Mirtone MA-401B Intercom Amplifier - repair/recap

[redwire]

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.

[CapLeaker]

Re: Mirtone MA-401B Intercom Amplifier - repair/recap

Nice repair redwire. Here in the sticks I haven’t seen many of these, but I remember that they always had a little hum.

[redwire]

Re: Mirtone MA-401B Intercom Amplifier - repair/recap

This uses a 4-wire system so all stations are in parallel, cable just daisy-chained (3 of the 4 wires) through an entire building.
I've repaired many apartment/condo intercoms and it's terrible when a suite has a stuck button (open door, listen, talk) you have to go to each unit to find it. Try that in a 30-suite building, tenants aren't home or you don't have permission to enter. Only the front vestibule Call button is a separate wire to each suite.

One building had a guy do reno's and a nail hit the cable, the door solenoid was randomly going on and off. I disabled it and then the Post Office guy couldn't get in to deliver the mail. Took a month to find that nail.

Including my other schematics for completeness. Mircom IS-401 station wiring, uses oddball speaker 07420043 15 ohms 1W I think 3" just use a 16 ohm part. The vestibule is a 45 ohm speaker though.
Mircom/Mirtone looks like a Canadian company.

[dantm]

redwire I see you've had a bunch of experience with Mircom systems...I have here a 9 unit apartment building with a Mircom panel, door striker, amplifier (MA-485A) and individual inside apartment units. Symptoms we have include: static on the audio (both ways), inability to hear anything (a few apartment units), intermittent door striker option = opening the door is 50-50. The door striker activation is the silent type and I feel the activation of the striker is not as strong (or loud) as it used to be years ago but I may be wrong.

So far we replaced internal units after talking to Mircom and it improved audio in some cases but not all -- it did not solve all the issues...should we consider the amplifier next? And maybe debug the door striker with an external battery?

Thanks!

[redwire]

What you are describing, it's many problems that are not so related.
I did see a building like yours and one owner was doing reno's and had damaged the cable. It daisy-chains from unit to unit. He drove a nail or screw through it which shorted the door striker intermittently it would just turn on all by itself or not work at all. Audio was lost for a few units. So consider that your cable got damaged, coinciding with when the problems showed up. Check for any contractor or owner doing recent work and put a drywall screw through the cable etc.

I would Ohmmeter the cable between the (end) MA485 to another other end, at an apartment unit. Look for shorts/opens in the cable. Or try wiggle it a bit to get the problems changing.

[dantm]

I would Ohmmeter the cable between the (end) MA485 to another other end, at an apartment unit. Look for shorts/opens in the cable. Or try wiggle it a bit to get the problems changing.

Any suggestion on how to ohm out ~100 ft distance? The leads are at most 2 ft and that's generous :-)

Thank you!

[redwire]

It's a multi-conductor cable and you are looking for both open-circuit and short-circuit between the conductors.
You need to first find a known good (reference) conductor.
At one end - person with multimeter on Ohms/Continuity on chosen wires (or if you can hear the beep, one person only at the other end).
At other end - person who can short chosen two of the wires.

Talking on radio/cell phone, chose a pair of colour codes and see if you can get loopback Ohms continuity reading. Once you have that, a known good conductor then just sweep through the other conductors to test them.
If you are still lost for a known good ref. conductor, I have used metal pipe ground or PE earth-ground on nearby electrical outlets as the loopback conductor.
Worst case use a long extension cord with a multimeter at one end and the jumper wire at the other end.

Sometimes the intercom cable can have spare conductors (not that they aren't also damaged) but you could move a suite to using those.
If there is a foil shield you can also use that as your common connection and test for shorts to the shield as well.

I have also seen intercoms poorly maintained where they just have a nervous breakdown. One unit has bad pushbutton switch, another the speaker is toast etc. and that can throw you when troubleshooting as well.