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mmerig 1st April 2012 10:16 PM

Arcam CD72 power supply
 
My Arcam CD 72 cd player stopped working, as if the fuse blew. After some sleuthing, I found that the transformer primary winding is open. I want to replace the transformer (which is a proprietary, sealed shileded thing) with some off-the shelf ones to run the clock, motor, digital grid, display, and display filament. The audio and DAC have their own power supply that I put in a some years ago as a modification. Can someone please give some advice on transformer ratings?

Attached is a schematic from the manual. What I am unsure about is what the AC voltage should be between transformer points 11 and 13. The latter looks like a center-tap, but if it was, why wouldn't the digital grid ground (DGND) connect to point 13? Is point 13 not a center tap?

Assuming that point 13 is a centr tap, here is what I think I need:

1. A center-tapped transformer with 30 volts AC on either side of the CT (parallel) . This would power everything needed except the filament The motor only needs +7.3 VDC and only +5 VDC for each of the others coming of of #13 and #15 is needed, but finding a transformer with asymmetrick windings may be tough. I figure 0.5 amp current is enough

2. A simple (no center-tap) transformer with 8 or 9 volts output in series to run the filament. the current could be vey small, say 0.1 amps.

I am just guessing on the current ratings.

I don't know what caused the winding to fail. The transformer does not smell and I did not see any obvious failed components in the power supply. To be safe, I replaced all of the capacitors and voltage regulators and checked the diodes (which were okay).

There was about 6.5 volts AC at all of the transformer output pins while on the pcb. I tested the pulled transformer and linked the proper pins and the primary tests open. I don't know how there was even 6.5 volts.

mmerig 1st April 2012 10:21 PM

1 Attachment(s)
Here is the schematic

mmerig 2nd April 2012 03:38 AM

As for my last comment about where the 6.5 volts came from: the winding between pin 1 and 2 is okay. It's the winding between pins 5 and 7 that's bad.

godfrey 2nd April 2012 04:45 AM

As far as I can see:
There's two sets of windings; the top pair marked 1/2/7/6 and the bottom pair marked 4/5/10/9.

It looks like for 230V both sets of windings are used in series, but for 115V only the top set is used. If that's the case, perhaps you can rewire it to use the bottom windings instead of the top.

What's the Molex connector for?

Edit: Disclaimer - I wouldn't rush out and start rewiring it without a second or third opinion.

AndrewT 2nd April 2012 01:11 PM

Check for continuity through each pri & sec winding.
Check for short to earth from any winding.

mmerig 2nd April 2012 04:02 PM

Great idea to try the bottom windings. To check, I connected 5 & 10 together and checked for continuity between 4 and 9, and there was none.

Resistance for the separate windings are:

1-2: 25R (R = ohms)
6-7: open
4-5: open (darn!)
9-10: 57R

11-13: 1.6R
13- 15: 36R
19 -20: 2.2R

There are no shorts to ground from any winding (checked with the transformer removed).

P.S. I had an error in my last message. It should read: " It's the winding between pins 6 and 7 that's bad" and not ". . . . pins 5 and 7".

The molex connector (SK6) is not connected to anything in the stock CD72.

Pehaps when the 7-6 winding failed, a fuseable link to pin 4 broke.

The transformer is in a shielded case filled with epoxy or something like that. It would be a nightmare to get it apart, and I would not trust a repaired one from a fire-hazard standpoint anyway.

Years ago, my house wiring was stuck by lightening, and the DAC on this cd player was ruined. I wired in a dac (RAKK DAC) and audio output stage and it had worked fine for years. I want to repair the cd player, which sounds good.

AndrewT 2nd April 2012 04:17 PM

Asymmetrical transformer?
maybe not.
Is it possible that it is a 30Vac CT transformer? With a high current rating on the +ve side winding?

What about buying a 24Vac CT and adding a few dozen turns to give the 12+18 for the same 30Vac or so?

Mine is a Delta 70.2
Is there any point in opening it up to measure some voltages?

mmerig 3rd April 2012 01:00 AM

Using a variac, I hooked up a center-tapped transformer with 30VAC on either side of the tap, to points 11, 13 and 15 on the pcb and applied 8 VAC to points 19 and 20on the pcb. Within seconds, I smoked a resistor (R48) and some small ceramic caps near it. The voltage across points 11 and 13 were too high, so the simple off-the shelf transformer option is probably not viable.

I will not pursue repair any longer and try to get a used player and stick the RAKK Dac in it.

Thanks for the advice. I did learn something.


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