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NutmegCT

NutmegCT

Great Pumpkin
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My 1930 Atwater Kent is showing its age. (and so am I ...)

Atwater-Kent Model 74.jpg


Power on, adjust the volume about midway, sound is excellent. Good for voice as well as music.

Then after about 30 minutes, suddenly the volume doubles. Wham - have to race to turn down the volume.

I'm guessing it's a heat issue, but need expert opinion on diagnosis and repair suggestions. Seems strange that heat might cause an *increase* in output, without cracking, popping, or static.

Thanks.
Tom M.
 
There might be a "resistance ladder" / "voltage divider"
> Atwater Kent 82 < (scrol down)

A bad solder or worn control can change the outcome.
 
Thanks! That forum article is excellent - I'm going to start with cleaning the tube pins (better) this morning.
Tom M.
 
I naturally assumed it wasn't the radio but his tube hearing aid finally warming up. :D

1735479543691.png
 
Good idea - but those critters are pretty rare these days.

I remember going with my dad to the neighborhood 7-11 back in the late '50s and early '60s, where they had a free tube tester (and they also sold tubes!). You had to check that the tester would test the entire circuitry of the tube - not just the power filaments.

tester jpg.jpg
 
NutmegCT said:
Good idea - but those critters are pretty rare these days.

I remember going with my dad to the neighborhood 7-11 back in the late '50s and early '60s, where they had a free tube tester (and they also sold tubes!). You had to check that the tester would test the entire circuitry of the tube - not just the power filaments.

View attachment 101490
Click to expand...
When I was in High School I was into CB radio (with amplifies that may have run a tad afoul of FCC rules). I spent a lot of time at the local "U-Tot-M" (a now defunct convenience store similar to 7-11) where they had a tube tester and also sold some of the more common tube types.
 
Tried the "clean the tube pins and sockets" idea.

Removed each tube, ran 800 grit sandpaper over the pins. Used a rat tail file to clean up the socket holes.

Sudden volume increase problem has apparently disappeared.

Still need to eliminate the 60 cycle hum - but the A-K sure sounds better now.

(Also found a Heathkit TC-3 tube tester for sale!)
 
NutmegCT said:
Tried the "clean the tube pins and sockets" idea.

Removed each tube, ran 800 grit sandpaper over the pins. Used a rat tail file to clean up the socket holes.

Sudden volume increase problem has apparently disappeared.

Still need to eliminate the 60 cycle hum - but the A-K sure sounds better now.

(Also found a Heathkit TC-3 tube tester for sale!)
Click to expand...
if you have 60hz hum, it is a sign that the capacitors are leaky.

I would start by inspecting resistors to see if any of them are cracked- if they are in the voltage control, a resistor opening could increase voltage significantly. Likewise leaky caps will change value as they heat. A cap shorting out in the tuning could significantly increase voltage to the amplifier stage.

In any case... to prevent destroying the tubes / power transformer (I assume it has a transformer and isn't like a AA5 which puts the tubes in series) you need to start with capacitors. Leaky caps (electrolytics) will significantly increase current flow on the transformer and overheat it. You want to look for the round caps, which if they are old likely will have wax leaking out of them - other caps (mica etc) most likely are just and since they may be very accurate / chosen values don't mess with them since it will likely affect the radios performance. Electrolytics can be replaced with modern "close enough" values with no problem.

to run the radio for diagnosis, you can (should) put it on a "dim bulb" this is just an incandescent bulb wired in series in the power lead. Properly working, it should start fairly bright ( cold filaments) then start diming as the filaments heat and finally get a bit brighter as the tube current starts. You need to experiment with different values of bulbs.... since they will change voltage to the radio

Next inspect resistors for any signs of cracking. when you have the chassis out of the radio and turned on, take a chopstick and gently tap on resistors one at a time.. if they are cracked you should suddenly hear noise, or the increase in volume. just KEEP YOUR HANDS OUT you don't want to experience the high voltage!!! Taping can help find loose connections.. If you have a dim bulb, you may even see a change in brightness if you tap a problem location.

Here is the little Fender I built a few years ago
 

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NutmegCT said:
Tried the "clean the tube pins and sockets" idea.

Removed each tube, ran 800 grit sandpaper over the pins. Used a rat tail file to clean up the socket holes.

Sudden volume increase problem has apparently disappeared.

Still need to eliminate the 60 cycle hum - but the A-K sure sounds better now.

(Also found a Heathkit TC-3 tube tester for sale!)
Click to expand...
This has been a very interesting topic, I said at 11:09 EST ;)
 
Tom here are schematics for your radio. looks like there were 3 different version though so need to figure out what you actually own.
Also... electronics were developing at the time and many changes just were introduced if they figured out a better way so really old electronics rarely exactly match the schematics.

early type

Later

Atwater Kent 60 | Antique Electronic Supply

www.tubesandmore.com www.tubesandmore.com

In any case it volume control is regulated by voltage
1735805512571.png


just guessing which circuit version (and they would be different) but the volume control potentiometer center goes to ground - sending center to ground probably didn't change over the design so that would be a very good place to look for bad solder joint or something with the potentiometer wiper (unless it is the 1st version most likely a double ganged pot) look there for solder / wire anything that would cause the center to come ungrounded. I think that if the in/out came loose you would lose all sound.
1735805863545.png

Anyway very interesting circuit, an old TRF circuit instead of the "modern" superheterodyne. With 8 tubes that thing cost a fortune.
 
Happy new year Yisroel. I sure wish I lived closer to you!

My volume potentiometer has four wires; all connections are tight. I'll send you a PM with a photo of the pot, as well as a photo of the entire chassis, so we don't overload the forum with details.

Thanks very much - sounds like you once worked for Atwater-Kent! The model I have was known as the Cadillac of the A-K world at the time.

Tom M.
 
Beware, This old type of radio has a Field coil speaker in it. That means the speaker should have 4 wires going to it. 2 of them have fairly high voltage DC that can REALLY hurt. I found this out 40 years ago and still remember!! The DC for this can also be the source of your hum also. George
 
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