air king 325B
 

Trying to find a schematic for this old Berns Air King Intercom system. Turn it on I only get a hum. Im sure I have a bad capacitor, but unable to see making on the capacitors.

We're going to need some more information. When researching most anything a model number and chassis number (one or both) are handy. There may be some service information for it.

Tube complement also, and whether it has a power transformer.

The model is a 325B transistors, hand wired, has a power transformer.

Wow, I did not know Air King lasted into the transistor era!

Air King products became CBS Columbia and quit building TV's and radios around 1957.
Berns Air king was a fan manufacturer in, IIRC, Chicago. It might have been made by Talk-A-Phone, or another Chicago firm.

Unable to find any other name on the intercom, say Berns Air King 325B and so far no other markings. If it was made by another firm, wouldnt it have another name on it?

That's one of those deals where you'd have to recognize part numbering schemes.
There's nothing in Sams and I doubt that any concern that is associated with that unit is still in business.

Is there by chance a basic wiring diagram of an old AM/FM radio that was hand wired, with transistors

Germanium or silicon?

How do you tell the difference?

Germanium usually are PNP and run from a positive ground power supply. Opposite for silicon. Earliest solid state were germanium.

If you can get to the leads, measure with a VOM to see which polarity they are. Take some pics and we can make an educated guess.

My VOM is at my Son in Laws, will go over tomorrow and get it, will check the leads and take pictures tomorrow.

Here's some pictures
 

Here's some of the ID's on the transistors. 2N261, 2N649, 5orSK3003, 2N1180, 2N1177.

When I move the capacitor that right next to the power transformer, the hum is reduced.

I only looked up the 2N261 and it's a PNP germanium power transistor. You are going to have fun replacing any of those but with luck you won't have to.

Almost for sure that big capacitor is toast. Replace it and see what happens.

Additional Pictures
 

Hope these a helpful. Is there a place I could send this to get it up and going again, without it costing an arm and leg to get it down??

I was thinking it was the big capacitor, but I'm not seeing any markings on it to let me know what it is, reason I was looking for a wiring diagram or one close to it.

here's what I think is written on the capacitor
927-1974
2000 MFD 20 VDC
2406323

Know where I can find one of these??

Okay so you need at least 2000 MFD at 20V or more. Easy to find. Try Google or even go to Digi-Key or Mouser etc.

I found a 2000 MFD at 50 VDC, not finding a 20 VDC, how about one that is 25V found one of those, its all metal on the outside, the old one has cardboard wrapping around it, does the metal one need to be insulated for mounting??
As you have probably notice, I'm really out in left field on this stuff.

Rule #1 in replacing capacitors you can always use one with a higher voltage rating as a replacement.
Rule #2 Electrolytic capacitors are polarized meaning if you connect them backwards...BOOM!.
Rule #3 most Electrolytics can be replaced with another lytic that is within 20% of the value of the original....So the 2200uf 50V caps at ratshack will work.

Some of those black caps in your first picture may also be bad. If you have no sound try jumping a new cap of almost any value across each of them in turn, and see if you get any sound. If the sound comes in at all replace the one that jumping caused the sound return with a reasonable replacement.

re
 

Yes either of those will do the job. And yes, you need to insulate but you can use tape (make it two layers to be sure) or nearly anything insulating.

Since that unit uses PNP transistors, I suspect the power supply is negative, and that usually means the metal can of the capacitor is at power supply potential and needs to be insulated.

Thanks, I will order the cap and give it a try and go from there. Again thanks, will keep you posted on the progress.

While you're ordering, I would replace any other caps (such as those black ones) that you can read the values off of. They will not be expensive to buy, so if you want to order all of them and replace only what's shot, you can do that.

I had already ordered the 2000 mfd, before I read your post, I received it yesterday and replaced the old cap with new cap. Still have the same hum. I think I posted the hum will get louder with the increase of the volume control. On my way to Radio Shack to get replacements for the black caps, hoping this will take care of it.

That was a wasted trip to RadioShack, Very poor selection of caps. Onto the internet.

Ok, I'm surfing the net for caps, not sure what to look for. You said to replace all the black caps, so I'm looking for 50 MFD 10v, 10 MFD 6v, 5 MFD 6V, .05 MFD 600 VDC, to many types of caps. polarized, non-polarized, etc.

Not having much luck on locating the caps, as not sure what type I'm suppose to be looking for, here's what I need, 50 MFD 10 VDC, 10 MFD 6 VDC, 5 MFD 6 VDC, .05 MFD 600 VDC,
What can be substituted such as 50 MFD 150 VDC?

That could work, but a lower voltage 47MFD cap would be cheaper. If you search 4.7 and .047 you'll get many more hits.

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?

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.

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

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

Bob91343 what did it mean with the volume test??

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.

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

I would love to find the diagram for this unit.

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.

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.