As the subject says, I have a Denon PMA 250 with an open circuit mains transformer primary - no doubt the thermal protection has failed. Can anyone tell me the secondary voltage?
RonUK
Assuming you have tried excavating into the wrapping to find a th f and not found it.
Is the secondary wire/s carried out to the terminals ? Then measure diameter of the wire/s , measure DC resistance of wire/s and
1000Hz inductance, guesstimate the average diameter of turns and find wire gauge to resistance table and isn't there a formula for the ratings of the secondary then.Failing that, try those measurements on some other known or powerable similar transformer.
-- Diverse Devices, Southampton, England electronic hints and repair briefs , schematics/manuals list on
Hmm. I just found a "datasheet" for a Denon PMA250SE. It quotes the output power as 2x50W into 4 ohms. And I'm wondering: how feasible is it to "reverse size" a power transformer based on an amp's output rating? For example in this case, 2x50W =3D 100W x some_fudge_factor (to account for losses, powering preamp circuitry, etc.), so maybe a PT capable of outputting 150W (at a reasonably proper voltage) might do the trick? Or is there some large loss or conversion factor I'm ignoring?
If you can't get a definitive figure for the secondary voltage, how about the old 70% of the main filter caps' rated voltage, times 0.707 guesstiformula ?
So, for instance, 63v rated caps, times 70% = about 44v, times 0.707 = about 30v RMS. As for current rating, take output stage efficiency at say
50% so for total output power of 50 watts per channel, stage input power around 200 watts. With 88v of supply across it, that's around 2.3 amps. 30-0-30 at 2.5 amps sounds reasonable. Just as a matter of interest Ron, why not a 'genuine' replacement ?Arfa
That`s what I`ll do, I was just hoping someone had a service manual to hand.
It`s actually rated at 30 watts p/c, the smoothers are 50 volt
£115+vat is why not ;) The amp's worth about a tenner CheersRon
**I only have a manual for a PMA350-II. It is rated at 50 W/ch. Secondary Voltage is rated 34-0-34. I reckon 30-0-30 sounds about right for your amp. I had the same problem with a Denon a couple of months back. Bloody potted toroidals!
Cheers for that, it`s about what I figured using the 70% 'rule'
Ron(UK)
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