldered to one of the old can terminals. I have come across a few out of tolerance resistors but not too many. There are a couple of "sand-ohm" resistors in this set that I'm not too keen on. I'll try to find replacements for those. I have had one fail in the other porthole I restored. That one fed the screen voltage for the audio output tube and caused the sound to be distorted. I've noticed one of these resistors has some oxidation on its leads going up into the body.
I purchased a couple of NOS flybacks from ebay the other day. They arrived yesterday. It turns out that one of them is an exact replacement for the flyback in this set. The original flyback does not look too good. It has quite a bit of its original wax coating falling off. I will pull it out and recoat it. It might be fine but it is always nice to have a spare just in case.
I also stripped the escutcheon yesterday and will be priming and repainting it soon. Here's a photo of it stripped:
May 19, 2004: I continue to recap the chassis. I have just a couple of paper caps and one electrolytic can left to do. I mentioned that there are a couple of "sand-ohm" resistors in this set that I don't trust. One supplied plate and screen voltage to the audio output tube, the other is in the low voltage supply. The plate supply resistor is a 3.5k 10W unit which measures 77k. The unit in the power supply is 400 ohms but measures about 475 ohms. That's close enough but I went ahead and replaced it anyway. The 3.5k unit I replaced with a 3k 20W wirewound resistor I had. Below is a scan of the two original resistors.
The one on the left is the plate supply resistor and the one on the right is the one that was in the power supply.
There's one paper capacitor hiding behind the flyback that needs to be replaced. I will have to remove the flyback to get to it and it looks like to do that I will have to unscrew the main power transformer. I'm not looking forward to that.
By the way, in case your interested here is the schematic.
May 24, 2004: I have one more capacitor to replace. It was buried under the flyback which I have now pulled out. I will melt the original wax off and recoat it with some RTV silicon sealant.
I found a maroon spray paint that is the closest I have found to the original color of the escutcheon. It is Dark Garnet Red automotive paint made by Dupli-Color. It isn't as dark as the original but it looks better than other paints I have tried. Here's the repainted escutcheon.
So, I have the cabinet all ready to re-install the chassis when its done.
June 2, 2004: Success!! I now have a full bright raster (with a brightener) and can get a picture displayed. I finished recapping the chassis, recoated and reinstalled the flyback, cleaned contacts and controls, checked the tubes, and replaced a few resistors. I am unable to get sound,though. I can get a buzz which is more than I was able to get before I replaced those two "sandohm" resistors.
June 9, 2004: I found the cause of
having no sound. It turned out to be a peaking coil feeding the
plate of the intercarrier sound amplifier. I can get decent sound
but I can't get both a good picture and good sound. I need to
fabricate a new fine tuning control and align the IF. I decided
to shelve the set until I can get a control made. Until then I
have reinstalled the chassis into the cabinet so it doesn't look
so bare, and to regain some room in my apartment.
January 20, 2008: I finally
got back to working on this set. I moved into a new house about 3
years ago. I built a larger bench this past summer so that I
would have the room to work on TV chassis'. So here is what I
have been doing to this set. I pulled the chassis and put it on
my bench to check the IF alignment. I pulled the 12UP4 metal CRT
and replaced it with a 10BP4 glass CRT so that I wouldn't get the crap
shocked out of me while adjusting the IF (for some reason Zenith put
the IF line almost right under the CRT). While I had the set on I
noticed that I kept having blooming of the raster when I adjusted the
brightness control. I measured the high voltage and it turned out
to be a little low, about 7kV instead of 9.5kV. I traced the
cause to be a 560k resistor in the plate of the horizontal discharge
tube that was reading about 1meg. Replacing this resistor brought
the high voltage back up and stopped the blooming. Next I noticed
that the picture was really snowy. I thought that maybe this was
due to the IF being out of alignment. I performed an alignment
but it didn't help the picture. Maybe the problem lied in the RF
section. I thought that I would try swapping out the RF amplifier
tube (a 6BJ6) to see if that helped. Removing the RF tube didn't
really make much difference in the quality of the picture nor did
putting a NOS tube in its place. I removed the cover to the RF
shelf and started checking voltages. The plate voltage was
reading about 154V which was a little higher than the 132V shown on the
schematic. This might have been OK but the screen also had 154V
on it when it should have 100V. I started looking at the circuit
to see if a component could have failed that would cause the screen and
plate to become shorted. I could find no single component that if
it failed would create this situation. I measured the voltage that is
feeding botht he plate and screen. This was 154V. Ah
hah! The tube isn't conducting, but why? I looked at the
cathode circuit. The cathode of the tube (pin 2) is connected to
ground through an 82 ohm resistor. I went to check to see if that
resistor was OK but what I found was absolutely nothing attached to pin
2. There wasn't even any solder that would suggest that something
had been attached to it at one time. This set left the factory
with the cathode resistor missing from the RF amplifier.
Installing a resistor improved the picture by quite a bit. I
still have what appears to be sound bars in the video so I will check
the alignment some more.
March 15, 2008: I finished the alignment of the IF and RF. I had picked up a Heathkit IG-57 post -marker / sweep generator and used this to perform the alignment. One of the drawbacks of this generator is the lack of a sweep output control. The generator originally would have come with a step attenuator used for this purpose. I improvised and fed the output to a 10k potentiometer to vary the signal going into the TV. This generator has crystal controlled marker oscillators to generate a number of markers at the same time. However, a couple of the frequencies I needed are not available on this generator. I had to feed an external marker from a signal generator into the IG-57. Here are a couple shots of the IF response curve. The first photo shows the corner markers at 43.5MHz and 45.0MHz. The second photo shows the picture carrier markers at 42.75MHz and 45.75MHz. I could not put all four markers on at the same time without generating a multitude of secondary markers created by the markers hetrodyning together.
The RF oscillator was tweaked to get it at the right frequency for
channel 7 as the service manual says to do. All of the other
channels were adjusted for best picture and sound using the "Bulls-eye"
adjustment on the front of the tuner. I was also able to adjust
the quadrature coil and buzz control to get good sound with almost no
buzz at all. I have heard that these Zenith sets with the 6BN6
quadrature detector always have a problem with buzz in the audio.
Now I plan to re-install the 12UP4 CRT and align the picture.
Then I will bench test the set for awile to make sure everything is
working fine before I put it back into the cabinet.
This site created and maintained by Sean Barton.
Last updated 3/15/2008.
