AG-1970 TBC Appears to Have No Effect - Troubleshooting?

[jbd5010]

So I bought a lot of 3 "as-is for parts or repair" AG-1970's for dirt cheap. I was pleasantly surprised when one of the units played/ejected tapes with no problem, the display and all inputs/outputs worked, and all controls appeared to work. Even S-VHS recording seems to work great.

The only problem I've found is that when I switch the TBC on/off, it doesn't appear to do ANYTHING to the picture, it just lights up the little yellow LED on the front panel. From my existing 1970, I know that the TBC typically makes the picture a bit darker when switched on (and hopefully improves playback!), or at the very least you see the picture change slightly in SOME way.

Does anyone have suggestions? My quick-and-dirty attempt at a fix might be to steal one of the mainboards from another one of the 1970's in the lot (those power up but have mechanical issues), and see if another mainboard will have functioning TBC circuitry in it.

To be clear, all of the other switches on the flip-down panel work, and the little LED does light up, so I don't think it's the physical TBC switch itself.

[BrainStorm69]

I'd get a tape that I know plays like crap and needs TBC help and try playing with the TBC on and off to see if there is a difference before I assumed it wasn't working.

[orsetto]

The AG1970 has a TBC which is almost invisible in operation: it isn't as apparent as the TBC of the later AG1980 or the JVCs. The 1970 was in production for quite a long time, so there is some unit variation as slight upgrades evolved (some 1970s exhibit a slight darkening of the image as you noted, other show no change with the TBC on or off unless a tape is loaded that contains the narrow range of issues the 1970 TBC can "fix" (primarily blown-out overexposed camcorder tapes, or dubs of them). The main impetus behind the AG1980 introduction was to replace the weak AG1970 TBC, nobody in the last 15-20 years has seriously relied on the 1970 for its TBC. Thats no slam against the 1970: it remains a great semi-pro VCR for most purposes that don't require major TBC intervention, has a phenomenal tracking and adjustment range, and is among the most mechanically reliable VCRs ever sold (better than the 1980, actually).

Its your call how to proceed, if you really paid a pittance for the three VCRs its certainly worth trying a board swap to see if it nets you a stronger TBC for the functional 1970. But if I paid more than $60, I would probably leave well enough alone, settle for the one functional VCR, and keep the others for parts or sell 'em for scrap. Odds are the functional 1970 has a working TBC, it just has the typical little or no obvious effect.

[jbd5010]

Thanks for the response guys. Brainstorm... that's exactly what I did. I have a standard crap tape that came leftover in a VCR I bought one time, horribly recorded; that's my torture test for any VCR I buy when I want to test it out. However, I don't have any tapes with ridiculous time base errors. The 1970 that I have for regular use darkens the image a bit, though. However, this newly-received one has no apparent affect on the crappy tape, or any others for that matter. Orsetto, I'm guessing this could be due to what you explained. Like you said, it's probably not worth the risk of breaking the working VCR just to see what happens with a different main board. (The other two in the batch power up but don't even attempt to accept tapes).

A handwritten sticker on the back of the working VCR reads "works, but funny noise when recording" dated May 2010. I tried recording to an S-VHS tape from my cable box via RCA audio and S-video... no such noise It just occurred to me, though, that they may have tried it using the RF tuner connector, so maybe I'll test that too. My plan will be to sell this unit, and I just want to discover any problems so I can either fix them or list them when I sell it (I'd rather be honest and lose a few bucks).

I wouldn't be comfortable selling it as "works perfect!" if I didn't think the TBC was functioning, but now I'm not so worried.

While we're on the topic, do either of you guys (or someone else) know what could cause an AG-1980 to power up briefly and then turn off after a few seconds? I managed to snag one for $0.99 plus $32.00 shipping (damn shipping!)

[BrainStorm69]

If you want to troubleshoot vcrs, here's one good place to start. Pretty comprehensive.

http://www.repairfaq.org/sam/vcrfaq.htm

[jbd5010]

Great link, thanks!

[orsetto]

The AG1980 was one of the first consumer electronics products to feature a then-new type of compact power supply module. While it does have certain advantages, this PSU design was prone in early versions to break down with symptoms such as you describe. The fix was usually to swap out the entire PSU module. While I believe these replacement modules are still available, they probably cost $100 or so and would defeat your intention of acquiring "bargain" vcrs. The PSU module can be repaired cheaply by those with patience and enough skill with a soldering iron to disassemble it and locate/replace the burned out parts (usually blown capacitors). Caps usually cost a few cents to a dollar each, assuming you have repair skills the actual cost often totals $5. Before delving into the power supply, its worth checking some other trouble spots. The unit might also power on/shut down quickly if the "pilot light" that tells it a tape is loaded has burned out, or a gear in the loading mechanism is stuck/dragging.

[jbd5010]

The PSU was my first thought. Coincidentally, there is an AG-1980 PSU listed for only $40 or so with shipping right now. Acutally, he has "3 available" as of my post -

http://cgi.ebay.com/PANASONIC-AG-1980-1980P-POWER-SUPPLY-WARRANTY-TESTED-/370481044395...item5642641bab

I might buy one just to hop on the good deal. If I feel like it, I might try inspecting the 1980 PSU. I am an electrical engineer, but I don't deal with board-level hardware very much any more, my college lab skills are probably getting rusty

Also, do you know if a 1970 PSU would run a 1980? I realize the form factor is slightly different, but maybe it could be modified? I was going to test voltages, etc to see what I can discover, but I'm having trouble removing the PSU from one of my junk 1970's that powers up... missing a clip or screw somewhere.

I also just found a link to the AG-1980 service manual. Unfortunately it's page-by-page images, but still a good find -
http://www.scribd.com/doc/11755577/Panasonic-VCR-AG1980P-Service-Manual-VSD9509M264

I'll dig through that and see what I can find.

[lordsmurf]

You can download the 1980 service manual as PDF here
http://www.digitalFAQ.com/forum/showthread.php/service-manual-panasonic-2600.html

Scribd sucks.

[jbd5010]

Agreed, LS. Thanks for the better link. I wasn't looking forward to downloading 108 separate images and creating the PDF myself.

Do you happen to have a AG-1970 service manual link or file? From what I understand, the mechanics are different enough that having both manuals is probably necessary. As I said earlier, I'm also having problems removing a 1970 PSU.

Thanks!

[jbd5010]

After all this discussion about power supplies, etc... it was the stupid POWER BUTTON.

I played with my 100% working AG-1980 for a while and noticed that if I plugged the power in, it too shut off after a few seconds and I had to push the power button to turn it on. So I *carefully* removed the PCB that had the power button on it from my working 1980, plugged it in to the broken 1980 and VOILA! It works. So I guess I need to track down a new PCB if I really want to fix the other 1980; shorting the terminals of the power pushbutton on the broken PCB does NOT turn on the VCR, so it's not the button itself.

[orsetto]

The AG1970 uses slightly different mechanicals and a totally different PSU, which extends a couple inches out from the rear panel. Its almost certainly simpler to repair a faulty 1980 PSU than try to hack a 1970 PSU to run a 1980 (if they're even the same spec).