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Old 8th May 2012, 05:40 AM   #1
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Default Mission PCM4000 comments and questions.

I note there are quite a few Mission versions of the CD160 around, one of which I have in front of me. This one arrived purporting to be in need of a new laser but actually, as usual, that's not the problem.

After several attempts by the previous owner to "fix" it, the drawer was jammed. After removing the grease and fabricating a lost spacer, the tray now blunders its way happily through its flip-up door.

The Mission differs from the CD160 in a few small ways. First and most obvious from the inside is the much larger transformer. Mission trumpet this as a good thing but mostly it's for powering the huge display, and it seems to be inadequately loaded. It is inclined to buzz if the load is decreased whilst testing.

Another problem is that the loaded voltages to all the other supplies are rather high, so the regulators get hotter and cap for the -6V supply is closer to its voltage limit. Worse, the unregulated supplies to the CDM4 servo are very high.

The servo board for the CDM2/4 mechs expect + and - 9V within 20%. The CD160 manual says 10V. The Mission delivers +12.3 and -14.7V: the offset is because there is no centre tap and the ground point is largely determined by the +5V and -6V regulators.

In the middle of the servo board is a transistor which is the final element of the laser control servo, and which draws the laser current directly from the unregulated supply. It is small and has no heat sink and gets scalding hot because of the high voltage it has to drop.

For the time being, I've inserted 10V regulators ahead of the existing ones, so the -6V one doesn't get so hot either, and the mech gets the right voltage and the transistor is considerably cooler. Laser current adjusts itself so no need to tamper I hope. Worth doing but probably there's a better way with more extensive rearrangement.

The fundamental problem with the machine, which only works when it's warm, is elsewhere. I have seen it described in several places but the solution never seems to be found.

From cold it reports a no disk error and behaves accordingly. When it's a bit warm it reads the TOC and shows the correct info on the display. In play mode it appears to find the track and sits there like it's playing but the display doesn't advance and there is no output. When it's warmer it plays OK at first but gets fidgety as it moves towards the edge of the disc. Finally it works OK as long as it's working, but reverts to step 1 if it's switched off for more than a few seconds.

Two further clues. When a bit warm, it can be provoked into playing if the fast forward or backward keys are pressed momentarily. Examining the circuit as per the CD160 manual, the only thing that seems strange so far is the signal that suppresses drop-out correction when it isn't required, such as when searching. AFAICS it is high when it shouldn't be.

Ring any bells? I've checked all the connections to the ropey piggy-back board with the MAB processer on it, and those to the M803A which serves as an SAA7210. The only other thing that gets hot is the display itself, and its seperate power supply. Could the problem originate from there? Manuals don't say much about the I2C and I2D connections between the front end processor and the main board, or how the processors react to them. Datasheets seem skimpy too.

Sorry this is long. Maybe it will help others who have recently acquired such machines in the great post-CD sell-off.

The other different thing, BTW, is the use of 5532 opamps. Oh, and the deletion of the remote RC5 receiver port. I wonder if the processor still supports it?

Oh, and there's a full-size galvanised, PVC-coated steel sheet bonded to the bottom polystyrene panel.

Ian

save the CDM2!
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Old 31st May 2012, 02:56 AM   #2
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As I recapped around the mistyped M4804A, the fault became more clearly defined. The machine now played either perfectly, when warm, or not at all when cold. Seemed the only explanation for this was a failing chip.

So, I bought an SAA7210 from a chap in China. It was very cheap, arrived remarkably quickly and respectfully packed, and does the job.

Sounds good with all its new caps and reworked voltage regulation. Added my DIY remote receiver too.

Unfortunately, in the meantime, I've fallen in love with a CDP337ESD, so the Mission will have to go.

Ian
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