When was the last time it was used? If it sat for years your caps are probably shot.
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It sounds like possibly a capacitor is letting go. They stopped making the 1900 almost 40 years ago so it’s way past time for service. If the amp had been sitting around not being used and you fired it up this can be a problem. I think the speakers are 8 ohm but if they go low at any frequency I know the amp is not happy at all. I liked McIntosh integrated amps but there inability at low impedances was a deal breaker.
I see that you're looking into an insurance claim but when you buy something as old as this amp it's as-is unless he gave you something in writing.
I would strongly suggest disconnecting the speakers, turning the receiver on, and measuring the DC voltage on the speaker terminals to see if it is producing any DC voltage. I know this from personal experience. Since the 1900 receiver is Direct Coupled with no safety (i.e. speaker relay, etc.) if output devices fail it will feed DC voltage directly to the speakers. My Father had a 1900 and we connected it to a set of old Advent speakers. We turned it on but there was no sound so we assumed it had no output. That assumption was wrong because after about a minute of looking around and checking connections, the speakers literally started on fire. The woofer coils got so hot they started the cheesecloth on fire inside the sealed cabinets. We removed the woofers with the cheesecloth burning and dumped water inside to put out the fire. Needles to say the speakers were ruined. I took the receiver to the local repair shop and it was putting out 60 VDC on Left and Right channels. I had the receiver repaired and listened to it afterward and it sounded great, but I used a speaker protection device between the receiver and the speakers from there on.
Over 40 years units can go bad any minute any time. So claiming refund from seller would not be legit.
Smoke also goes from transistors mainly driving transistors.
I usually ask $200 + high priced parts if overhaul is needed.
I void shipping turnarounds and so advice you do same.
If you’re 1...2 hours of driving from descent tech, you should take the ride and give your unit for repair.
Be ready to stretch to $150...200 ball park. Also some claim examiners are pretty smart so they will note the age of unit and deny your claim.
Are you building a antiques-only system? If so, maybe get the old Mac receiver an expensive repair, which will not be cost-effective IMO, but then you'll have the sentimental glow of an Olde Audio Experience. Surely the eBay (basically flea market) purchase was understood as an As-IS thing? Let's be reasonable about this. No way you paid a lot for this Olde boat anchor, right? Chalk it up to experience and the inevitable risk of buying old stuff sight unseen. Sometimes eBay luck turns bad. If you want real warranty and real buyer protection, well that costs real money, as it should.
Or maybe spend a few more bucks and get a way more satisfactory modern product that doesn't need to be re-capped and will sound better? Many options out there, and not all of them super-expensive.
Eli - anything greater than Millivolts is too much, but you shouldn't even have to go that far because if there is any smell of heat or burning there is something wrong with the receiver. When these receivers are in proper working order they don't generate any substantial heat even when driven hard, and they definitely should not be generating any smell. I am guessing there is something wrong with the receiver to make these components get hot, and it probably should not be used until the problem is resolved or it may damage your speakers.