Rescued an MI-3

Discussion in 'McIntosh Audio' started by miniman82, Jul 28, 2017.

  1. AdamAnt316

    AdamAnt316 Collector of heavy things Subscriber

    Finding a replacement transformer is going to be difficult, as it's a rather specialized type (1,200V with a LV tap at 400V, IIRC, plus two 6.3V filament windings, one of which needs to be able to handle -1,100V or so riding on it). You might be able to salvage one from an old tube oscilloscope, though those often used 5" CRTs so the HV may be much too high (and it might not have the same sort of LV tap). Your best bet would be to have the original transformer rewound; this company apparently can do it, but it's not cheap. Good luck!
    Little Nellie likes this.


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  2. Little Nellie

    Little Nellie Well-Known Member

    SF Bay area
    I did, one talks about the filament transformer, non about the HV side other than miniman82 able to fix from a break he can see. If you have any info other than a transformer rewind as Adam suggest, please share. Thanks
  3. w5hro

    w5hro Member

    Hey, thanks for posting that link. I've purchased stuff from Radio Daze before, but it looks like they expanded what they sell now.

    I was actually looking at maybe trying to find a MI-3, but I would need to modify my mac 1700 receiver with the TP-1 and TP2 outputs so it would work. I think it can be done though by tapping off the circuits using buffer transistors.
    cduarte likes this.
  4. miniman82

    miniman82 Go Navy!

    Great Mills, MD

    If you can get it removed I could try repairing it for you. The HV winding has tiny copper wires, they are very hard to see. If they are broken near where the large lead out wires are like what happened in mine it may be possible to fix it, you have nothing to lose really. If I can’t figure it out, I could ask heyboer if they would be able to rewind it for you.
  5. twiiii

    twiiii Addicted Member

    west Texas
    DSCN0645-07.jpg Long ago copy (1).jpg I had a MI-3 I bought when I traded my MR-67 for a MR 71in 64 ???. I added a rotor to my FM antenna stack and the MI-3 allowed me to get the best performace. At the time the college station had a 2500 watt effective power rating and the transmitter was 100 miles away. When I Long ago copy (1).jpg Upgraded to a MR -77 I traded the MI-3 for a MPI-4. Watch out the MPI-3 doesn't have the protection circuit the MPI-4 has so you can easily burn the display coating on the inside of the tube if you set the intensity to high. As you can see when the MR-80 and C-29 had come along the Knobs had been changed.
    Last edited: Jan 11, 2018
    AdamAnt316 likes this.

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