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Old 6th October 2005, 07:06 AM   #1
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John I have a Discmagic 1 and it's got a problem, I hope you can point me in the right direction. It has 14.9mhz that I measured on the clock instead of 16.9mhz sometimes it reads and plays, most times it does'nt spin up and just says disc in the readout after about 5 seconds of not spining up.
One of the two regulators with sheet metal aluminimum heat sinks on the clock/output board, that are side by side almost touching the rear panel is very hot, the other is warm, is this natural?

Cheers George
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Old 6th October 2005, 12:50 PM   #2
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George,

Both regulators on the PCB should be at the same temperature - the fact that one is getting hotter then the other, and that the Clock Frequency is incorrect suggest a fault somewhere on the board bringing down the PSU rail.

Thanks to Dan on this Forum - I can forward to the Circuit Diagram of the DOTX board - PM me if you need it.

John
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Old 7th October 2005, 12:46 AM   #3
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John or anyone else. Question on this transports power supply for the clock/output board.
Why is the 12v negative regulator U1 output being shorted to ground via the 30mh winding of the L2 choke, it is very hot and is only giving .5v output, because of the near short.

Cheers George
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Old 7th October 2005, 01:35 AM   #4
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I just looked up the specs on that 30mh Murata PLAA3030R3D0181 choke, each of it's coil windings has a DC resistance of just 4ohms! no wonder the poor negative regulator is getting stinking hot, getting it's output to ground via 4 ohms.

Cheers George
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Old 7th October 2005, 01:48 AM   #5
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George,

In fact the -12V is not being shorted to ground - its and error on the Cambridge audio circuit Diagram - "Ground" for the board is taken AFTER the CM choke - you should see 12V after the 7812 & 5V after the 7805’s - if one of these supplies is "low" then you need to find out why.

Even if the one of the supplies is low - I would be very surprised if the XO circuit was oscillating at such a low frequency (you said 14.9MHz), I would suspect measurement error - or failure of the Crystal (did you change the crystal type?). The XO circuit is standard Colpitts – not much to go wrong here.

If the problem is with the DOTX output board, then have a suspicion that somebody’s tried to ‘Upgrade” the clock or PSU and messed things up –has this unit been modified in anyway?

I would confirm the Clock Frequency measurements again – if indeed it’s correct, then I would look around the servo board, these units are more then 10 years old – and the Sanyo / Sony laser and chipset was never the best – developed before CD/R or CDRW, or even longer playing CD’s.

If this unit has been modified, check that the small ceramic “RF decoupling” Caps are still in-place between the servo PCB and the copper shielding.

Best of luck in repairing the unit,

John
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Old 7th October 2005, 01:58 AM   #6
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Come on George, Pls. give some credit here, No designer is ever going to be crazy enough to needlessly short a regulator to ground via a 4 ohm short - Why?

I repeat again: -

"the -12V is NOT being shorted to ground - its and error on the Cambridge audio circuit Diagram - "Ground" for the board is taken AFTER the CM choke"

The PCB "area" before the CM choke is Isolated from the "Ground plane" after the CM choke - if this is not the case, then there is a short-circuit, but its not by design - it’s a component fault somewhere on the PCB.

John
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Old 7th October 2005, 02:47 AM   #7
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Sorry John, I did not mean to imply that you designed this error into the prodution models, but maybe the person who owned this unit gave it an earth thinking it should be there because of the diagram. I've yet to dismatle it to have a look under the board to see why it may go to earth. I've measured it from the top and sure enough it does go to earth after the choke. I'll keep you posted.

Cheers George
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Old 7th October 2005, 03:01 AM   #8
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Yes this unit has be looked at before, many times by the look of the screws and solder jobs, it's the 7912 reg that's getting hot, the crystal looks original and all the components around it, and it is giving 16odd fluctuating mhz, my new second hand Fluke digital counter is of and needs a recal after checking another 16mhz transport it also read 14mhz. So the cyrstal is ok, now to find out why the choke is getting earth on the other side of it.

Cheers George
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Old 7th October 2005, 04:10 AM   #9
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George,

Before you spend too much time searching for a fault that may not exist - with the unit powered-off, check the resistance between the transformer centre tap (incorrectly indicated as GND on the circuit diagram. and the Ground plane AFTER the common mode inductor - say the Ground tab on the 7812 Regulator on the black heatsink - if a short does not exsist between these nodes then its OK.

I would suspect the disc reading problems you describe have nothing to do with the DOTX board (the Digital output board), but a worn laser (if still original, then 10 years is very good going for this laser head).

John
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Old 7th October 2005, 06:36 AM   #10
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John just checked what you asked, and yes as you suspected the centre pin of CN1 marked ground (centre tap of transformer), is a short to the centre (ground pin) of U3/7812reg. you say that's wrong?

Cheers George
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