Mirtone MA-401B apartment intercom amplifer that would squeal/yelp/chirp when you pressed the Listen button. Now it just oscillates a steady tone when you press Listen and you can't hear anyone from the vestibule.
Date code 1980 with 10 mostly Marcon electrolytics and 13 discrete transistors and a triac for the door solenoid. It has a 7812 for +12VDC power and 16VAC input. I found one open 22uF cap but replaced all of them, it is running 40 years 24/7.
Capacitor list:
1 of 470uF 35V
3 of 47uF 16V
1 of 22uF 16V
4 of 10uF 35V
1 of 0.47uF 50V
If you have hum problems, "T2" is earth/chassis ground so you need an extra wire for that. "S2" is also shield GND for vestibule mic/speaker.
There is still some hum but they used the wrong wire (not twisted pair or shielded) in the building.
Mirtone company was renamed to Mircom, and the MA-485 is the 2nd generation replacement module.
Date code 1980 with 10 mostly Marcon electrolytics and 13 discrete transistors and a triac for the door solenoid. It has a 7812 for +12VDC power and 16VAC input. I found one open 22uF cap but replaced all of them, it is running 40 years 24/7.
Capacitor list:
1 of 470uF 35V
3 of 47uF 16V
1 of 22uF 16V
4 of 10uF 35V
1 of 0.47uF 50V
If you have hum problems, "T2" is earth/chassis ground so you need an extra wire for that. "S2" is also shield GND for vestibule mic/speaker.
There is still some hum but they used the wrong wire (not twisted pair or shielded) in the building.
Mirtone company was renamed to Mircom, and the MA-485 is the 2nd generation replacement module.
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