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Old 20th Sep 2023, 4:22 pm   #1
Join Date: Aug 2023
Location: Leicester, Leicestershire, UK.
Posts: 21
Default Sony MDP-850D Laser Disc Player Fault

Hello there,

I have a Sony LD player as the description.

Its `story`:
I bought it as spares & repair about 3 years ago. It turned out to be fully working.
Not used very often, but about 6 months ago, it stopped reading - suspected laser unit.
Laser unit replaced - still not working. I found out I needed to tweak many settings. (I think the replacement was a dead laser - see next)
I eventually bought another same model, a couple of months ago, again spares & repair - no power. Easy fix - Thermistor dead on the PSU.
But - this player is cosmetically POOR! So I swapped over laser carriage assembly from this to my first player. It worked!

Unfortunatly I didnt spot until too late, this assembly had a crack in it, so when the laser travelled to the top to play side B, it was too low, and scratched the disc!
So - the latest thing I did... I swapped the assmebly back to the original one, but swapping the laser unit (so mine now had the working laser).

Now here lies my problem.
Sometimes..the player is fine... plays both sides no issue.
`Most` times, When I press play on side A or B, it spins up then the display flashes `OFF`. No function on the buttons at all - I have to remove mains power, but trying aagin, can sometimes play that same disc that caused it to shut down.

Any ideas, before I strip both players, and swap parts again?
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Old 24th Sep 2023, 11:43 pm   #2
Join Date: Aug 2023
Location: Leicester, Leicestershire, UK.
Posts: 21
Default Re: Sony MDP-850D Laser Disc Player Fault

Any help on this please?
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Old 25th Sep 2023, 10:43 am   #3
G6ONEDave's Avatar
Join Date: Dec 2019
Location: Owston Ferry, North Lincolnshire, UK.
Posts: 1,538
Default Re: Sony MDP-850D Laser Disc Player Fault

Not worked on one of these but have worked on plenty of CD players, you have to be very careful in sourcing a laser unit. There are a lot of sub standard ones out there, as well as fakes being sold as replacement parts. Always buy a replacement from a reputable seller that g/tees the part. That way you should be able to get a replacement or refund supplied for a faulty device.

I would check that the laser from the damaged mech has not suffered any damage to it's lens and or focusing assy. If the laser lens stands proud of it's mounting, it could have been scratched at the high point when it collided with the disc.

The sequence when a disc is loaded is that the laser should first try to focus, if that fails then the machine could well shutdown or give a display of 'no disc'. Once focussing is achieved the TOC (table of contents) should be read and displayed in the data display screen. Again a faulty laser could focus but fail to read a disc. Something else to check is that the disc spin motor is spinning freely and not catching the mech. The height of the disc platter will effect the focus/reading of a disc. Likewise also check that the disc clamp is not allowing the disc to slip when spinning.

If you have access to an oscilloscope and have the service manual, you can check the various waveforms in the manual against what your machine is doing when playing a disc. The service manual will emphasize that the electrical adjustments be done when the laser is replaced. On some models this always had to be undertaken but there are some where the tolerences of the replacement laser assy are so tight that nothing actually needs adjusting.

Quote "All is hyperthetical, until it isn't!" (President Laura Roslin, Battlestar Galactica)
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Old 27th Sep 2023, 1:16 pm   #4
Join Date: Jul 2020
Location: Dundee, Angus, Scotland, UK.
Posts: 17
Default Re: Sony MDP-850D Laser Disc Player Fault

Do the power rails look OK while it's spinning up? I had a Pioneer LD player with thoroughly-cooked PSU capacitors that would try to spin up and reset because the 5V line dipped too low - LDs are heavy, so spinning them up takes a lot of current!

I don't know if Sony used the same logic, but the Pioneer players try to get initial focus first before they spin the disk up, so if they're trying to spin up then they're reasonably happy with the laser already. They then start looking for sync pulses from the demodulated video, and spin down again if they don't find them within a reasonable period of time. (LDs don't necessarily have a TOC - you can have one as part of the CD-style digital audio track but it's not required.)
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