Hi folks,
On this Philco 38-7 the tone pot has me confused. On the back of the pot there are two terminals. These are the AC on/off portion of the pot. No biggie. There are two other terminals. With the pot full ccw (this is also the off position for AC) these two terminals are common to each other. Go one click cw (now power switch is on) and these two terminals are again common to each other. The next two clicks cw leaves these two terminals open from each other. If you compare what I have said to the schematic, something doesn't make sense. Can someone explain to me what's actually going on? Like I say, I'm confused with it. Thanks,
Johnny

:Hi folks,
: On this Philco 38-7 the tone pot has me confused. On the back of the pot there are two terminals. These are the AC on/off portion of the pot. No biggie. There are two other terminals. With the pot full ccw (this is also the off position for AC) these two terminals are common to each other. Go one click cw (now power switch is on) and these two terminals are again common to each other. The next two clicks cw leaves these two terminals open from each other. If you compare what I have said to the schematic, something doesn't make sense. Can someone explain to me what's actually going on? Like I say, I'm confused with it. Thanks,
:Johnny

Unles someone has made some change in the circuit, the print shows a 3 position switch as tone control, not a pot at all. Even says so at the top of the print. One position is off, then two tone selections in the AC on mode.

::Hi folks,
:: On this Philco 38-7 the tone pot has me confused. On the back of the pot there are two terminals. These are the AC on/off portion of the pot. No biggie. There are two other terminals. With the pot full ccw (this is also the off position for AC) these two terminals are common to each other. Go one click cw (now power switch is on) and these two terminals are again common to each other. The next two clicks cw leaves these two terminals open from each other. If you compare what I have said to the schematic, something doesn't make sense. Can someone explain to me what's actually going on? Like I say, I'm confused with it. Thanks,
::Johnny
:
:Unles someone has made some change in the circuit, the print shows a 3 position switch as tone control, not a pot at all. Even says so at the top of the print. One position is off, then two tone selections in the AC on mode.
:
:My bad Warren. Looks like a pot so I called it a pot. When tone "switch" is full ccw position a/c is off and there "is" continuity between the other two contacts on the tone switch. One click of the tone switch c/w and there is "still" continuity between the two contacts and a/c is on. In the other two positions c/w on the tone switch there is "no" continuity between the two contacts. So exactly what is the tone switch doing to the tone circuit if there is continuity between the two contacts when power is off? According to the schematic the second and fourth switch positions short two seperate signals to ground. Why is one of those being shorted to ground when the radio is off? Something isn't making sense there.
Johnny

:::Hi folks,
::: On this Philco 38-7 the tone pot has me confused. On the back of the pot there are two terminals. These are the AC on/off portion of the pot. No biggie. There are two other terminals. With the pot full ccw (this is also the off position for AC) these two terminals are common to each other. Go one click cw (now power switch is on) and these two terminals are again common to each other. The next two clicks cw leaves these two terminals open from each other. If you compare what I have said to the schematic, something doesn't make sense. Can someone explain to me what's actually going on? Like I say, I'm confused with it. Thanks,
:::Johnny
::
::Unles someone has made some change in the circuit, the print shows a 3 position switch as tone control, not a pot at all. Even says so at the top of the print. One position is off, then two tone selections in the AC on mode.
::
::My bad Warren. Looks like a pot so I called it a pot. When tone "switch" is full ccw position a/c is off and there "is" continuity between the other two contacts on the tone switch. One click of the tone switch c/w and there is "still" continuity between the two contacts and a/c is on. In the other two positions c/w on the tone switch there is "no" continuity between the two contacts. So exactly what is the tone switch doing to the tone circuit if there is continuity between the two contacts when power is off? According to the schematic the second and fourth switch positions short two seperate signals to ground. Why is one of those being shorted to ground when the radio is off? Something isn't making sense there.
:Johnny

I looked again at the print. Seems when the set is turned off, that switch cuts the audio, so there is no noise. The volume control is left in a preset mode, and not turned down as the radio goes off. If you have ever just pulled the plug on some radios, the volume goes up with a blast, or it makes some kind of noise due to the volume being left up.

Some radios have a jack in the back to use the audio amplifier portion of the radio to play a phonograph, etc. Is that what this control is for?

The switch should have 4 positions/terminals. The wiper is connected to the chassis via the metal shaft bushing. The 1st terminal isn't used because it is only contacted when the power switch is off. The 2nd terminal goes to a resistor that goes to a tap on the volume control. When the switch is in that position, the resistor will put a load on the audio circuit that will reduce the time it takes for the capacitors in the input circuit to charge, which will make them less responsive to bass. This is your 'speech' position. Incidently, the resistor that connects to this terminal also goes to a small value condenser (38) that goes to the chassis (other side of the audio circuit). Normally, when the tone switch isn't in this position, and isn't shorting across this condenser, the resistor and condenser together respond best to the mid-range tones, and somewhat to the high-range tones. Together they mute these tones somewhat, and curve the audio for a pleasing full-tone sound. You will notice that as the volume control is turned past the tap, the audio becomes harsher and more powerful because these tone balancing components have less effect on the circuit.

The 3rd terminal connects to nothing, and so this will be your 'full tone' position. The only tone correction that takes place in this position is that permanently built into the set, such as the resistor and condenser that go to the tap on the volume control (above), or the two condensers that are in series shunting the output plate circuit (as described below).

The 4th terminal connects to the junction of the two output plate tone condensers. They are wired in series, and with the lower one being of very small capacitance, they both equal an even smaller capacitance. I believe, if I remember correctly, that the upper condenser is labeled as .03 MFD (significantly sized as far as audio frequencies are concerned at the impedances found in your amplifier). The two in series are mathematically the same as two resistors in parallel: 1/(1/C1 + 1/C2)=CT. Shunting across the lower condenser will put the upper condenser into full action, buffering out much of the high frequency tones. You can think of this as your 'low noise' or 'bass' position. The condenser(s) doesn't actually short out the higher frequencies, since condensers don't actually pass anything, but rather it neutralizes the upper frequencies much like the filter condensers in the power supply neutralize hum. When there is a high (crest), the condenser charges that way, and when there is a low (trough), the condenser charges the other way. During the transition from high to low and low to high, the new polarity meeting the condenser is met with the opposite polarity already charged in the condenser. The two neutralized and the net result is 0. A smaller condenser holds less of a charge, and so it is going to be charged and discharged more effectively with high frequencies than low ones, which is why small value condensers don't cut out all of the audio, but rather just the high frequencies.

T.

http://www.nostalgiaair.org/PagesByModel/256/M0013256.pdf

Hmmm..... I was looking at item #26 and I should have been looking at item #39. Sorry about that. If the control only has 2 terminals, though, one should go to the resistor that goes to the tap on the volume control, and the other should go to the junction of the two output plate tone condensers. I can't see why the two terminals would ever have continuity to one another unless either the switch is broken internally, or the capacitors in the related tone circuits are shorted.

T.

:Hmmm..... I was looking at item #26 and I should have been looking at item #39. Sorry about that. If the control only has 2 terminals, though, one should go to the resistor that goes to the tap on the volume control, and the other should go to the junction of the two output plate tone condensers. I can't see why the two terminals would ever have continuity to one another unless either the switch is broken internally, or the capacitors in the related tone circuits are shorted.
:
:T.
:
:Thank you Thomas! That explanation is exactly what I was looking for. I have just never seen a tone switch before. As far as the two contacts being common to each other at any position, I was mistaken. They are as you say, never common to each other. Thanks again!
Johnny

You're welcome. For a second I thought that I had gone overboard.

T.

Glad to see you back and posting! With so much negativity as of late, this is what this forum should be about :)

:You're welcome. For a second I thought that I had gone overboard.
:
:T.