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.

__________________
1985 380SE Blue/Blue - 230,000 miles
2012 Subaru Forester 5-speed
2005 Toyota Sienna
2004 Chrysler Sebring convertible
1999 Toyota Tacoma

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."

__________________
1998 C230 330,000 miles (currently dead of second failed EIS, yours will fail too, turning you into the dealer's personal human cash machine)
1988 F150 144,000 miles (leaks all the colors of the rainbow)
Previous stars: 1981 Brava 210,000 miles, 1978 128 150,000 miles, 1977 B200 Van 175,000 miles, 1972 Vega (great, if rusty, car), 1972 Celica, 1986.5 Supra

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.

__________________
1985 380SE Blue/Blue - 230,000 miles
2012 Subaru Forester 5-speed
2005 Toyota Sienna
2004 Chrysler Sebring convertible
1999 Toyota Tacoma

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 !!!

__________________
Grumpy Old Diesel Owners Club group

I no longer question authority, I annoy authority. More effect, less effort....

1967 230-6 auto parts car. rust bucket.
1980 300D now parts car 800k miles
1984 300D 500k miles
1987 250td 160k miles English import
2001 jeep turbo diesel 130k miles
1998 jeep tdi ~ followed me home. Needs a turbo.
1968 Ford F750 truck. 6-354 diesel conversion.
Other toys ~J.D.,Cat & GM ~ mainly earth moving

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?

__________________
1985 380SE Blue/Blue - 230,000 miles
2012 Subaru Forester 5-speed
2005 Toyota Sienna
2004 Chrysler Sebring convertible
1999 Toyota Tacoma

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!

__________________
1998 C230 330,000 miles (currently dead of second failed EIS, yours will fail too, turning you into the dealer's personal human cash machine)
1988 F150 144,000 miles (leaks all the colors of the rainbow)
Previous stars: 1981 Brava 210,000 miles, 1978 128 150,000 miles, 1977 B200 Van 175,000 miles, 1972 Vega (great, if rusty, car), 1972 Celica, 1986.5 Supra

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.

__________________
1985 300D 198K sold
1982 300D 202K
1989 300E 125K
1992 940T

"If you dont have time to do it safely, you dont have time to do it"

"The democracy will cease to exist when you take away from those who are willing to work and give to those who would not."

Top view:


Bottom view:

__________________
1985 380SE Blue/Blue - 230,000 miles
2012 Subaru Forester 5-speed
2005 Toyota Sienna
2004 Chrysler Sebring convertible
1999 Toyota Tacoma

It's also got these funny lights that I can't find anywhere. Measured 8v at their sockets.

__________________
1985 380SE Blue/Blue - 230,000 miles
2012 Subaru Forester 5-speed
2005 Toyota Sienna
2004 Chrysler Sebring convertible
1999 Toyota Tacoma

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.

__________________
1998 C230 330,000 miles (currently dead of second failed EIS, yours will fail too, turning you into the dealer's personal human cash machine)
1988 F150 144,000 miles (leaks all the colors of the rainbow)
Previous stars: 1981 Brava 210,000 miles, 1978 128 150,000 miles, 1977 B200 Van 175,000 miles, 1972 Vega (great, if rusty, car), 1972 Celica, 1986.5 Supra

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.

__________________
1985 380SE Blue/Blue - 230,000 miles
2012 Subaru Forester 5-speed
2005 Toyota Sienna
2004 Chrysler Sebring convertible
1999 Toyota Tacoma

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