Transformer replacement worries

[richlooker]

I just purchased a Denon DP-47F turntable; a fully automatic, quarz lock direct drive thingy from 1985. The transformer was humming loudly, so I decided to replace it. According to the service manual, it's 2x12VAC, and the mains voltage here is 230V, so I ordered a 15VA 2x115/2x12VAC RKD toroidal. The sticker on the back of the turntable says it consumes 8W; 15VA was the smallest non-PCB transformer I could find and I thought a little overdimensioning would be just right - don't want it to run hot.

Before I removed the old transformer, I measured it's secondary to 2x14.4VAC with power off, and 2x13.0VAC with power on. The turntable was upside down and partially disassembled, so I could not check with the motor running. I didn't have the whereabouts to measure the DC voltage after the rectifier. Then I measured the secondary voltage of the new transformer without load; it's 2x15.9VAC.

1.5VAC higher than the original transformer, plus I would expect the voltage drop under load to be smaller. This made me hesitate. Even though the schematic says the unregulated power supply should be +/- 12V, I'm certain it would (also originally) be a bit higher than that - at least before the motor starts - 13 x 1.4 - 2 diode drops? +/- 17V? With the new transformer, assuming it's voltage drop under load is corresponding to the old one, the unregulated DC voltage will be +/- 18.8V.

The unobtainium motor controller chips run off the regulated +5V supply; the unregulated supply feeds the servo and motor control circuits (the lateral and vertical arm movement is also motor-driven, if you can call coil-and-magnet arrangements that rotates <75 and <15 degrees, respectively, motors)

Is there reason for worry?

• Should I just go ahead and install the new transformer?
• First reduce the secondary voltage with some opposite-phase overwinding turns?
• Make it a regulated supply with +/- 12V fixed voltage regulators? LM2940-12 +12V/0.8V dropout plus L7912CV -12V/1.1V dropout?

Cheers, Richard

[mag_therm]

Could you solder pair of antiparallel 1N4... diodes in each lead, in heatshrink, at the power supply terminals?
That would give about  2 * 0.6 V drop, and would be reasonably neat.

Otherwise over-wind as you mention.

[Paul Rose]

It would likely be OK with the new transformer, but I can see why you might not want to try.

I like the antiparallel diode idea.  You could even do two such pairs (in series) per lead first, and remove one if it seems to too low once under load.

I think the pre-regulator idea is overkill, and you start having to worry about dropout of the per-regulator itself.

[andy3055]

If the new transformer is of too large a capacity, it may not drop sufficiently as you expect. Your old transformer must have loose laminations. If you have a nearby motor winding shop, you can get it baked and re-varnished at hardly any cost. If you can get the varnish, you can even do it at home.

[richlooker]

If you have a nearby motor winding shop, you can get it baked and re-varnished at hardly any cost. If you can get the varnish, you can even do it at home.

I don't know of any motor winding shop. The transformer casing is spot welded anyway, so I guess this is a no-go.

Could you solder pair of antiparallel 1N4... diodes in each lead, in heatshrink, at the power supply terminals?

I really like the diode trick; simple and effective.

In the meantime; I have made a couple of discoveries; first, the transformer was wired for 220V (white-red) but I have measured the mains voltage here to 238V, so I will make some more measurements, with mains wired for 240V (white-yellow) - this will make the difference with respect to the new transformer larger. I will find a suitable resistive load to draw the 220mA as indicated in the service manual. Then see how many diode pairs I need with the new transformer to get the same voltage with the same load.

There's something fishy with the original transformer, BTW - google finds zero matches for the part number, and it's not the same as in the service manual - the part number in the service manual gives plenty of hits, both pacparts et.al. and eBay. And the "YODA" designation on my transformer is strange.

There is one original EU transformer on German eBay, photo attached. If everything else fails I will buy this. But everything else won't fail 

Cheers, Richard

[richlooker]

Using the primary winding for 240VAC, with no load, the secondary voltage is 2x13.4V. With a 250mA current draw (one secondary winding only) it drops to 12.3V. I'm guessing 4 antiparallel pairs of 1N4003 should be just right. I don't have these diodes available now; just ordered them and I expect to have them (plus replacements for every electrolytic; 35 in total) on Thursday, will breadboard it then and post the results.

Cheers, Richard

[richlooker]

One secondary, 4 x 1N4003 pairs, 260mA, 12.9VAC RMS. Good thing I bought 20 diodes, I'll need 16 here

Cheers, Richard

[richlooker]

Done and happy with the solution. I added a 2x12VAC/10VA PCB transformer to the component order, measured and compared with the toroidal with diodes. While the AC RMS is almost exactly the same, both without and with load, the PCB transformer gives 1.5VDC higher under load. Probably because the diodes deform the sine, giving a lower peak-to-peak voltage.

Never mind the noise in my scope traces; it's generated by my fluorescent tube magnifier lamp.

Cheers, Richard