Hello everyone! I was looking around at one of the local hamfests when what should I spot sitting on a seller's table but a McIntosh MI-3! It was sitting on top of a Heathkit AJ-15 tuner; the seller was willing to sell it separately for $200. According to the seller, it has HV and/or CRT issues. He told me it had worked fine with the Heathkit tuner until one day, when the MI-3's fuse blew.
He was under the impression that the HV selenium rectifier may have been to blame, took it out-of-circuit, and replaced it using three silicon diodes in series (1N4007s, I'm guessing) along with a resistor in an attempt to make up for the difference voltage drop. This had apparently cured the fuse blowing (not sure how many times it happened), but the CRT still wouldn't might up, which made him think that the CRT may have been blown. He picked up a replacement 3RP1 from Sphere Research, which was included.
Using a cheapo DMM, I checked the filament of the CRT, which turns out to be OK. I also ohmed out the HV taps on the power transformer: The main HV tap (1200V?) measured around 1,100 ohms, while the blue wire for the center tap (400V?) measured around 80 ohms to the red/yellow wire. Does this seem out-of-place? Is there a known source for a replacement, or might there be a company out there which would be willing to rewind a transformer like this? Anyway, here are some photos:
Front of the unit:
The front panel looks to be in good shape overall:
A view of the top of the chassis, seems pretty clean:
Underside of the chassis, again looks pretty clean:
Closeup of the chassis; note the modification in the HV section:
Close-up of the modification:
Another angle of the mod:
Other than the diode mod, it looks pretty good inside. I visited a local electronics supply store, and they had a rather large NTE-517 diode there meant for microwave ovens. It's rated for 15KV@550mA, so it should be more than enough to replace the old selenium diode. Might the old diode going bad have caused the issue, or is it more likely to be the power transformer? Any suggestions on how to go forward? Thanks in advance!
-Adam
He was under the impression that the HV selenium rectifier may have been to blame, took it out-of-circuit, and replaced it using three silicon diodes in series (1N4007s, I'm guessing) along with a resistor in an attempt to make up for the difference voltage drop. This had apparently cured the fuse blowing (not sure how many times it happened), but the CRT still wouldn't might up, which made him think that the CRT may have been blown. He picked up a replacement 3RP1 from Sphere Research, which was included.
Using a cheapo DMM, I checked the filament of the CRT, which turns out to be OK. I also ohmed out the HV taps on the power transformer: The main HV tap (1200V?) measured around 1,100 ohms, while the blue wire for the center tap (400V?) measured around 80 ohms to the red/yellow wire. Does this seem out-of-place? Is there a known source for a replacement, or might there be a company out there which would be willing to rewind a transformer like this? Anyway, here are some photos:
Front of the unit:
The front panel looks to be in good shape overall:
A view of the top of the chassis, seems pretty clean:
Underside of the chassis, again looks pretty clean:
Closeup of the chassis; note the modification in the HV section:
Close-up of the modification:
Another angle of the mod:
Other than the diode mod, it looks pretty good inside. I visited a local electronics supply store, and they had a rather large NTE-517 diode there meant for microwave ovens. It's rated for 15KV@550mA, so it should be more than enough to replace the old selenium diode. Might the old diode going bad have caused the issue, or is it more likely to be the power transformer? Any suggestions on how to go forward? Thanks in advance!
-Adam