Another Zenith "bugeye" restoration

[bandersen]

Yes, I'm working on another bugeye set. This time it's a 16Z25 chassis and a 14XP4A CRT.





There are some odd old repairs. Like two fusible resistors where only one is needed.
Also found a ceramic also in the AGC/IF shorted to ground and resistor added in parallel with the HOT bias resistor. I guess someone was trying to improve the brightness or contrast ?




I restored the circuits to their original values and wiring using a Sams and another, restored 16Z25 for a reference.

After a quick recap, I bypassed the CRT filament with a power resistor, popped the HOT cap off and powered it up. B+ seems fine, but I only get sound for a few seconds before it cuts out.


I figured it's and AGC issue and tried adjusting the AGC control but nothing happened. I unmounted it for a closer inspection and found it's shot just like in the other bugeye I'm working on.


I temporarily tacked in a new control, but it made little difference.
Next, I did a resistance check on the tuner and IF and sound tubes and compared to the table in Sams. The only odd values were on pin1 of the RF tube and pin1 of the 1st IF tube. Both are about 40% high and part of the AGC buss. I'll start check the associated resistors.

[bandersen]

It took a while, but eventually, I tracked the problem down to this 3.3M resistor that had gone open. I took a while because I tried measuring it in circuit and by coincidence it measured about 3.3M across it.
After exhausting other possible bad resistors, I came back to this one and disconnected one end. Sure enough - infinite resistance. That got me good sound, now to try for some video.


First though I had to replace a busted brightness pot


Then using an 8XP4 test CRT only produced a faint, ghostly image.


Next I'll try the 14XP4.

[dieseljeep]

Regarding the CRT heater, I just used to use a brightener to complete the circuit. I would also try the brightener on the test CRT to compensate for the heater current difference. It might be a little more voltage for the test CRT, but it's only for a short time.

[bandersen]

Great idea! I'll have to remember that.

[Glenz75]

Hi, you seem to get getting through these Zeniths at a good pace. At least servicing and access is easy. It seems those pots are the biggest headache with that chassis. I've struck that phantom resistor syndrome before where across the open resistor it reads roughly the same... But having the identical resistance would have been a pain though... These things are sent to try us...... This chassis looks like its coming back to life Cheers Glen

[bandersen]

Yes, these are much easier to work on than the Predictas. Only took a couple hours to recap

I remounted the 14XP4 and added a little aluminum foil to bridge an area where the aquadag had worn off,


I remembered to position the width sleeve correctly this time


After some ion trap magnet fiddling, I got an image, but the vertical hold is nonexistent. Time to hunt for more bad controls, resistors or tubes.

[dieseljeep]

Do both of those sets belong to the same owner?
They were both in really rough condition.
I had a set like that only 50 years ago. Mine had the handle that folded below the front bezzle. The set worked perfect and had UHF.

[bandersen]

Originally Posted by dieseljeep

Do both of those sets belong to the same owner?
They were both in really rough condition.
I had a set like that only 50 years ago. Mine had the handle that folded below the front bezzle. The set worked perfect and had UHF.

Yes, and a 3rd as well that was in the best condition of the three. I got that one running without much trouble.

As for this set, I found a fried resistor in the vertical circuit. I think it fried because I didn't install the focus strap correctly and put 245V across it.


I figured the focus (pin 6) could connect to either pin 2 or pin 10 (whichever gave the best focus) and had it going to pin 2.

However, the schematic shows it going to pin 10.


That got the vertical working better, but it was taking a long time for the oscillator to startup. I went to swap out the 8CM7 and discovered I had installed a 6CM7 by mistake


It's working much better now

[josephdaniel]

I have a set like this that I may get around to fixing one say soon it needs filter capacitors that I never got to changing.

[Reece]

Great picture now!

[bandersen]

I picked up a B&K 415 a couple months ago that was barely if ever used. This seemed like a good opportunity to finally fire it up.


It works


Zenith aids alignment by making all the key connection points readily accessible. After a bit of twiddling coils, I got the response close to what was shown in Sams.


The picture is noticeably sharper and sound clearer Here it is hooked up to my modified Sencore VA62.


Here are the multi-burst and crosshatch patterns. The two rightmost bars in the multi-burst are 4.0 and 4.5 MHz. That's beyond the bandwidth of this B&W set. I'm curious to see how they come through in a pre-color set like the 630TS.


Here's some shots of MeTV. The closing credits for Combat! make a good test pattern too


All that's left now is to put it back together

[marty59]

That Pic looks great! Just how off would you say that the IF Alignment was?

[bandersen]

The video response was lopsided and shifted about 0.5 MHz when I started. Also the audio was a bit weak and had a buzz.

[Sandy G]

The amount of technical know-how, & the ability to ferret out intractible problems by the membership here NEVER fails to AMAZE me..You guys are WIZARDS...

[bandersen]

Thanks! I used a little Meguires wax on the cabinet and cleaned the safety glass. Now, all it needs is a couple metallic knob inserts for the front controls and a few more of these little knobs for the top controls. I'll dig my big box o' knobs out the the attic and might get lucky.