[OZ4]

[Image: http://philcoradio.com/phorum/images/smi...on_eek.gif]Hi everybody. Yes I am still around. I been lurking around without much success. Meanwhile Ive been working on old radios.
And currently it's a 40-140 electronic restoration (well restomode anyway).

After replacing the customary paper caps. and tired resistors and the volume pot.  I found  the audio sounded like garbage   like arking audio input to the amp. very weak. Oh and it seemed that the AGC was varying it up and down while monkeying with the connections.

After much investigating the only thing I could find was the radio didn't match the schematic drawing at the volume control item #35. It was missing the ground for the low side of the pot. Now mind you it was made that way. There was no evidence that there ever was one. The terminal never had any solder on it, no wire, nothing. And to top it off I have another radio that has the same chassis and it has the same missing ground.  So I got daring and added a ground which made it work.

Now to the question. Has anybody seen this issue before and how could this detector circuit ever work like it was. Of course both radios had been monkeyed around with by some previous orangutans.

Lokie Here. [Image: http://philcoradio.com/phorum/images/smi...on_mad.gif] http://philcoradio.com/library/download/...20Book.pdf
( "Foxit reader" for PDFs.)

Meanwhile doing all this exercise I decided to try and figure out how the Detector/Demodulator works so I redrew it as semiconductors.Now it make some sense.

It appears that this radio has two stage demodulator.  The 7C6 tube has three separate little goodies inside. Seen as semiconductors   Two diodes and NPN transistor with a common lead at the cathodes/emitter hard to ground.  On the drawing. Caps 33C and 33D (both of which are inside the IF can) are the two stages I speak of.

It looks like both diodes rectify the IF. One is used for the AGC level. And the other rectifys the same IF and charges Cap 33C which charges into C 33D which discharges into the volume pot. The audio is then tapped off by the volume pot and sent to the base of the NPN transistor with the collector level being amplifyed copy of the audio. All this by one little tube.

So without the ground  the audio "DC" level of the caps must have been floating up not following the audio level. 

When I was a kid I use to think that radios were some kind of voodoo. Especialy those little square cans with screws in them. Twistie twistie.



Oh and I have been playing around with my SDR radio. Boy is it cool. I can see the oscillator sail accross the the noise floor and I can find the exact freq.
I wish it had a tracking counter. Using a calculator I set the exact freq for the dial readout. How long it will stay that is another question. (all this with the $200 SDR and a second hand laptop).  I set the IF with my rather unstable generator.

I will try and Post some pics and maybe some videos in a few days. Can I post videos here? I never tried.

Chris

[radiodale]

See if the pot has been replaced. Often the original will have one terminal with a built in ground.

[OZ4]

Yes the pot that was in it has Philco on it with the part number. Of course it's no good. both ends are trashed. Open.
I looked for an internal ground but found none. So it's still a mystery.

I tried to post some pics this morning but the phorum software says the files are too big. I don't know whats up, my largest image file is only 237k while the limit is 1536k
Maybe somebody knows how to do it.

[OZ4]

Here's some pics


   


The volume knob doesn't fit because of the temporary pot with too short of a shaft.
The chassis on the left is the poor doner.



   

Here you can see the volume pot with the unused terminal.



   
   

Next is a connector for the loop antenna.  It's interesting that the antenna is also the RF preslector. The other radio is missing this and without it the radio won't work properly.



   

My collection of test gear.

[OZ4]

More analysis theory. Hopefully this is easier to understand

After redrawing the circuit again. It looks like two functions are going on.
   
In my drawing I have the resistor #33e as 47k which is wrong it should have been #34 at 2.2meg.

On the diodes side of the transformer any positive voltage is shorted to ground which pushes all the negitive voltage to the detector side. This charges the caps 33C and 33D which act as filters but only at IF speed. There voltage discharges fast enough to follow the audio voltage.

While the caps 33C and D hold their charge and the IF changes polarity the voltage on the other side of the transformer sits on top of voltage of the caps which is now doubled and stored in C7 for use to control the gain of the IF stages.

Its interesting to note that both sides of the transformer are driving the two circuits negative which reinforce each other as voltage doublers.

All this still doesn't help me to figure out how this worked without the ground on the volume pot.

[OZ4]

Back to the open ground issue at the volume pot. I must confess there is a ground connection from the low side of the pot to the shaft/mounting portion.

Now my excuses. There is no evidence a external connection. And the carbon track appears to have burned out, its open. So the radio underside wiring was open. Even after removing the back cover of the pot  no connection can be seen. It's hiding underneath the mount.   Rookie mistake  I thought I checked it.

[amptramp]

I think there is a difference between the radio schematic and your drawing of an equivalent circuit.  The two diodes to ground are separated by a capacitor, not a resistor and the AVC side has a resistor which is returned to the negative supply.  A lot of sets use a single diode and get the AVC off a lowpass filter from the top of the volume control.  The problem with this is that it is possible for the AVC to back-bias the detector.  Suppose for example, the AVC voltage was returned to ground rather than the bias supply and was at -2.7 volts.  If this voltage was developed at the top of the 500 K volume control, the top of the volume control would be at -0.5 volts due to the divider effect of the AVC resistor and the volume control.  But if it is back-biased by this amount, the diode would not begin to conduct as soon as the transformer output goes above zero because it has to overcome the reverse bias, in this example, 0.5 volts.  This becomes important when you get near full modulation.  Keeping the AVC diode and the audio detector diode separate means you get the same audio regardless of the value of the AVC voltage.


The connection between the diodes cannot be a resistor, it has to be a capacitor to get this effect.  The separation of the VAC and audio diodes allows the audio to be returned to ground whereas the AVC is returned to the negative bias supply.

[OZ4]

I definitely screwed up my drawing  but the Philco drawing shows a resistor #34 between the two diodes. In my drawing I have the resistor in question as #33e 47k which is wrong it should have been #34 at 2.2meg.

We must be looking at different drawings. The one I am looking at is from the link above.

Which capacitor do you speak of?  What's the circuit number?

[amptramp]

Nostalgiaair shows a different schematic.  I assume this one uses the capacitance between the parallel plates of the dual diode to couple IF into the second plate.  The schematic from the link above has the disadvantage of still back-biasing the detector.  I was using this:

http://www.nostalgiaair.org/pagesbymodel...013329.pdf

What circuit do you actually have?