Admiral 26R12 restoration

[Glenz75]

You too huh? I seem to be having a run of high resistors lately...Maybe they're conspiring against us?

Originally Posted by bandersen

Something else I'm working on is the washed out picture. At first I thought it might just be a worn out CRT, then I got a scope on the 6AC7 video output tube.
It turns out the signal on the grid is about 33% low and so is the output going to the pix tube. Now, I'm going through the IF stages hoping to boost the gain.

As with every Admiral I've worked on lately, most of the resistors are way off value. This 1K resistor is measuring almost double that.

[Electronic M]

Originally Posted by Glenz75

You too huh? I seem to be having a run of high resistors lately...Maybe they're conspiring against us?

And so the resistance movement has begun!!!!!!

LOL.

[bandersen]

I'll get those resistors in line!

As for the IF alignment, I tried two different sweep generators and what I'm seeing just doesn't make sense. The gain is lousy and the response curve is nothing like it should be. I'm starting to think I might have a bad coil or cap in there somewhere. I still have the set I just restored with the same chassis so next I'll compare them side-by-side.

I also cleaned out the HV cage. Pretty nasty in there.



Terrible idea to put the AC power plug so close to the fylback
Amazingly, the fuse is actually the correct 1/4 Amp. I usually find one more like 10 Amps

[kvflyer]

Originally Posted by bandersen

...

Terrible idea to put the AC power plug so close to the fylback
Amazingly, the fuse is actually the correct 1/4 Amp. I usually find one more like 10 Amps

The porthole Zenith that I have (still in queue) had exactly that! A 10 amp fuse in the Horizontal circuit. Needless to say, the flyback looked like a burned pork chop. I was lucky to find an NOS Merit thanks to one of the guys on ARF.

[dieseljeep]

I remember reading one of the production changes in the chassis series was including that fuse to satisfy U/L requirements.

[Mars777]

Wow!

I would imagine, from the factory, that the wires were dressed a little better than that! At least I hope so. Does the other duplicate chassis look the same in that area?

Anyway, I zoomed in via flicker. The outer flyback coating looks like it is completely "punched through!" Are you sure the outer windings are 100% OK. Did you test it with your EICO flyback and yoke tester? I think that you said that you had... It wouldn't have taken much more abuse before that flyback ended up as toast!

[bandersen]

I slid some heatshrink tubing over the AC wires and pushed them out of the way. It seems to be OK. I've had the set playing a few hours with no HV problems. I should melt some new wax over it just to be safe.

Here's a production run #5 chassis (this set is run #2). Copper plating, but same bad design.


Finally, by production run #14 they had the sense to relocate the AC plug.

[holmesuser01]

I hope you have decided against rejuvinating the CRT for now. You could easily kill it. I've got a CRT rejuvinator/tester, and have never used the rejuv feature with it. I've never seen a rejuvinated tube that lasted more than a few months at the most.

Your set is looking fantastic! I'm really enjoying the progress you are making!

Bruce

[bandersen]

Thanks. No, I'm not going to rejuv. this picture tube. It's very watchable if you dim the lights a little.

My Sencore CR70 has an auto-restore and several rejuvinate functions. I've only used them on CRTs that were really weak. It helped a little - couldn't say for how long though as I don't play my sets that much.

I received my filter parts over the weekend and finally had a chance to try it out tonight. Here's where I wired it in.
I'll do a neater job once I'm done tweaking it. I figure I can use one of the unused damper socket lugs for a tie point.


It definitely reduces, but doesn't eliminate the ringing. I'll try tweaking the capacitor value to reduce it even more.


Unfortunately, the bars are still there but not as bad as before I think.


I also tried several different 6W4 and 6AX4 damper tubes with little change.

[bandersen]

I re-read those Admiral servicing notes more carefully and discovered that the filter must installed inside the HV cage not under the chassis. I'm not sure if that affects the circuit or it's a safety issue, but I went ahead and moved it. I mounted a terminal strip to an existing hole and soldered in the components.


I also tweaked the capacitor value. I figure the ripple is a harmonic of 15,750 Hz. The closest to what I saw on the scope are 173 and 189 kHz. A little experimenting found that 189 kHz was correct so I went with about 5900 pF in parallel with the 120 uH inductor.

Unfortunately, while mucking around in the HV cage I managed to snap off a flyback lead. I melt back the wax a little and managed to solder it back on then coated it with corona dope. Seems to work fine after 10+ hours of playing.


Then on to cleaning out the tuner and going through the alignment procedure.
I found this date stamped inside the tuner.


Here's the IF response.


I spent a lot of time fighting with the focus coil, centering lever, ion trap magnet and yoke. It's not perfect, but certainly better than what I started with.


Here are a couple screenshots. You can see how the bottom right-corner bends up a little.
Another issue is an occasional hiss / crackle in the audio. it sounds just like a HV leak, but I can pull the HO tube and the hiss remains

[Glenz75]

Those pictures looking pretty stunning Bob Are those bars still there even after moving the modification to the high voltage cage area?

[bandersen]

Originally Posted by Glenz75

Those pictures looking pretty stunning Bob Are those bars still there even after moving the modification to the high voltage cage area?

Thanks. I guess if you have to ask then they're gone
Seriously though, they are still there but much fainter than before. Relocating the filter didn't have much effect but switching from 6800 to 5900 pF did. I put a couple caps in parallel (5600 + 300) to get that oddball value.

[kvflyer]

It is about time you got back on this TV! What's going on, did you have a holiday or something???

Seriously, looks good and I am glad that you have it going so well. Please keep us posted Bob...

[bandersen]

Lol - I actually have been working on it every day. Easily the most frustrating chassis I've worked on to date.
Now, I need to go back to the last one I restored and try installing the filter. And I have two more sets with the same chassis to restore some day

[bandersen]

I'm still chasing down the hiss and squeal in the audio. I pulled all the tubes so I'm down to just a functioning tuner and sound output. The squeal remains.
My next thought was bad mica caps, but they all appear to be ceramic. Even so, I'm going to test each one.