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I am servicing a Pioneer Audio amplifier (PMA-320) which sadly has an open primary winding in the mains transformer. As you may know, original replacements are unobtainium therefore, I am planning to retrofit a conventional toroidal transformer, therefore, I would like to ask you for feedback to confirm if I am on the right track.

According to the technical specs from this amplifier, it is capable of delivering 60W+60W into 8 ohms (0.05% THD) and a maximum of 90W+90W into 4 ohms (1% THD). The VAC output voltage before rectification is 36VAC per secondary winding. Once rectified it should be of about +/- 49VDC.

Transformer section PMA-320

Taking a maximum power delivery of 90W per channel, the transformer to be fitted should have a minimum VA rating of 180VA and two secondary windings of 36 V (when loaded). If my simple math is correct... ;)

Would a 225VA transformer be close enough to substitute the original transformer? Taking into account some current overhead for transients when listening at high volume levels.

The closest output voltage that I could find here in the UK is 35VACx2 which I suppose won't make a great difference on B+/B-.

The next important point to consider is that the original transformer was potted from the factory and shielded in a metallic enclosure:

enter image description here enter image description here

I am totally aware that I should mount the new transformer to the chassis using the right mounting kit (rubber insulator and washer) to avoid shorting the outputs, however, my next issue is going to be dealing with the tight space constraints inside the amp. Having said that, two questions come to my mind:

  1. Should I try to shield the transformer using a metallic harness/enclosure?
  2. Should I consider potting it myself?

Cheers

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    \$\begingroup\$ As for shielding, thin sheet metal will only affect E-fields, and whenever I hear shielded transformers, I think about clueless audiophiles using gold plated mains cables. \$\endgroup\$
    – winny
    Commented Jan 30, 2022 at 21:15
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    \$\begingroup\$ It might be worth checking the original transformer to see if it has an internal thermal fuse. It might be repairable. \$\endgroup\$
    – Transistor
    Commented Jan 30, 2022 at 21:23
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    \$\begingroup\$ Agree with Transistor, if the gunk is removable you'll probably find a blown thermal fuse or maybe a bad solder joint between the transformer coil and the colored wires. \$\endgroup\$
    – bobflux
    Commented Jan 30, 2022 at 21:27
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    \$\begingroup\$ @Winny Don't know about Denon but I certainly know of mu-metal shielding on some other hi-fi amplifier transformers. But it doesn't like a lot of mechanical shock : removing the transformer may damage the shielding properties unless it's annealed again (which involves 800C in a hydrogen atmosphere). \$\endgroup\$
    – user16324
    Commented Jan 30, 2022 at 22:18
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    \$\begingroup\$ Got a feeling 225VA would be too lightweight to drive 2x90W into 4 ohms, but probably OK if you restrict it to 8 ohm speakers. If that casing is mu-metal, expect some hum without it. \$\endgroup\$
    – user16324
    Commented Jan 30, 2022 at 22:22

1 Answer 1

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I just wanted to update you and close my post with the final result of my repair.

I decided to fit a new transformer. The specs from the one I chose are 300VA 2x35V, precisely this one.

Here is the final look:

300VA mounted on a custom 3D printed bracket

Mounting it was a bit of a challenge due to space constraints. The original transformer was quite slim and thus it had to be raised about 1 inch to clear the capacitors, wires and other boards. It was safely secured and isolated with the included mounting plates and rubber washers.

To do so, I designed (in Onshape) and 3D printed a custom support with an off-centre mounting hole:

custom riser

As a result, the amplifier is now back to life and working even better than with the original transformer!

I evaluated the power output and I was able to squeeze a clean output at 120W per channel onto a resistive load (8 Ohms, though it got quite warm :S). Testing it with my pair of Denton 85th the sound was very crisp and warm. Success!

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    \$\begingroup\$ Great post. Thank you. Can you advise how you wired the grey, red, yellow and black wires to the rectifier board? There are 3 pins on the board and 4 wires. \$\endgroup\$ Commented Jun 2, 2023 at 4:02

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