JVC HR-7100U: Heavily distorted picture

[ESigma25]

I spent my day today working on my JVC HR-7100U, after finally getting the belts I ordered almost a month later. But it has a problem, a real nasty one.

This is what the picture looks like when you play a tape:

https://i.imgur.com/yXywhpF.jpg

It's black and white and constantly rolling and trying to adjust the tracking knob on the front of the machine just makes it worse. I've already cleaned the heads on this VCR at least three times and I do get sound perfectly fine, so what's stopping me from getting a stable picture? I can't even find a service manual for this VCR anywhere to see if there's a tracking adjustment on one of the boards in the machine.

[old_tv_nut]

Not an expert on these machines, but I'd hazard a guess that either the tape or the head is running at the wrong speed, causing a constant tracking problem and loss of color.

If this is a HIFI unit, you are also probably getting only longitudinal sound with tracking this bad.

[Ed in Tx]

If the head drum was running at the wrong speed you'd not see a picture in the noise, you'd be seeing horizontal lines across the screen. Looks more to me like either loose tape guide rollers or no back tension on the tape.

[ESigma25]

Originally Posted by Ed in Tx

If the head drum was running at the wrong speed you'd not see a picture in the noise, you'd be seeing horizontal lines across the screen. Looks more to me like either loose tape guide rollers or no back tension on the tape.

I actually had to partially disassemble the mechanism around the tape drum as one of the guides had come misaligned from its track, but that picture I took was after doing that and reassembling the VCR, and the guides are definitely moving in sync now and aren't loose.

What exactly is "back tension"? I do notice the friction band for the left reel is in very poor shape, but would that affect playback?

[Ed in Tx]

Originally Posted by ESigma25

I actually had to partially disassemble the mechanism around the tape drum as one of the guides had come misaligned from its track, but that picture I took was after doing that and reassembling the VCR, and the guides are definitely moving in sync now and aren't loose.

Likely need a proper alignment if a guide assembly was out of the track, or something is broken that caused it to come out.

What exactly is "back tension"? I do notice the friction band for the left reel is in very poor shape, but would that affect playback?

"Friction band" is probably what supplies the back tension. Back tension is the tension on the tape as it wraps around the cylinder, usually measured with a "Tentelometer" or a special back-tension tape with a scale on it, value usually specified at 25 to 30 grams as I recall.

Without proper back tension the tape won't be pulled up against the heads making contact with them and possibly causing the loss in signal off the tape like you show.

[ESigma25]

Nothing was broken, the screw for the guide had just gotten loose and thus the metal post on the underside wasn't being pushed into the plastic slot underneath like it should be. It's tight now and both guides move in sync when it comes to both threading and unthreading the tape.

I don't know how I would replace the friction band, I'm sure that replacements for such an old VCR just don't exist. A lot of the felt on the inner surface of the band is just...gone, likely from excessive wear as the belts wore out.

[Ed in Tx]

That's the old top-loader with the colored buttons? (Like in the intro of "The Goldbergs") Must have been a million of those made. HR-7300 should be similar. Maybe look for a donor unit to make yours work right. Unavailability of parts is why I don't work on the things anymore.

[ESigma25]

Yeah, I've had a decent experience buying belts since people can just buy belts in bulk and put the necessary ones for a specific machine in a baggie, but I've never even tried to search for specific parts like that before. And I had my terminology wrong, the band is actually called a "back tension band" so if my problem is back tension, that's the cause.

[redk9258]

According to this page https://encompass.com/item/4588583/JVC/PU51390A/ The backtension band is P/N PU51390A.

https://kpcomponents.co/products/pu5...c-tension-band

[ESigma25]

Originally Posted by redk9258

According to this page https://encompass.com/item/4588583/JVC/PU51390A/ The backtension band is P/N PU51390A.

https://kpcomponents.co/products/pu5...c-tension-band

Oh wow, thank you! I had no idea it was still available anywhere!