Motorola 7-VT2 worth restoring ?

[jr_tech]

"While taking that photo in total darkness, I noticed the 25L6 driving the flyback glowing blue. Should I be concerned?"

To me, the blue glow looks like one would expect from electrons striking the glass of the tube...it is at the internal surface of the glass, right? and not inside the tube structure between the cathode and plate (that, I might take to be an indication of a gassy tube).

Have you tried swapping the caps back one at a time to determine if it is the 100 pf or the 250 pf That is causing the problem?

Incredible job, I would never have attempted to restore a set that is that far gone...Standing ovation from here!

[bandersen]

Originally Posted by jr_tech

"While taking that photo in total darkness, I noticed the 25L6 driving the flyback glowing blue. Should I be concerned?"

To me, the blue glow looks like one would expect from electrons striking the glass of the tube...it is at the internal surface of the glass, right? and not inside the tube structure between the cathode and plate (that, I might take to be an indication of a gassy tube).

Have you tried swapping the caps back one at a time to determine if it is the 100 pf or the 250 pf That is causing the problem?

Incredible job, I would never have attempted to restore a set that is that far gone...Standing ovation from here!

Thanks! Now that it's actually showing signs of life, it's a lot more fun to work on.

Yes, it's the internal surface of the glass that's glowing. It's the side facing the HV cage. It's a NOS tube, but I suppose it could still be gassy. I'm also wondering if the nearby HV or grounded surface could be affecting it. I'll try swapping it later tonight when it's nice and dark.

Very good point about those caps. I'll try swapping them one at a time. I've got some others to try as well. I did a little thinking while at work today and it occurred to me that the voltage on those caps might very well be exceeding 500v at the peak of the sweep voltage.

Maybe one of them was breaking down and conducting causing the clip with those 500v micas?

[bandersen]

At long last, it's actually working

Here's a really poor quality clip. The battery in my good camera is dead and so am I - it's 4am

So what was the final problem? Well, I dusted off my B&K 1077 TV Analyst manual and put it to work. I already knew if I injected video at the grid of the video amp, I got a picture. So, I worked my way through the Rf and IF stages. That's one of the great things about the B&K - you can generate RF for most VHF channels and the right IF for older sets. Around 23MHZ in this case.

Here I'm poking around in a functioning set for a comparison.


Well, no picture at any of those stages, but I could see on the scope that my signal was making it up to the grid of the 3rd IF amp, but no further. That tube was good and I swore I had check the voltages already so I figured the problem must like inside the demodulator box (dotted line).

Interesting to see (at least to me). I've never opened that box before.


I couldn't find any problems though so I rechecked the 3rf IF amp again. Finally, I discovered a bad 220 ohm resistor to B++. It was open! I replaced it with a cruddy used resistor (can't believe I'm out of decent 220s), powered the set back up and was treated to snow on the screen for the first time.


Hooked up the B&K and got a decent picture. It might not look like much, but keep in mind that's coming from the antenna input.


Hooked up a video modulator and got a decent picture. Finally, I tuned in the last OTA station in Chicago on CH. 6. That's what you see in my video.

I'll tackle the hum in the audio tomorrow. Time for bed

[zenithfan1]

Great job my man! I knew you'd figure it out! Lookin' good!

[bandersen]

Thanks Mark

I've swapped out that 25L6 and the blue glow is gone. I also replaced those ceramic tube caps with some higher rated mica ones.

The video is looking really good now


Audio not so much

Here's a YouTube clip taken with my good camera. There's a brief clip from a Cubs game so I hope YouTube doesn't pull it!

There's a buzzing that gets really bad when these bright white on the screen. I've had this happen with other sets, but nowhere near this bad. The schematic indicates that shielded cable should be used for the contrast control, but my sets just has plain wire. I'll try putting in some coax and see if that helps tonight.

[Sandy G]

Never ceases to amaze me to see one of these old veterans brought back from the dead...You've got some "tweakin'" to do yet, but you're 95% there...Great Job !

[jeyurkon]

Good work!
John

[bandersen]

I finally licked the audio problem by using some shielded cable to the contrast control, aligning the audio coils and using a really good 25L6. I also soldered a little extension on the fine tuning control and grabbed the knobs from another set. I'm still on the hunt for a complete set of gold controls. Anyone? Anyone? Bueller ?

Otherwise, I'm calling this project done. Thanks for all the compliments and help guys

Here's a little video.

Here are some before and after pictures. Ooops I forgot to clean some of that crud off the CRT before installing it



There are a few minor details left like filling in the recessed lettering with gold paint. I'm also going to print out some reproduction labels based on some from other sets. I think I'll try to make a new back and making an "M" logo too.

Time to finish that Admiral 20X122 finally!

[bandersen]

Oh, almost forgot to mention something I've been wondering about while working on these Motorolas from circa 1948. They use a 1N34 germanium diode for a detector. They very new back then - introduced in 1946 by Sylvania. Was this the first set to use them ?

[bandersen]

I've scanned and reproduced the labels for the high voltage cage and channel alignment. Take your pick - faded or new looking, print them out and glue them on

Here's the HV cage 3" X 1.125"
Scanned
Reproduction

Here's the Channel Alignment 4" X 0.625
Scanned
Reproduction

Tube placement 3.25" X 3.25"
Scanned
Reproduction - N/A

Here's where they go:

[Eric H]

Originally Posted by bandersen

I finally licked the audio problem by using some shielded cable to the contrast control, aligning the audio coils and using a really good 25L6. I also soldered a little extension on the fine tuning control and grabbed the knobs from another set. I'm still on the hunt for a complete set of gold controls. Anyone? Anyone? Bueller ?

Otherwise, I'm calling this project done. Thanks for all the compliments and help guys

Here's a little video.

Here are some before and after pictures. Ooops I forgot to clean some of that crud off the CRT before installing it


There are a few minor details left like filling in the recessed lettering with gold paint. I'm also going to print out some reproduction labels based on some from other sets. I think I'll try to make a new back and making an "M" logo too.

Time to finish that Admiral 20X122 finally!

See, just like I told you way back in post #9, this set's not worth fooling with!

[bandersen]

Originally Posted by Eric H

See, just like I told you way back in post #9, this set's not worth fooling with!

That's the best way to motivate me - tell me not to do something

Other than that annoying vertical line on the left, the set has been working perfectly all week.