70’s - 80’s model NuTone intercom system

[Wadew]

We moved into an older home that is in overall good shape but many of the fixtures and equipment in the house is original to when it was built in the 70s. Not sure when the intercom system was installed but very likely came from when the house was originally constructed. Currently it powers up the tape deck rotates. it does make an audible tone that is constant but picks up no radio and the coms between the hub and the remote stations throughout the house do not work. How do I start troubleshooting this system? Will it be easier to gut it and replace with modern components? We want to keep the original aesthetic look but bring it up to a modern, or at least functional standard. Thanks in advance!

 

[Ylli]

Can you perhaps find a model number on it? If we are going to have *any* chance of helping you trouble shoot this, we are going to need to find some service info on it.

 

[Ylli]

Can you perhaps find a model number on it? If we are going to have *any* chance of helping you trouble shoot this, we are going to need to find some service info on it.

Found it!. http://northsideforyourhome.squarespace.com/nutone-intercom-system-service/2542service.pdf
Let me study it for a bit and I'll get back to you.

 

[Ylli]

I've seen more convoluted schematics, but not by much..

Is the tone coming from the speaker on the control unit? Does changing the volume or tone control setting have any effect on the audible tone?

What do you have available for test equipment? Digital Multi-meter? Scope? How about your ability to solder/unsolder parts or connections? Can you follow the schematic at all? If I ask you to measure the voltage from either end of R58 to ground is that doable for you?

 

[Wadew]

I do have a multimeter, my soldering skills are minimal. I am a novice with this kind of stuff. I can tell you the tone is coming from the speaker on the control unit. It can be directly controlled with the volume knob and it cuts out when either of the “talk” switches are engaged.

 

[Ylli]

So the offending tone drops in volume when you reduce the volume control, and increases when you turn the volume up. That tells me the tone is coming from before the volume control. Let's back up to the mode switch. Is it any different if you select AM, Tape, or FM?

 

[Wadew]

No the tone is the same in any setting.

 

[Ylli]

If the volume control does not have any effect on the tone, then it is something in the audio amp, or there is a feedback somewhere. It could be that the unit is trying to talk and listen at the same time. Physically examine the talk-listen switches the best you can. If these have open contacts, look for any bent or broken contacts. You could try using some contact cleaner on them.

To go any further, we'll need to dig in to the circuitry. The service manual seems incomplete (no layout drawing for the 'main' board) and does not seem to match your pictures exactly. You'll need to find some parts on your own. Let's see if we can find a feedback path.

Locate "D7". It is near the corner of the board that is closest to the speaker and cassette. (lower right in your pix). Gently move the two adjacent disk caps out of the way and using a jumper wire, short out D7. This will disable the intercom preamp. Does the tone go away with D7 shorted?