Rescued an MI-3

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

  1. miniman82

    miniman82 Go Navy!

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    This was a cheap $200 eBay find, but it did have some serious problems.
    IMG_1280.JPG
    Obviously the glass is toast, at some point the scope test switch took a hard impact and broke the wafer. No trace on the CRT either.

    The power transformer in these uses hair fine copper wire for the -1300v CRT supply winding, and this one was open circuit. With nothing to lose I opened the transformer, and sure enough I saw a break in the thin wire right near the surface. It's caused by corrosion near the point where the thin winding connects to the normal size lead wire. I was able to carefully fix the break, then potted the new connection in epoxy so it hopefully never happens again.

    Now I had a green trace, but brightness didn't work. That wound up being a bad CRT (3RP1), must be a cathode/G1 short or something. I pulled a good CRT out of a scope I had in the garage and installed it, now brightness functions normally again.

    Then all I had to do was replace the scope test switch with one I picked up on eBay and it's functioning electronically fine again, next paycheck I'll order and replace the glass and the MI-3 will be perfect once again.
     
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  2. VintageMac

    VintageMac McIntosh Junkie Subscriber

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    I'm not sure McIntosh makes the MI-3 glass....

    Larry
     
  3. damacman

    damacman Blown and Injected Subscriber

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    Job well done! Always nice to see such a unit rescued.
     
  4. miniman82

    miniman82 Go Navy!

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    Tom has a pair, I'm ordering on Monday.
     
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  5. VintageMac

    VintageMac McIntosh Junkie Subscriber

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  6. 62caddy

    62caddy Trust but verify Subscriber

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    Nice to see these older Mcs saved. Glad everything worked out.
     
  7. kevzep

    kevzep Its all about the Music Subscriber

    Really nice work man!! Love to see some out of the box thinking and using some basic intuitive skills....

    Very impressive!!.
     
  8. AdamAnt316

    AdamAnt316 Collector of heavy things Subscriber

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    Nicely done! :thumbsup: I recently went through a similar debacle with my own MI-3. In this case, the CRT filament winding had apparently shorted to ground, which caused HV to go where it shouldn't, killing the HV rectifier and frying parts of the intensity control. To remedy this, I purchased a filament transformer with a split bobbin, and wired it in place of the CRT winding wires from the main power transformer. The results don't look overly elegant by Mc standards, but the unit no longer goes up in smoke. My question is, how easy was it to get to the CRT filament winding within the transformer? Might I be able to perform a fix similar to what you did? I have yet to attempt work inside a transformer, so I'm not sure quite what to expect, or exactly how to go about it.......... :dunno:
    -Adam
     
  9. miniman82

    miniman82 Go Navy!

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    Yeah, filament shorts are bad news here since the heater is biased to the same potential as the cathode (-1300v). It's not hard to dig into a transformer to get at the windings, you just have to be uber careful since you have to use a sharp razor blade to peel back the insulation paper like an onion. Danger there would be slicing into an adjacent winding, cutting a wire altogether. This transformer has a copper strap around the whole thing, which you have to heat up with a small propane torch in order to get the solder to melt. Once you have that off there's a layer of paper you have to cut through, bend that back and you'll begin to uncover the individual windings. Delicate work, but not impossible. To make sure I didn't break the thin HV wiring a second time I epoxied the wire to the core before soldering, then covered everything to keep it from moving around.
     
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  10. AdamAnt316

    AdamAnt316 Collector of heavy things Subscriber

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    Thanks for the reply. I may end up leaving this one to the professionals, since it sounds like it'd be far too easy to make things worse than they currently are. For the time being, the outboard filament transformer makes the CRT's filament glow nicely, the intensity control doesn't smoke anymore (need to replace it at some point, since it has several dead spots), and I get a nice 'trace' on the screen:
    [​IMG]
     
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  11. miniman82

    miniman82 Go Navy!

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    If it works as is, i'd just as soon leave it. "do no harm", as the doctor's motto goes.
     
  12. miniman82

    miniman82 Go Navy!

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    Now I'm starting to get pissed off...

    This is the third one this year these assclowns have broken! First a MC2002, then an MC7100 now a brand new MI-3 plate. I have a new policy: if I can't pick it up in person, I'm not interested! It's simply not worth the hassle anymore, I'm done shipping Mac gear.

    Not like these things grow on trees.

    :rant:

    IMG_1289.JPG
     
  13. damacman

    damacman Blown and Injected Subscriber

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    Dude ... That totally sucks!! Didn't you say Tom had another? Maybe he could triple box it ...
     
  14. 62caddy

    62caddy Trust but verify Subscriber

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    Well they say bad things come in threes....:rolleyes:
     
    Last edited: Dec 20, 2017
  15. miniman82

    miniman82 Go Navy!

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    Success! Kudos to Tom, he filed the claim with fedex and it went through. Said I was the first in 11 years to have an issue, which caused him to add extra packing material to glass he ships. I added the MI-3 to my bedroom system, looks nice and works great.


    IMG_1378.JPG
     
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  16. Little Nellie

    Little Nellie Well-Known Member

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    Has anyone having a need to change the Se. Rectifier? SR1 to SR7.. If yes, what type did you changed to?
     
  17. cduarte

    cduarte AK Subscriber Subscriber

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  18. AdamAnt316

    AdamAnt316 Collector of heavy things Subscriber

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    SR7 (the HV stick diode) had to be changed in my MI-3 because an internal short in the power transformer took it out. It was originally replaced with three 1N4007s in series by the person I bought the MI-3 from; I replaced those with a NTE517 microwave oven diode which far exceeds the specs of the original EDAL diode (15KV@550mA as opposed to 3KV@3mA!), and looks similar to the old one. I have yet to replace the other selenium rectifiers in my MI-3 because that part of the supply is fine.

    Old:
    [​IMG]
    (also note the aforementioned 1N4007s)

    New:
    [​IMG]
     
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  19. Little Nellie

    Little Nellie Well-Known Member

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    The HV winding of my transformer is dead. Took the transformer out and cover off to see if there is a easy fix and can't find any. Called McIntosh for part and was told non available. Anyone know of where to get a replacement transformer to get this working?
     
  20. damacman

    damacman Blown and Injected Subscriber

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    there are a couple of MI-3 threads here that discuss solutions to this - you may search them.
     

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