Good luck with your troubleshooting, don't give up on that unit its worth keeping. it could be another piece of equipment or a ground loop introducing the hum. If the hum goes away, start connecting all the other gear such as tape deck, Phono etc, one at time testing as each one is reconnected. A quick internet search reveals a handful of MAC-1900s ranging in price from 700 to 1,599 for a near-mint specimen recently acquired by New Jersey’s SkyFi Audio, a joint venture with New York City’s Stereo Exchange that deals in vintage and esoteric audio gear. If you have an oscilloscope, its a breeze to pick up hum on VCC buss lines, again suspect failed lytic's, after all this baby is 40 years old Best of all it includes the original owners manual, service manual, AND the schematics. Since the hum is present in both channels, the defect is one that has to be shared by both left and right channels, most likely suspect is power supply. The MAC-1900 is quickly becoming a great collectible piece, mostly due to ts great features, solid tuner section, and amazing looks. Is the hum still present? If so you have proved it to be the rcvr and not anything connected to it. Since you never mentioned which input you are using there are several variables.ġst, disconnect all other equipment(except spkr's) connected to the rcvr, set to aux in and turn on. #BUY VOLUME CONTROL FOR MAC 1900 DRIVER#This rules out an amp or driver circuit issue as the hum is present before the vol control. In your post, you state the hum increases or decreases with the volume control setting. The actual manufacturer was Fosters (Japan) under contract to McIntosh, as I recall they only made a few thousand pieces, so its a bit of a rare bird. A fellow StereoTech owner! I bought mine in 1976 while I was service manager for a McIntosh dealer.
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