Help Troubleshooting Intercom

Discussion in 'General Electronics Chat' started by itsinthepast, Aug 25, 2010.

  1. itsinthepast

    Thread Starter New Member

    Aug 25, 2010
    Hi all, I'm new here but not new to electronics. I've been working with electronics for more years that I care to mention, but as all things go, my job took me in a different direction so some of my electronics knowledge has faded so I'm hoping to get some help.

    I have a whole house nutone intercom system. I've attached the schematic. The unit is about 15 years old at least. Recently it started buzzing, to be more specific, it has an intercom and radio built in. When in intercom only mode, when you push the "push to talk" it starts buzzing and stays that way until you push the "end call" button. If you put it in intercom/radio mode, it buzzes constantly even if you push "end call". Turning the radio in that mode, you can hear the radio, but barely under the buzzing. The clock works ok on the LED display, but the frequency for the radio on the LED display is garbled. I've disconnected the radio from the circuit, so effectively its not connected at all, as well as the remote speakers, and the problem stays, so its something in the main unit.

    My initial thought is something in amp section or the freq & timer board, since parts of the circuits, like the clock, work ok. I have a full compliment of test tools, (meters, scope, etc).

    Any hints, suggestions, ideas on where to look would be appreciated.

  2. someonesdad

    Senior Member

    Jul 7, 2009
    When you say "buzzing", what frequency is it? Look at the signal to the speaker with your scope. I'm going to bet this buzzing is at the line frequency (or double the line frequency). If so, start tracing back through the circuits to see if you can determine where it's coming from. It's a good bet to look for a bad capacitor in the power supply, which is not doing its filtering job.

    You can use the scope in an AC-coupled fashion to trace the noise, but it's also useful to look at the DC voltage levels, so DC coupling is also useful. A common first step in troubleshooting something is to first check that the requisite voltages from the power supply are present and have proper values. Trigger the scope from the line and if the noise voltage is stable, it's almost certainly coming from the line somehow.

    The first suspect in older equipment is often a bad cap in the power supply.
  3. marshallf3

    Well-Known Member

    Jul 26, 2010
    Nutone equipment was rugged, but as with anything their power supply filter caps fail over time.

    They are also notorious for the switches and pots getting dirty so while you have it apart hit them all with some contact cleaner/lubricant. Sometimes this alone will solve the buzzing problem you describe but since the display is also affected that really points more towards the power supply itself.
  4. itsinthepast

    Thread Starter New Member

    Aug 25, 2010
    It was c116, the 2200mf cap. Thank you both for the advice and feedback, I'll also hit it with tuner cleaner. And, I had fun getting back into it...

    Thanks again
  5. edjack11

    New Member

    Dec 15, 2012
    Sorry to be bringing this post back from the dead, but it was very helpful to me for fixing my IM-3003. So I wanted to come back and document the fix for others in the future, since information on these old intercoms is just about non-existent. This is a common problem on the Nutone IM-3003. Symptoms: The volume being very low, a buzzing noise on all the speakers, and/or the LED display being garbled or randomly changing numbers. All of those symptoms have the same common cause. They are because of the capacitors for the amplifier power supply going bad. The original poster itsinthepast found the specific cap that went bad, and seems to have only changed that one bad one. But if you are going to do this, you might as well do a full rebuild on the amplifier and replace all ten capacitors. They are all near the tall black heat sink, there are seven in front of the heat sink, and three behind the heat sync. This is the parts list for the 10 capacitors:

    100MF 10V - quantity of 2
    1.0MF 50V
    100MF 16V
    10MF 16V - quantity of 2
    470MF 10V
    220MF 25V
    1000MF 16V
    2200MF 25V

    If you replace that set of 10 capacitors, which will fully rebuild the amplifier on the IM-3003, that will fix the intercom 95% of the time. It is almost certain that is the problem with these models, regardless of the various weird symptoms you have, they all point to the same common problem.

    These extra steps are not needed, but optional, while you are there you can also replace the four diodes on the amplifier power supply, these usually don't go bad, but I figure replacing them might help to clean up the power flow, then are:
    GP15B, 100 Volt 1.5 Amp, silicon rectifier diodes. Quantity of 4. These are the ones that are in two sets of two, near the heat sink.

    If you really want to clean up the sound nicely. Then also go to each remote station. Replace the two 16volts 47uF capacitors, per station. Then blow out the Pots and Push buttons with compressed air, then spray them with some Deoxit Cleaner, then Deoxit Fader Lube. (Or other comparable cleaner and lube). Then the remote stations will sound very clean and nice. If the stations make a screeching noise when you push the buttons, that's because the contacts for the buttons need to be cleaned/conditioned/lubed. Then they will be nice and clean and clear sounding when you push the buttons, or adjust the volume control on the Pot.

    Do all these things, and you will have a brand new condition IM-3003 that will last another 15 years. (Then you get to do it all over again 15 years from now...)
    Last edited: Jan 18, 2013
    absf likes this.