air king 325B

[bob91343]

Hum isn't necessarily caused by defective capacitors. A poor ground is a good possibility. Since it varies with the volume control then it's coming from an earlier stage. Short the middle volume control lug to ground and see if the hum clears up. It also may have more than one source. A leaky heater-cathode in a tube, a corroded ground lug mounting screw, lots could do it.

Describe the hum. Is it very low pitched, smooth? Raspy with lots of high frequency content? More like a buzz?

[insurone]

I shorted the middle volume control lug to grd, it reduced the hum. can still hear the hum, but very low. The hum to me is a low pitched and get louder with volume increase. No tubes, this is a transistor unit.

[insurone]

Is there anyway to post a sound recording on the site??

[insurone]

Just a thought, I could video tape the hum with the iphone and email it to you.

[insurone]

Bob91343 what did it mean with the volume test??

[bob91343]

There will always be some residual hum if it's run off the power line. If the residual hum with the control shorted is acceptable, then you need to discover its source, which obviously is before the volume control.

It's starting to look like a power supply issue. Else there may be a bad ground in the detector circuit. You can disable the detector and see if the hum is gone. More than this, I'd need a diagram.

There may be a poor ground. Trace the 'cold' end of the volume control and make sure it's got a clean connection to chassis.

[insurone]

I've been checking all the grounds, they are all solder to the frame of the unit. Have not got a clue what the detector circuit is, or where it would be, what does it consist of?? Would the rectifiers cause the hum. And I WOULD TO FIND THE DIAGRAM

[insurone]

I would love to find the diagram for this unit.

[bob91343]

The volume control has three points. One is ground, and the middle one goes to the audio section. The remaining one comes from the detector and apparently there is some hum there.

Finding a diagram shouldn't be difficult as long as you don't insist on the exact brand and model. Radio engineering has largely been a me-too field, and if you can find a radio of a similar era with similar parts you probably can use that diagram.

[insurone]

I disconnected a wire from the volume control to this transformer and the hum was gone, looks like a audio transformer of sort. Here's a picture of the area.

[insurone]

As for a diagram of something similar, Where would I look for era 1957-1960, of a whole house intercom, AM/FM radio, with transistors.

[bob91343]

What brand is this unit? One of the big brands was Nutone, so you could start there to see what looks close.

RCA made PNP power transistors in great quantity at low prices and it appears yours uses one. What's the part number? I can't quite read it in the pic. I have old RCA transistor manuals so maybe I have the data on it.

[insurone]

Berns Air King intercom AM/FM radio, the transistor is 40254, could this be causing the hum?

[bob91343]

The 40254 (yes I have the data) is nothing special, just a PNP power transistor for audio purposes. We really need to have a diagram of at least something similar. And you need to trace the circuit with an oscilloscope.

Many of these intercoms, being sealed up in a wall, were not isolated from the power line. So be careful.

[insurone]

oscilloscope! I guess I will need to find someone to fix this unit, as I don't have one and not even sure how to use one. I could possibly find one to use.