Home intercom power supply
 

Our house has a '60s era intercom system with a radio. We like to use it in the morning to catch the traffic and weather report, but lately it has started to hum pretty badly. The power comes from a 24v AC transformer hard wired in the basement.

As a test, I disconnected the power supply and connected the intercom to a 12v DC wall wart. It actually sounded very good with no hum at all, just a little quiet, I'm guessing due to the lower voltage. So I'm wondering if I can just replace the old AC transformer with a 24v DC one? Hopefully somebody knows more about this stuff than I do. The unit is a Rittenhouse RM-2 if that makes any difference.

A 24v ac replacement will probably be your best bet. Cheaper too.

Just an educated guess here... I am guessing that the rectifier or more likely the filter caps are going bad. The transformer is likely not bad, so a new one won't help. The DC supply is a neat idea. You might try an adjustable power supply as a test (an electronics guy will have one). You might find 18 or 20V works better than 24V. It is hard to say since the voltage ratings of transformers are not absolute, but are "nominal."

Thanks. I don't know enough about circuitry to identify the parts you mentioned, but I bet it would cost a fortune to get it repaired. I have a DC output model train transformer that I'll try next to find the 'ideal' voltage.

I dont think the model train power pack will be much better, they are normally 12V. It does sound like the rectifier in your existing unit is on the way out & by supplying 12V DC to it you have overcome the problem. If you can find a 24V DC power supply, You may get the result you want.

Good Luck !!!

Get it repaired.

Just using a DC external power supply and leave the bad components in is a very bad idea.

'60s electronics is easy to repair and parts are cheap.
A bad power supply can cause a short circuit and start a fire.

Rob

I agree with you in principle, but the odds of getting something like this repaired at a reasonable price around here are close to zero. The intercom itself is a low voltage system, and I would buy a new DC power supply. Is there really a danger?

Yes there is a danger, at least one of you old components is bad.

If you can get a DC power supply you can remove the old rectifier and stabilize components (Diodes and capacitor).
It just takes a little soldering.

Rob

Post a picture of the circuit board and we can probably tell you what parts to change!

Hum which was your complaint is usually caused by a bad filter capacitor.

Most radio shacks have a good selection and if you know what it is your looking for ebay works well too.

Top view:
http://i286.photobucket.com/albums/l...r/IMG_2884.jpg

Bottom view:
http://i286.photobucket.com/albums/l...r/IMG_2882.jpg

It's also got these funny lights that I can't find anywhere. Measured 8v at their sockets.
http://i286.photobucket.com/albums/l...r/IMG_2887.jpg

You might be able to use these:

http://www.mouser.com/ProductDetail/Chicago-Miniature/2107/?qs=sGAEpiMZZMucm%2f%252bFOY0TQinNCJWEhTEP8HYDmwK4UEM%3d

Hard to say.

As far as the capacitor, I could guess the big black one in the lower left of the first photo marked 1000MFD 35V. These days known as 1000uF 35V.

I don't see the rectifier in the photo.

Thanks. If the rectifier is a black square item, I don't see one on the chassis anywhere. However there is a silver can shaped component to the left of that big capacitor and about the same size.

Given the age of this device the rectifier will be 4 separate diodes (marked with an arrow ending in a minus sign).

The schematics will look like this
http://en.wikipedia.org/wiki/File:RC_Filter.png

Rob