Tapping into the AG-7650 TBC possible?
 

I found a cheap dump condition Panasonic AG-7650. I read in the forum how difficuld and unfit these machines are for VHS transfer even though they have a 3,000 dollar TBC. I wondered if one could tap into the TBC only and use it from an external S-Video signal bypassing the transport/servo system of that machine. I looked into the block diagram for the 7650/7750 and found a few Chroma and Luminance signals going into the TBC. I have very little knowledge in this area (very little used as an euphemism for none whatsoever) but I'm sure there are lots of good people on the forum that could answear this. I just want to know how feaseble this would be.

For now, I'll use that machine as a learning tool for cleaning contacts, grease things and hopefully see it turning again. I have no expectations it will ever be up to spec by my own hands - but that TBC really enticed me.

Thanks!

I have no personal experience with this mode, but this machine is likely from the era of failing SMD capacitors in Panasonic VCRs, so the electronics might not be up to par. No way to confirm this if the machine is not otherwise usable.

IMO: It would take a very skilled technician to tap into the electronics to inject/extract signals. It would also require some supplemental custom circuitry to pass though signals pulled from another VCR. If you have those skills it might be an interesting hobby project, but that's about it. If you don't have the skills, or a skilled friend interested in doing it as his hobby with you, its not worth it. You could probably buy a good VCR, ProcAmp, and TBC for much less cost.

I contacted a specialist in Australia - found some articles he wrote on modding these units to correct some bad design issues - and he kindly answeared me:

"Without looking at the circuits (they're somewhere else, at the
moment), it's possible that you can feed in external video signals.
But you'd need for that source to be vertically sync locked to the time
base corrector (with advanced sync). It's not a floating window TBC.

I know that just piping video into the main inputs will go through the
TBC, and if the input signal is sufficiently out of vertical phase with
the TBC, it flutters."

So it would surely need some sort of specialized electronics. My idea is not to tap into the source VCR but get the S-Video signal from the regular output. The AG-7650 I have in my hands shows a good image when in pause, moves frame by frame with the jog shuttle, but shows an image that looks like it's with dirty heads (unlikely from the pause image, I think) or can't track properly. I'll remove all boards - very easy on these machines - Deoxit all contacts, do some lubing and see how it behaves. It seems the TBC is working properly.

My idea is that if this is feaseble and if the comunity manages to design the necessary circuitry to enable us to use these units' TBCs we could have access to good TBCs (an ever rarer device to get our hands on) at low prices. Lets see how the discussion goes.

PS: According to his answear, the 7750 (which the one capable of recording) could be used as a TBC. The service manual has schematics, PCBs and block diagrams for both units.

Thanks for your considerations!

The recorder version, AG-7750, has video signal input circuits, the playback only version probably does not. That makes it a much more complex undertaking.

Using the proposed method to get a "good TBC at a low price" only applies if labor is free and parts, tools, and supplies come from a well stocked shop and spare parts bin, and AG-7750/7650 with good TBC boards cost next to nothing.

Keep in mind that the TBC may have been "worth $3000" when new 20+ years ago, that is not the case today.

But I wish you luck in your quest, and please share results as you progress.

I have a AG-8700 which also has pass through TBC. Anyone know if the two models are related? Anyway, I had to rebuild my 8700 with all new capacitors, a task which too around 12 hours total. Thankfully the PCB's are resilient so no lifted tracks etc. I now use my 8700 as pass through TBC with great results. These machines are well worth rebuilding if the heads and mechanics are working well.

Quasipal,

Did you have to change all of the caps or just the thru hole electrolictic ones? These machines are really good! I read on the forum that they don't behave well with damaged tapes but even my dump unit can put out some images from a good tape. Also, good to know about the 8700 - it's much lighter than the 7650/7750 units therefore cheaper shipping!

Thanks!

Interesting thread. :)

I was unaware (or forgot) that any VCR was passthrough capable. However, like the ES10/15/20 (as opposed to all Panasonic recorders, or all DVD recorders), it's a unique feature. What I'm curious about is the potential side effects and signal cleanliness (as all TBCs tend to add some degree of processing noise).

It does seem this is curtailed to PAL gear, nothing NTSC that I've ever read (or again, forgot).

The TBC in the Sony PVW-2800 betacam vcr we got here also works on passthrough. Though, it seems to only work when both the input and the VCR is genlocked to the same signal (otherwise the image slowly rolls), so I've only gotten it to work properly with a Panasonic AG7350 (cheaper or older? VCR in the same line as the one OP has, but has no TBC) broadcast-VCR as it can sync the playback to an external signal. Sounds similar to what the engineer replied about passing through the AG7650.

It was all surface mount ones - the through hole ones on the Audio board and control board were perfect still. The unit smelled of fish when powered up and gave out no picture because all surface mount caps had gone bad.

As to not playing damaged tapes well personally I don't see that - and they play home recorded tapes just fine (apart from LP of course). Using the tracking control you can set each recording optimally. I do wonder if these findings are based on using units that are worn out or are out of alignment - as many probably are. I got mine on eBay and drove over to collect it - they are HEAVY units. If it was transported and bashed something would break or bend. My unit was purchased from the original buyer and they sold me that and a mint Panasonic HS950 for £40 for the two.

Now saying all this - and I do love the pro decks - it is not a huge lot better (and no LP of course) than a good home deck concerning playback picture quality. Its sharpening is better controlled though so no halos or ringing. Where it does win out is linear audio quality where the sound is MUCH better on all tapes compared with consumer SVHS decks. And its linear stereo too. And with switchable Dolby...

As a stand alone TBC mine works great - and all levels are adjustable. I'm glad I have one but I would never buy one that had done the rounds on eBay with knobs missing and dented.

Mine is just dirty, and is producing image though it seems it's off tracking. When it's on "stop" it produces a perfect freeze image and I'm able to jog dial though the frames!

Yep ... the 7650 weghts 17Kg (~34 pounds) and the 8700 11.5Kg (~23 pounds). Even though I have purchased things like Apple IIgs color monitor from the US and it arrived here working and in one piece, I think the mechanics on these units are much more sensitive.

I guess I should try to at least make the sound work, then!!! Nice info! My unit will be a resotration project ... if it does not work, at least I will larn somethings. I also read on Tapeheads someone mentioning these units have very good mic preamps.

I think my Idea would involve something more complicated than just injecting the S-Video signals into the board. I'll look for the connectors while I clean it - check wheather they come from cables or from a backplane board - and try to inject the Y and C signals to see what happens!

I found a TBC project online from an engineering PhD student somewhere near Sweden: it's a very simple device, actually: 1 Philips video decoder, memory chips for the frame buffer and another Philips video encoder chip. His design can also be used as a transcoder and can produce a set of component outputs. His page had not samples showing how well it worked nor the Philips ICs settings - I guess I'll have to read to 140 page datasheets and try to figure it out. One other thing I want to do is make it possible for the user to play around with the Philips ICs settings. If/when I manage to have it working I'll post here.

Thanks, Quasipal!

PS: The engineer from Australia sent me another reply telling me to check the capstan lubrication - liquid might have been used to clean it and this could have removed the lubrication from the motor. Another thing he mentioned is that the heads could be superseded by wear. I'm not sure "superseded" is a technical term or just another term for completely busted.

Is it this one? I mentioned it in the TBC sticky a while back, the video decoder is actually the same as in the Cypress (AVT8710, TVOne etc) TBCs (and they have a similar setup). I wish he shared some of the ROM/FIrmware stuff, given that it contains off-the-shelf components.

Yep, that one! The firmware is just setting up the SAA7114H registers. What runs on the PIC is the LCD and setting up those registers. The PIC could be replaced by a Atmega (Arduino). Apparently, all the informatino is in the 7114 datasheet - that is 144 pages long. What I want to do is build the hardware and make the Atmega allow us to fiddle with the registers in an easier way.

Good to now it's the same chip on the other TBCs!

No, now wait a minute. "one of the chips" on the best TBCs (namely Cypress). There are more chips inside. I don't want you making the mistake that you just need to Philips chip. That's not the case.

I also do not trust the interwebs blackhole not to open and swallow that page of schematics, so I'm attaching the 2 JPEGs, the 2 PS files, and the HTML pages (as PDF) into a zip on this post. I'm not sure how useful that data will be long-term, but saving it is wise.

I did share this a while ago but can't find the post so thought I would add...

Have a listen to this linear soundtrack from one of the AG Panasonics. I think its pretty impressive and very usable in place of HiFi.

https://www.dropbox.com/s/k4bde0mrcl...001_2.WAV?dl=0

The pro machines carry some fine circuits.