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

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

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:

http://videokarma.org/attachment.php...hmentid=179003

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.

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?

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

Never had issues with my Beta decks before.

-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.

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).

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.

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.

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. :(

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.

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...

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.

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.

Dang it, I really want a TBC, now. And also an S-VHS machine and a SuperBeta Hi-Fi, haha. The thrift stores around here don't really have anything cool (though I guess I should check the one by the recycling center sometime, as I often ignore that one), but sometimes the pawn shops have nice stuff. I wouldn't expect the pawn shops to sell me anything that could be used as a TBC for less than $50, though.

Surprise, surprise! I went to a pawn shop today, and they sold me a DVD recorder for $15!

Actually, I'm not sure it's a DVD recorder -- it says it's a Digital Media Recorder on the front panel, but it does have a DVD drive, so I'm kind of assuming it would record to that by default rather than to the USB port on the front panel. I do NOT know if it has a TBC, but I thought $15 was worth the risk. It does have a component output, so at the very least it would have to have some kind of transcoder for the video inside.

EDIT: Huh. The damn thing's a DVR. I opened it because the DVD tray wouldn't open (even though the machine powered up and seems electronically intact), and found a 40GB hard drive inside.

EDIT 2: It does have some sort of TBC, but not a very good one. It works well for 'normal' video, but I tried running a tape of '80s music videos through it, and when they have sections of the screen that are blacked out next to sections that have a full picture, there's still some pretty funky bending going on.

EDIT 3: I've figured out what it is. This crazy thing has some sort of automatic brightness control or something. It seems to be constantly adjusting the picture to maintain a consistent black level, which is honestly kind of annoying on my main CRT, and is probably the cause of the funky distortion I'm seeing on my 1959 Zenith. It's still not a bad purchase for $15 though, and I'll probably start using it to record some of my TV shows (since my VCR is honestly terrible at it -- it probably needs a cleaning, badly).

It likely doesn't have a TBC. The brightness thing is likely overzealous Marcovision protection kicking in.

No, it's doing this while watching regular TV stations. The effect is actually far less pronounced when watching tapes.

You're probably right that it doesn't have a TBC, though. I think what the passthrough feature is actually doing is digitizing the video, writing it to memory, then playing back whatever is written in memory.

I'd love to have a professional one, but I could not even afford a 20$ used pro unit at a ham fest a week back(low on my priorities compared to caps for my restorations).

I always thought of a TBC as only cleaning synch issues and not the video that makes it to the screen...That site stretches the definition I've always had of a TBC.

I mostly use a Toshiba D-VR5 for elimination of flag waving/ bends at the top of the screen on my tube sets. I feed it S-video from my JVC HR-S3912U SVHS-ET deck(the Toshiba is a VCR/DVD recorder, but I don't like or use the tape section of it). I also have a Panasonic DMR-E80H that may have a TBC, but the current state of my system wiring prevents me from easily testing that.

I do have a NEC brand DS8000U mkII SVHS deck with a three setting "Digital NR" that improves video noise issues(I can't find any info about this deck online). It does get motion issues at it's highest setting, but the lower settings seem to get the job done without doing that to an objectionable or noticeable extent.

I almost never transfer video tape to digital formats...As I prefer to watch my tapes on a CRT TV with a tape deck(In fact aside from my computers, which I rarely watch video on, I only use CRTs for the video I watch). Also most of my tapes are first gen time-shifts made on decks I still own and keep working, so anything more than synch correction is not all that noticeable or important to me.

Encountering issue on my ctc-38. Played one tape without an issue then the next one had massive bending at the top, tried a third and same thing. Horizontal AFC circuit is a bit different than the zenith earlier in this thread - any ideas which parts to mess with ?

C106 and R156 are the main AFC loop filter components, in conjunction with R155.

I would try reducing the size of C106 first. How much is hard to say, but try 1/3 the size (about .05) first and see what you get. The bending should extend less down into the picture, but if it gets squiggly, then you need to play with the damping resistor R156.

This kind of modification is actually easily made by "decade box engineering." if you are observant. If you have a switchable capacitor box and a switchable resistor box, just substitute them for the C and R in the circuit, start with them set for the schematic values, and then note the effects of changing the values. It's wise to shut off the set before changing the switches, as you may have infinite resistance and infinitesimal capacitance between switch positions.

If you don't have decade boxes, the next easiest experimentation is substituting parts using clip leads - be careful that the connections are secure!

Thank you!! This is exactly what I will do, very appreciated advice!

Changed C106 to 0.047 and problem is a little better but far from watchable. In other words the bending did extend less far down and I did get squiggly. Going to have to get an assortment of capacitors and resistors and dig into this farther. Luckily havibg C106 at 0.047 does not affect anything with a dvd signal so the TV is still absolutely watchable. I am getting fast at pulling the chassis!

Okay so I took the cop out solution - stopped by a thrift store today while doing something else and picked up a lite-on dvd writer for $9.99 CDN. Ran through that the picture is perfect. I still would like to make it work properly but the urgency is gone.

Yep, that's squiggly! But you can see that you could zero in on the correct values pretty quick if you had a variety handy to try.