04-19-2019, 09:38 PM
So, on closer inspection, I see a couple more interesting things about the tone control in my 71B.
First, I didn't realize, but discovered that I have another 71 chassis, a Code 221, which probably had the single position tone switch installed, but someone put a pot for a variable tone control, so I still don't have an original two position tone control to compare, but I'm glad I have it for other comparisons, and a possible parts donor if I need it.
On the schematic, I was trying to make sense of the capacitor to the left of the output tube (39), as it relates to the switched tone capacitor to the right of the tube (coming off the plate), also labelled 39. This confused me until I looked at the modern wiring diagram by "Geoff", which I think I got from this very forum. It shows both capacitors inside the original tone control, which explains why they both had the same callout number! So, there is always a .01uF cap from plate to ground, but another one can be switched into the circuit to cut more treble.
Now, on mine, (Code 125), the tone switch did not have a cap always to ground. Instead, it has an additional capacitor block in the open space between the power transformer and the line filter block (item 49), with a .01 uF cap always to ground off the plate of the output tube, and then whatever my 4 way tone control does is in parallel to that capacitor. I believe this block is original, because someone bodged a .025 uF cap (too much) off of it to ground. This seems to suggest this radio did in fact come with the 4 position tone control, and it was still designed to have .01 uF to ground in addition to the tone control.
Barring any Code 125 specific information being available, I'm hoping the rest of the radio is the same as the Code 121, because there is such a rat's nest of bodged floating components, "air joints", and other atrocities, it's going to be a challenge to restuff the blocks (thankfully still there), and wire it like it was originally supposed to be.
(Some people just have no respect for old radios or the voltages exposed on those floating, bare joints!)
I thought readers (or me, if I find myself working on another one years from now) might find this interesting.
Dan
First, I didn't realize, but discovered that I have another 71 chassis, a Code 221, which probably had the single position tone switch installed, but someone put a pot for a variable tone control, so I still don't have an original two position tone control to compare, but I'm glad I have it for other comparisons, and a possible parts donor if I need it.
On the schematic, I was trying to make sense of the capacitor to the left of the output tube (39), as it relates to the switched tone capacitor to the right of the tube (coming off the plate), also labelled 39. This confused me until I looked at the modern wiring diagram by "Geoff", which I think I got from this very forum. It shows both capacitors inside the original tone control, which explains why they both had the same callout number! So, there is always a .01uF cap from plate to ground, but another one can be switched into the circuit to cut more treble.
Now, on mine, (Code 125), the tone switch did not have a cap always to ground. Instead, it has an additional capacitor block in the open space between the power transformer and the line filter block (item 49), with a .01 uF cap always to ground off the plate of the output tube, and then whatever my 4 way tone control does is in parallel to that capacitor. I believe this block is original, because someone bodged a .025 uF cap (too much) off of it to ground. This seems to suggest this radio did in fact come with the 4 position tone control, and it was still designed to have .01 uF to ground in addition to the tone control.
Barring any Code 125 specific information being available, I'm hoping the rest of the radio is the same as the Code 121, because there is such a rat's nest of bodged floating components, "air joints", and other atrocities, it's going to be a challenge to restuff the blocks (thankfully still there), and wire it like it was originally supposed to be.
(Some people just have no respect for old radios or the voltages exposed on those floating, bare joints!)
I thought readers (or me, if I find myself working on another one years from now) might find this interesting.
Dan
"Why, the tubes alone are worth more than that!" (Heard at every swap meet. Gets me every time!)
Philcos: 90, 70, 71B, 610, 37-61 40-81, 46-420 Code 121 to name a few.
Plus enough Zeniths, Atwater Kents and others to trip over!
