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 Post subject: PR-7820 Focus Issue
PostPosted: 26 Aug 2022, 16:31 
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Received the unit with a bad clamp ring. Replaced that, removed the battery and now I have a 7820 where the speed seems consistent, the carriage is fully functional, the controls work and the laser fires but once the disc is ingested the main focus just hunts up and down.
Did a voltage rail check from the PSU terminals and they seemed to be okay according to the service manual. Pulled the boards and did a general cap check and connector cleaning and didn't find much that was notably out of tolerance. Carefully cleaned the mirrors and tried multiple discs both Discovision pressings and later titles. Same issue. I made a video to better demonstrate the problem. Fast forward to 7:18. https://youtu.be/DHCD0MMAPUM?t=438
I discussed the issue with the arcade crowd over at Vintage Arcade Preservation Society because the player had a strong tie-in with Dragon's Lair among other titles and was told to recap the PSU as they are known to be problematic. Well when I asked for extra details on that I got no helupful replies and because the power supply is underneath the optical deck and I'm fairly certain the 3-pole mount it sits on is part of the alignment, so that isn't happening until I get a clearer answer.
Scanning through the service documentation seems to indicate that a service mode can be entered and you can manually operate the focus but I'm not entirely sure what that might accomplish as I don't know where to attach a scope to monitor the signal form the detector, or what I'm expecting to see on the scope without a test disc.
I see the focus clicking has been a problem at least once before for someone else so perhaps it's a common issue. Do we have any ideas on where to start?
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 Post subject: Re: PR-7820 Focus Issue
PostPosted: 26 Aug 2022, 16:47 
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I think if you didn't have issues before you do now from cleaning those mirrors.
You need a special jig from what I remember or have read in the past to put the mirrors in their proper place.
Also depending on how much the player has been used your gas tube may be dying and need a replacement.

I had one once and it was a super sweet player, I do hope some others can help, only two I can think of that may know how to fix or have an idea
would be happycube or blam1.

These are just too old for many still alive to know how to fix.
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 Post subject: Re: PR-7820 Focus Issue
PostPosted: 26 Aug 2022, 17:03 
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Quote:
I think if you didn't have issues before you do now from cleaning those mirrors.
You need a special jig from what I remember or have read in the past to put the mirrors in their proper place.

The mirrors were not removed. I know better than to alter or adjust anything in the optical assembly, especially since the set screws are still factory set in the lock-tite which means nobody before me as altered anything either. I treat them as front-coated surfaces and I have a special cleaning kit I use for my Nikon negative scanners as to not scratch the surfaces.

I do not have a laser wavelength or power output meter, so I agree it is possible the tube is weak but at the same time I'll be optimistic as I have at least two other VP-1000's with crazy hours and use the same Toshiba LG-3217 tube and those both still work.
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 Post subject: Re: PR-7820 Focus Issue
PostPosted: 30 Aug 2022, 13:12 
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Very nice video. I have thought of doing them, but the time required to make something that polished is extensive.

mips wrote:
Scanning through the service documentation seems to indicate that a service mode can be entered and you can manually operate the focus but I'm not entirely sure what that might accomplish as I don't know where to attach a scope to monitor the signal form the detector, or what I'm expecting to see on the scope without a test disc.


I have never worked on this model, but I read through the SM.
When you press play, FOCUS ON is sent from CONT to VSOP and FOCUS LOCK is sent back. If focus can't be locked (thus no FOCUS LOCK response is sent), it tries four times at one second intervals. You should see (or hear in your case) four attempts. LASER ON is sent at the same time of FOCUS ON.

As the focus motor raises the lens, tracking error is created and then focus error.

TP1 is Focus Error, TP5 is tracking error. You can turn off tracking by grounding TP8 and tangential (on later players, this is called tilt) by grounding TP 1/6 (Not sure if 1/6 means pin 1 on a 6 pin connector).

Referencing the troubleshooting section of SM (Page 69), check for a "S-shape curve" at TP1 of at least 2V peak to peak. The other note is something about a "bight" and paper I do not understand in regards to slider beam axis. TP-7 and 12 should be within 20mV of 0V. It then mentions adjusting focus gain, but I would skip over that and check Z1, Z3, Q1 in focus servo. I read Q1 is closed when Q7 is low and Q6 is high.

More or less what I would try. Double check all of the above before actually attempting. It would be best to take a look at the focus motor to make sure something isn't mechanically wrong there first.

Don't worry about a test disc. Any CAV will suffice for the above, but you could still order GGV1016 from instrumental parts a few months ago which would be helpful.
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 Post subject: Re: PR-7820 Focus Issue
PostPosted: 05 Sep 2022, 13:58 
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The PR7820's always been a bear to fix I hear - I think there are some (awesomely) crazy people in Europe who have gotten something out of the Philips VLP600/700's (a very differently all-but-unfixable player), so it's not unthinkable that there's someone in the US who can still fix a 7820, but I have no idea who these days.

As for forum regulars, krbahr and grasshopper never worked on them, Duncan did but his brain flaked on him a decade+ ago, sadly.
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