Fix the bending and waving images when playing beta and VHS tapes on your old TV

[cbenham]

Here are some Horizontal AFC schematics for various sets from the 1970s with
slight additions that may be helpful in stopping the flag waving or jittering you see when you play beta or VHS tapes through your vintage TV set.

https://www.dropbox.com/s/1fw8311noq...for%20VCRs.PDF

These are not absolute by any means but are suggestions that may help you
modify your sets H AFC so it is more stable.

Cliff

[jmetal88]

Since this thread seems to have arisen due to a PM I sent Cliff asking how to deal with the problem on my TV, I thought I'd post how I wound up solving it (or at least getting it as watchable as I could, given the parts I had on hand).

Here is part of the horizontal section of the schematic for my Zenith C1415 set (16C20 chassis). I have circled the two components I changed in dark blue:



I noticed that if I disconnected C47 and C48, the picture became quite stable while watching VHS tapes (with only a very slight bend at the top, depending on where I had my horizontal hold set). So in order to leave everything connected and still solve the issue, what I ended up doing was making the C48 capacitor a very, very small value and making R70 a very, very large value (currently, I have less than 10pF for C48 and more than 1M-ohm for R70). An interesting side effect is that the horizontal hold locks in over a much wider range, now. I can actually sort of use the horizontal hold as a centering control now, and somewhat control the degree of bending at the top of a VHS signal.

EDIT: I might have overdone it, haha. The picture tends to bend a bit with sudden changes in brightness with an otherwise good signal, so I think I have a little more experimenting to do.

[Username1]

How much of an effect has it had on the G1 voltage? (-3.5v) and what happens with a no signal condition? You know all snow..... or a weak signal?

[Electronic M]

I just feed the VCR video signal through the digital TBC in my DVD recorder...

Never had issues with my Beta decks before.

[jmetal88]

Originally Posted by Username1

How much of an effect has it had on the G1 voltage? (-3.5v) and what happens with a no signal condition? You know all snow..... or a weak signal?

-2.64V with no signal, -2.97V with signal.

EDIT: I might be imagining it, but I think my set somehow started having more reasonable contrast since doing the mod.

EDIT 2: Oh, and also I changed the values again since the first post. It's much less 'drastic' now, and I just tweaked the values enough to get a stable picture from a VHS source, but not enough to totally remove the bend at the top.

EDIT 3: Actually, I don't think I was reading the voltmeter quickly enough. The voltage drops when I hold the probes in place, but it starts out around -3.9V before it drops.

[jmetal88]

Originally Posted by Electronic M

I just feed the VCR video signal through the digital TBC in my DVD recorder...

Never had issues with my Beta decks before.

I must have an awful Beta deck then, as my picture was bending just as badly as with a VHS source.

Ideally, I'd like to get one of those DVD recorders with a pass-through TBC and return my set to the original configuration, but I don't have nearly enough spare cash to do that right now. I think I'd prefer a standalone TBC, though, since I think that would help a lot more with recording tapes to my computer (even though they're a lot more expensive).

[Electronic M]

I've found multiple DVD recorders, and a stand alone TBC at thrift shops and garage sales usually for less than 10$. If you can't afford a new item go hit the pavement and look for a used one.

[zenith2134]

Thanks for the tip re: flagging with VHS
My XL100 does this with every deck i have and I'm going to dig in and see what can be done.

[jmetal88]

Originally Posted by Electronic M

I've found multiple DVD recorders, and a stand alone TBC at thrift shops and garage sales usually for less than 10$. If you can't afford a new item go hit the pavement and look for a used one.

Where do you live? No one ever sells anything cool like that around here. Closest I got was a Commodore PET on Craigslist, but an eBay flipper got to it the day before I did.

[Electronic M]

Originally Posted by jmetal88

Where do you live?

Seriously? Check the upper right corner of any post I make right below 'join date'.

The suburbs here are reasonably affluent. The thrifts are usually FLOODED with old VHS decks. I've bought 7 SVHS decks in the last two years and passed on some. I've scored 4 Betamax decks in the last year or so as well. In that time I've also scored 5 DVD recorders one of which also functions as a DVR.

There are also a lot of elderly folks here who are a source of tube gear. I've lost count of the number of tube radios and stereo consoles I've seen in garage sales and thrifts.

[jmetal88]

Originally Posted by Electronic M

Seriously? Check the upper right corner of any post I make right below 'join date'.

The suburbs here are reasonably affluent. The thrifts are usually FLOODED with old VHS decks. I've bought 7 SVHS decks in the last two years and passed on some. I've scored 4 Betamax decks in the last year or so as well. In that time I've also scored 5 DVD recorders one of which also functions as a DVR.

There are also a lot of elderly folks here who are a source of tube gear. I've lost count of the number of tube radios and stereo consoles I've seen in garage sales and thrifts.

Yeah, I realized you had your location listed right after I posted. XD
I didn't go back and edit it though, as I felt that removing that question would make my statement less effective, haha.

I used to find a lot of cool stuff in thrift stores when I lived in Fargo (that's where I got my SelectaVision video disc player with 20 or so movies for $20), but around here you only get clothes and kitchen stuff.

[Dude111]

Originally Posted by jmetal88

I noticed that if I disconnected C47 and C48, the picture became quite stable while watching VHS tapes (with only a very slight bend at the top, depending on where I had my horizontal hold set).

Hmmmmmm I dont know why you would have to do that buddy.... Most VCRs send a stable/SYNCED picture that the TV can lock onto,im not sure why your having to do this,im so sorry

[jmetal88]

Originally Posted by Dude111

Hmmmmmm I dont know why you would have to do that buddy.... Most VCRs send a stable/SYNCED picture that the TV can lock onto,im not sure why your having to do this,im so sorry

It's obviously a common problem, or the OP wouldn't have had five pages of examples for fixing the problem...

[old_tv_nut]

Originally Posted by Dude111

Hmmmmmm I dont know why you would have to do that buddy.... Most VCRs send a stable/SYNCED picture that the TV can lock onto,im not sure why your having to do this,im so sorry

Most VCRs do NOT send a time-base corrected signal. The problem then is that the H sync timing can change up to +/- a half line when the playback head switches near the bottom of the picture. Older TV sets had long horizontal AFC time constants to reduce horizontal jitter under noisy signal conditions. When VCRs came out, the time constant had to be shortened so the H phase could pull in during the vertical retrace time. This had the unfortunate effect of making the H sync more jittery on weak off-air signals, but that was way less objectionable than the flag-waving on VCR playback.

DVD player output is crystal controlled, is generating perfect sync locally (instead of playing it back), and does not have any horizontal phase jump.

[NJRoadfan]

Originally Posted by Electronic M

I just feed the VCR video signal through the digital TBC in my DVD recorder...

Never had issues with my Beta decks before.

This. A proper TBC will solve any viewing issues with old TVs and lead to higher quality video output as well. A great overview is found here: http://www.digitalfaq.com/forum/vide...time-base.html

When I do captures for DVD transfers, I usually use a SVHS VCR with a line TBC (true Time-base correction) to correct horizontal jitter and an external full-frame TBC (really just a frame sync/buffer as my unit does little jitter correction) to clean up the sync (front/back porch, color burst, etc) and output proper RS-170a to the capture card. It also provides a stable continuous sync signal to prevent frame dropping.

A popular DVD recorder with pass-through TBC is the Panasonic DMR-E10. Some of the JVC DVHS machines also have pass through TBC functionality as well.