VT-71 "upgrades and mods"
 

This is my first post on this forum. I have been an electronics engineer/technician for about 30 years. Most of my experience has been in pro audio but I have worked with a lot of vintage gear and a few antique radios over the years.

One thing led to another, I bought a VT-71 to restore, then another one, now a third one. (Two will be gifts to friends.) After doing a total recap, building a retrofit ballast etc. and getting them in more or less like new operating condition, I started thinking about some mods and upgrades.

The main things were adding 3 RCA jacks to the rear panel for direct audio and video inputs (along with a switch to select between the internal tuner and the input jacks), and changing the horizontal AFC circuit to make the set compatible with VHS tape players.

(Isolation transformers are hardwired to these sets, so there is no problem with patching the new RCA's to external electronics.)

After spending way too much time with my head buried in old Motorola schematics I finally got the mods working perfectly. I'll be glad to share the circuit details with anyone who's interested.

There's one more thing I'd like to do, and that's find a way to eliminate retrace lines. Does anyone have any thoughts about how this could be done on a VT-71? If so maybe you could save me some time.

I realize that many people who restore old electronics feel that vintage designs should be left as is and I generally agree. But the VT-71's and VT-73's are so common it's hard for me to see this kind of thing as a historical disservice and besides - the mods I did make the picture look really, really good. :cool:

I would be interested in any technical insights on these sets or any sort of "hotrodding" that others have done on them. Thanks in advance.

Welcome to AK!
I have read your posts over at A.R.F. and they are quite interesting.

I have several of the Motorolas and a couple extra chassis that I wouldn't mind trying your mods on, I'm not a tech though so you will have to speak slowly :p:

I'm sure many of us here would like to hear and see the details. :D

Eric

Welcome aboard out merry ship of misfits! I for one am all for hot rodding mods as long as they're reversible.....like installing disc brakes or electronic ignition on an older car....not readily seen but improve performance and are easily changed back to stock.

IMHO some more undercover mods can be quite tongue in cheek. Walk softly (quiet exhaust) and run a big block LOL!

Anthony

Show us what ya got! :D

I've got a VT71 that I'd love to improve beyond its present state of performance :)

I wonder if there is really any visible difference in the picture quality with direct video input, assuming the IF is aligned properly. The small screen size makes it difficult to resolve small detal.

Thanks for replies
 

OK guys, give me a couple of days to organize my notes and sketches and I will post the details of the changes I made to these sets.

Steve, by using a satellite box or VCR and feeding the same program to the antenna terminals AND the video input, it was easy to compare the two modes by using the Tuner/Video switch. The video input looks better, though it's hard to quantify the difference.

Even though I aligned the RF section very carefully, I guess I'd describe signal through it as looking just slightly "blotchy" or having a tiny bit of herringbone (maybe caused by neighboring channels), compared to the composite video in.

Using the video in you can more easily count the horizontal lines on the screen and everything looks a bit more defined.

The other thing is that with the tuner, I always had to fiddle with the fine tuning control to get the best combination of sound and picture and it was sometimes hard to make both perfect. With the direct ins, that issue obviously goes away.

Another factor may be that DC level is correct using the video input, because it is *almost* direct coupled to the picture tube (after passing through a small video amp module.) This would make the grey scale more accurate wouldn't it? I'm not sure but I don't think the VT-71 has any kind of DC restoration.

Also the audio is generally better. It should be, since all that's being used in the audio section is 1 1/2 tubes!

David