[ducretet]

Hello,
Some time ago I restored an Atwater Kent, model 82. I changed all the capacitors, non-tolerance resistors, et ...
There is a problem. When I turn on the radio the volume is practically at maximum. After a few seconds the volume drops and works normally. I've gone through everything, and nothing.
I do not understand.....!
I will appreciate if someone tells me that it can happen.
Thanks.

[Arran]

Not that it is related you your problem but I hope that you replaced the original wiring on the power transformer, if it still has the original transformer?
With regard to the volume level, where is the control set when this happens? I'm wondering whether it is a case of rather then the volume going up, and then dropping to a normal level, whether it might be that the level might be that the volume control is set high, and the gain is dropping after the set warms up?

[ducretet]

The transformer is not the original. Has been changed. I know. The early Atwater Kent transformers have quite a few problems.
As for the volume, it is very rare. It does not heat up. I do not understand. After a few seconds it works normally. But when I turn on the radio it is very loud.
Otherwise everything perfect. I had to change all the wiring. The rubbers came off. They were completely dried out. It was a danger !!!!
The volume is driving me crazy !!!!




It's the weirdest thing that has ever happened to me.

[Phlogiston]

Yes, you have to replace the original rubber insulated wire IN the AK transformers. I have never had one fail after doing this. And, rewiring an AK trans is easy since no splice is required.

Russ

[Eliot Ness]

When this happens if you just turn the volume control a hair one way or the other does it go back to normal?  That volume control (if original) is about 90 years old now so it might need an occasional, or maybe initial cleaning.

You might also want to clean the pins on your tubes (a very fine emery cloth works well for that).  Maybe lightly tap on various tubes with a non-metallic object (wooden chopstick maybe) when this is happening.  You might want to substitute in a different set of known good tubes. 

I also like to check and make sure the tube sockets are all tight and then I clean them with some contact cleaner on these little brushes I got from Harbor Freight for airbrushes:

https://www.harborfreight.com/airbrush-c...68155.html

   

I'd also suspect any original resistors that you didn't replace and also look for any cold or suspect solder joints, both old and new.  An intermittent problem that only happens for a few seconds can be hard to find.

[MrFixr55]

Sorry, I know that this is 4 months late but here goes:
The 82 schematic is interesting to say the least. Look into the 24A that is referred as the CONT (Control?) tube, This may be an AVC circuit. The volume control is part of a resistor "Ladder" Voltage divider for various B voltage, Check for voltage variation at either end leg of the volume control and all along the "ladder" for changes in voltage. Replace questionable resistors and caps. Check for changes at the wiper. However, the wiper voltage may change as the "Control" Tube warms up. Read on.

The volume control varies the cathode bias of the "Control" tube, which varies grid bias on the 1st detector (Mixer) and the IF Amp. Based on where on the voltage divider the cathodes of the IF and Mixer and the plate of the Control Tube are, the plate of the Control tube is negative with respect to the cathode. The more current that passes through the tube the more negative the plate gets in relationship to the Mixer and IF Cathodes, making the grids of these tubes more negative. Cathode voltage of the 2nd detector controls the grid voltage of the Control Tube, therefore AVC is obtained. As the current of the tube goes up, the bias on the mixer and IF amp is increased. Could be that slow warmup of that tube is the issue. Swap with the 2nd detector, or dig up a 24A from somewhere else. The 35 is a somewhat similar tube but is a remote cutoff (Super Control) tetrode, as opposed to the 24A which is the standard sharp cutoff tetrode tube.

[radiorich]

Hello Eliot Ness ,
what great idea far as those cleaning brushes I will pick some up!
Well, Ferran I look forward to your restoration of this set because this last year I bought myself an Atwater Kent Model 89 and I look forward to restoring it !

Sincerely Richard

[ducretet]

Hello,
I've changed the tubes, to no avail. I'll check the potentiometer. I have noticed that it is not the original. The former owner changed it. When I have time I'll look at it.
Thanks for the observations.

[MrFixr55]

Hi Ferran,

Note that the volume control is supposed to be only 550 Ohm. I don't know how many watts the volume control should be able to handle, but the more, the better. The control is part of a "voltage divider". The total resistance of this string is is about 30,000 Ohm. The percentage of this string that is the resistance of the volume control is only 1.8% of the total resistance and 1.8% of the approximately 250V across this voltage divider. The voltages on the tube are very small, being 16V for the plate, 8V for the grid and 4V for the Cathode. However, these readings were made with a 1000 Ohm / Volt meter instead of one of the modern supersensitive DVMs.

The point here is any change in resistance as these resistors and the 80,000 plate load resistor heat up will possibly cause the issue that you are having. In other words, consider changing all of the "bleeder resistors", the volume control and the 800,000 Plate Load Resistor for the Control Tube. In addition, change all of the capacitors in the radio.

Good luck, I hope that this helps.

Best Regards,

John "MrFixr55"

[ducretet]

It could be John. I will keep you informed.
Regards,
Ferran

[ducretet]

I have already seen the error !!!!!
You were right, John. The fault was related to the control tube circuit. The potentiometer is correct: 550 ohms. The fault was in the 670 ohm resistor. It read 1,000 ohms. Totally out of tolerance. I had not noticed.
The radio works perfectly !!!!
Thanks for the help.
Regards,
Ferran