Mac 1900 thumping sound

Johnc37

New Member
hi everyone I recently purchased my first Mac from a friend. It had been sitting in storage for a long time. After turning it on for the first time I noticed it had a low thumping sound in both channels with the volume turned down all the way. I've deoxit all pots and push switches. Reseated the driver boards and cleaned the preamp jumpers. It seems like it might have lessened but it's still audible. Pushing in the main speaker switch stops the thumping sound if the speakers are hooked up to main. Same thing occurs with the remote speaker switches when speakers are connected to remote. Everything else works properly. It sounds even and balanced. Just this faint thump sound at all volume levels. Anyone had this problem or have any ideas? Thanks
 
So when you have a closed circuit (Speakers connected and selected) you get a constant thumping, like a low heartbeat?

I never had that on my MAC 1900, but it sounds like something is oscillating. Only it is on both channels, so likely in the power supply. This is a SWAG, and I am sure someone more knowledgeable will respond. You could try looking in the Hints and Kinks part 1 and 2.
 
I also noticed the FM tuner below 96 is crazy staticky. It won't tune into anything below 96. The sound of the FM is like a short wave radio tuning. Above 96 it tunes fine. The thumping sounds like a low pitched helicopter rotor. I'm going to check the ground below the power supply board next. I've gone through almost 2 cans of deoxit cleaning switches and board connectors.
 
TWO CANS of Deoxit? More is not better and if put in the wrong place it will destroy adhesives that hold the carbon traces inside pots.

It needs a date with a tech's test bench.
 
Thanks I'm definitely bringing it to a tech to look at next week. It's a very nice receiver cosmetically. I usually screw up more than I can actually troubleshoot or fix. I bought it from a friend who never even turned it on and sat in his closet for 10 years. He got it from someone who was moving. Have all the original manuals that came with it.
I didn't test it out before I bought it. My buddy didn't have speakers to hook it up to! I would have purchased it regardless due to its cosmetics and reputation.
 
Just talked to a tech and he has a Mac 1900 in for the same issue-motor boating- power supply capacitor issue. He said the FM tuning is common on Macintosh units where it's the tuning capacitor contacts that are made of silver needing to be re burnished. I'm going to have him go through the whole unit and I'll update on what fixed it.
 
I took it to a place called Stereo Rehab in Chicago. I will say the Mac 1900 is now repaired. The tech there is Casper. Awesome guy. Super conscientious and truly loves his work and customers. He spent a lot of time with me as I also brought a couple of sansui 9090db driver boards in for repair. Personally showed me how to set dc bias and offset on both boards. The Mac 1900 needed a power supply cap and cleaning of the tuner. Reasonable price and was finished the next day. Casper is great and highly recommended.
 
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