Back to the VT-71H - Videokarma.org TV - Video

Chris K · #1 02-12-2023, 08:52 AM

Juggling multiple TVs this weekend that I have right on the edge of working. I'm still trying to get this 7" Motorola to produce a picture. I replaced the 1N34 germanium detector diode that seems to be the culprit in most of these TVs with a diode kindly sent to me by Don S. who has used them successfully in his VT-71s. I thank him so much for sending me 10 of these for no charge. Unfortunately, that didn't solve the issue.

I have a nice and bright raster on the screen. I'm trying to see if I can get a crosshatch pattern from my Sencore VA62 to go through the antenna with a 1KHz audio tone. I get sound OK but still no picture, just the raster. I'm not all that familiar with the 62 functions yet but I have used it as a pattern generator on other sets and it works fine. Any suggestions as to the next step should be? I suppose I should start with injecting a video signal through the video amplifier 6AU6 but I'm not exactly what I should try to put through it from the VA62. I've been told this is a pretty good tool and I suppose I should start to learn how to use it. Thanks all.

C

Chris K · #2 02-18-2023, 08:41 PM

The camera can't pick it up but there's a bright 4-3 raster on the screen. I'm feeding a crosshatch pattern with a 1KHz audio signal through the antenna leads. Sound is fine...no picture. I've tried a bunch of different RF levels with no change. Should I start by feeding a video signal through the video amp tube and work my way back? Is that a good strategy? All the caps and any out of tolerance resistors have been replaced and all of the tubes test strong or are NOS. Replaced the 1N34 diode with the recommended substitute. It's a shame I can't get an image on this. Very strong 7JP4...an RCA replacement that's almost NOS as well. If I can get this producing a picture, this will be a really nice piece.

jr_tech · #3 02-18-2023, 10:33 PM

Have you verified that the horizontal and vertical sweep frequencies are correct, or at least close to being correct?

jr

Chris K · #4 02-18-2023, 10:43 PM

No I haven’t. Should that be the first step before I dive into the IF series?

jr_tech · #5 02-18-2023, 10:58 PM

I would check the frequencies when the screen display is messed up...Looks to me like the horizontal frequency is way off.

jr

Gregb · #6 02-19-2023, 09:47 AM

+1 , looks like horizontal is off.

Greg b

timmy · #7 02-19-2023, 04:32 PM

I know with those vt71 sets if the horizontal hold is turned all the way to one side you get lines and looks like a horizontal problem until you turn it the other way try that maybe that’s it. There is also a few mica caps in the horizontal section I had to change because of really bad horizontal issues so look into those. these mica caps look like dominoes. There is a 680,200,900,mmf

Chris K · #8 02-19-2023, 07:37 PM

Thanks Timmy I’ll try that. I didn’t replace any of the dominoes because it’s pointless for most restorations I’ve been told but I’ve never been comfortable leaving old components in these things. Such a shame this is stuck where it is. Very strong CRT and a beautiful cabinet. Sometimes I just think I’ve got a black cloud over my bench. I guess I’ve seen too many videos where people change a couple of electrolytics and the thing fires right up!

timmy · #9 02-19-2023, 07:48 PM

Yes a lot say that the old dominoes mica caps don’t go bad but there’s more that go bad then not. I have 6 of those little Motorolas and iirc 3 of them needed the horizontal micas

Chris K · #10 02-19-2023, 08:51 PM

I'll give it a shot...not hopeful but I'll try. Thanks.

bandersen · #11 02-19-2023, 10:39 PM

Careful - those might be 1,000 volt mica caps. I'm assuming by the thread title this is a VT-71 with a TS-4H chassis?

Chris K · #12 02-20-2023, 06:23 AM

Quote:

Originally Posted by bandersen

Careful - those might be 1,000 volt mica caps. I'm assuming by the thread title this is a VT-71 with a TS-4H chassis?

That's correct. Most of what I've come across are 500v maximum but I have seen 1 or 2 1000v caps listed in the Sams.

timmy · #13 02-20-2023, 07:31 AM

Quote:

Originally Posted by Chris K

That's correct. Most of what I've come across are 500v maximum but I have seen 1 or 2 1000v caps listed in the Sams.

You can check the micas with a cap checker as I did with mine and the few 1 kv micas I did use 500v and they are in there for years and never had a problem.

Yamamaya42 · #14 02-20-2023, 07:42 AM

It's not like ya can't find 1kv ones if you really need to!

https://www.mouser.com/ProductDetail...vQNQEczw%3D%3D

but, they are very tiny next to the original! :O

Electronic M · #15 02-20-2023, 09:33 AM

Generally speaking don't touch micas in the tuner or IF system, but change sweep circuit micas with maximum prejudice...
Typically RF/IF micas have very little DC on them (worst case 200V) and maybe a volt of AC signal... They'll live forever like that.
Sweep stage micas can have 350-1000VDC on them while being whacked by 200-1000VAC pulses.
Logically you can see what is more stressed and thus more prone to failure.

The nice part about this is the sweep can be eyeballed back into adjustment (so no reason not to change sweep micas), where as the tuner and IF take special equipment and a lot of skill and effort.

Another thing to watch out for is I have encountered tin whiskers in Motorola pots in these sets.