Motorola 7-VT2 worth restoring ?

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Originally Posted by jeyurkon

Yes, that should be a coax on the contrast control. I have one on my TS-18A. There should be a dot where the wire from the brightness crosses also.

The Rider's is wrong. I've attached a scan from an original Motorola schematic.

John

My scan came from a Wallace's Telaide 1946-1951 volume one. Who knows where they copied it from. Those dots sure make a difference

That's some cool looking vintage coax in your set. I'll see what I can scrounge up.

Look how simple that contrast control was in the earlier TS-4 chassis. I think the extra components in the TS-18 are an attempt to suppress the vertical retrace ?

[bandersen]

Thanks for the help guys. I've got that control wired up properly now.
I used to not check lower value resistors all that often, but this set has changed my mind. Check out these.

This should be 82 ohms.


This should be 6.8K.


Ugh, more ooey, gooey capacitors Did they really need to use that much tar?

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Nah, it's not that bad. A little solvent (mineral spirits, lacquer thinner, etc.) will clean them right up

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I'm down to the sweep and HV sections and have a couple more questions for you guys.

The vertical sweep is missing two tubular ceramic capacitors. Here's what they look like from another set.


One of them had been replaced with a little disc cap and the other is missing entirely.


Working from the schematic, they don't appear to carry HV DC.
Is there anything special about those caps or can I use modern mica or disc ceramic replacements?





Finally, I need to rebuild the flyback.


Originally, it used a 1250pF mica cap like this photo from another set.


I dug up this more modern 1258pF mica cap. I'm not sure if that's close enough or the voltage rating is high enough.


Here's the schematic.


Interesting how the cage is now grounded whereas it's floating with a 470K to ground in the older TS-4 chassis.

[Eric H]

Unless there's a known problem with the ceramic and Mica capacitors I wouldn't change them.

They are unlikely to go bad in most cases and the values are usually critical.

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I agree, but they're missing from this set
I'd estimate about 20% of the components including the flyback and cage were missing overall. Seems someone was using this as a parts set

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I dug up an old copy of the ARRL Handbook and determined that the flyback mica from another set is 10% tolerance @500V. So I think my replacement will be OK.

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I pulled a marathon session last night and finished replacing components and rebuilding the flyback. I went with 500V mica caps for both the missing flyback cap and the two ceramic tube-type missing from the vertical amp. Here's a little photo diary I made of my progress.

Just how I received it:


Rebuilt tuner, pulled electrolytics and fixed up RF amp.


Restored Vertical oscillator and Video amp


Started on Clipper and rear controls


Restored audio amp and restuffed and rewired electrolytics


Finished Clipper and rear controls


Restored Horizontal amp and started on Vertical amp


Cleared out HV caps and flyback area


Finished Vertical amp, replaced resistors and HV capacitors for vertical and horizontal output


And finally, the flyback. I used some nice 8KV HV wire from the same old HP logic analyzer I salvaged for the CRT socket earlier.


Yummy vintage parts. Maybe I should put these on eBay


So does it work ?

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I did some preliminary test for shorts cold joints etc. Then, I hooked up my variac and isolation transformer. I also plugged into a few test sockets. They make it a lot easier to check voltage without poking around under the chassis. I also stuck a dummy load across the 7JP4 filaments.


I slowly powered it up and all seemed well. Voltages slowly increased and the tubes started glowing. At full AC input the voltages were in the right ballpark so I shut it down and hooked up a 7VP1 CRT.
A faint raster appears!


But it's short lived A friend noticed smoke rising from the set. I hit the power and started investigating. Argh. The main capacitor in the voltage doubler was smoking and bulging.


My first thought was the capacitor was in backwards, but it wasn't. It was the diodes. It seems that Motorola had reversed the capacitor and rectifier between the TS-4 and TS-18 chassis. I wired the capacitor by comparison to the original but the diodes by comparing to a TS-4 chassis.
Darn vintage symbols for a selenium rectifier!


Time to do a little repair!

[bandersen]

Ah, that's better. Kinda interesting that a capacitor will work for a while when reversed.
Here it is with the 7VP1


And with a 7JP4.


There's a fade from from top to bottom (bottom to top?). Any ideas?

I'm also not getting any audio or video on either a local station on Ch. 6 or from my Blonder Tongue modulator on Ch. 5.

I'll fire up my B&K 1077B TV Analyst tomorrow and start investigating.

Considering how chewed up this set was, I'm quite happy.

Again a big thanks to Steve for the donated flyback and cage. It's working just fine so far

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I spent a few more hours troubleshooting and found an 82 ohm resistor measuring 200 and a bad connection from the audio output tube to B+. That helped a bit. B++ and B+ and right on the money, raster is very stable and bright. However, no video or audio - just a faint hum. Volume control has no effect.

I left all of the old mica and a few mica-mold caps in place. I'm going to start checking those.

[bandersen]

I discovered a missing capacitor in the audio amp. Now the volume control seems to be working. While working on the set, I got a nice jolt from the B++ cap . Always discharge those caps before poking around!

I also fired up my B&B 1077B and feed some RF into the antenna. Nothing on any channels whatsoever.

Next, I fed a composite video signal directly into the 6AU6 video amp. I detected some signs of life and audio. I made some adjustments to the hold and contrast controls. Here's the best I could get. I've got some major vertical linearity problems. Any suggestions?
I hope one of the HV caps hasn't failed. I'm out of them



So now I know there's something wrong in the RF amp, converter or IF amps. Or maybe my tuner repair wasn't so successful after all.

I'll dig into that later tonight. Gotta watch the Bears game first

[Phil Nelson]

Again, I have to salute your persistence in this type of project. With so many parts missing, etc., I would easily get discouraged.

Strange that a number of lower-value resistors are so far off spec. I usually don't check those, either, unless some clue points to one.

Phil

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Thanks Phil! I think I'm getting close.
I hooked up a scope to the two vertical output plates. One is fine, but the other is clipped.



I've already swapped out that tube with several other known good ones, so I think I can assume it's OK.

Time to double check all the components in the Vertical Sweep Output circuit.

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All the components checked out in the vertical amp which got me thinking about Eric's comment to not replace the ceramic and mica caps.

Since the two ceramic tube capacitors were gone when I got this chassis, I didn't have much choice. I went with some 500V micas - maybe that wasn't right?

So I dug out my unrestored VT73 with a TS-4J chassis and pulled out these two capacitors.


Here's the resulting picture after some tweaking of the controls.


Surprising how much of a difference that made! The vertical isn't clipped anymore, but now I've got a "snake" and ghosting. Regardless, it's progress!

Does anyone know what's special about these caps? According to this great site, the 100pF is +/-10% and the 250pF is +/- 20% both with a temp coeff. of -330 ppm/C.

I would have though that 2% mica caps would be even better?

Unfortunately, I need to put these back where I got them, so I'll dig around my old parts and see if I can find something equivalent.

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