The Fine Things are Always Hand Made
It seems my newly acquired McMurdo Silver 15-17 ( https://antiqueradios.com/forums/viewtopic.php?f=1&t=365571 ) has been a little bit more tampered with than I thought after my initial inspection.
Yesterday I set it up on the bench to replace the electrolytic caps but quickly noticed something wrong with the volume pot.
It looks like someone forced it way out of its range, and it actually spun in place, which resulted in the wires and components attached to it breaking right off.
After taking off the knob and looking at the solder joints, it became evident this potentiometer might not be original at all. It does however measure 1.2Meg which is close to the original spec (1Meg).
Here are my three questions :
1. If I were to replace this potentiometer, should I pick a linear or logarithmic curve?
2. What should the tap measure? Right now it's at 500K , so pretty much in the middle.
3. What is this coil connected to the tap? It has no ID on the schematic and I want to understand its purpose.
I attached 2 pictures of the pot and a quick drawing of the volume circuit, but a full schematic of the set can be found at : https://antiqueradios.com/gallery/main.php?g2_itemId=155689
I strongly doubt that is an original control. I'll check my references to see what I can find.
The little circuit off the tap is intended to increase the bass response at the low end of the volume control, giving the set a richer sound at lower volumes. Once the wiper passes the tap, the little circuit has no effect on the tone.
A tapered pot for the volume control. You can watch the action of your control by putting the center and either end lug on a meter and turn the control shaft, something I do to assure the control action is smooth electrically to determine if it needs cleaned.
Look like a mica capacitor across the volume control taps, maybe an attempt to adjust for an improper replacement volume control.
Look at the volume control circuit for a Scott Philharmonic or Sixteen. The tap is pretty low, at 20,000 ohms on the Scott Sixteen 500,000 ohm volume control. Tap at 50,000 ohms per diagram for the Philharmonic which has a lot more audio power with the four 6L6s.
I have had two Scotts where the volume control resistance strip was broken at the 20,000 ohm tap. Difficult to source replacements, had to settle for a 30,000 tap control, but seemed to be good enough.
The circuit from the volume control tap results in more low frequency response, with the effect much like the "loudness" control on 1950's hi-fi.
Little volume increase until past the volume control tap.