I'm working on a Zenith 10h20.
Here's the print:
http://techpreservation.dyndns.org/schematics/51/51_Zenith_10H20_158.djvu

I'm trying to understand the phono input circuit.
What is the purpose of the "phono oscillator coil" L8 and trimmer cap C16?

Is the input as shown only for low impedance input or as it seems a part of an oscillator that works with the cartridge somehow??

If I try to connect the headphone output jack from a typical transistor radio to the phono input it seems to load it down and I can't hear the signal. So that can't be right.

However if I DO connect it AFTER C5 at the junction of R8 then the signal actually sounds fine.... actually works nicely (very well with volume control and all).

Now...my customer is asking me to add an RCA jack to that input to update it for other typical common audio inputs.

Should I connect it via another .05uf cap to the junction of C5-R8? or elsewhere in the circuit?
Any ideas are welcome.

And.. here's another couple curious questions.

1.) What is the purpose of the center lead coming from the other.."PHONO AC socket" ?
It goes to grid 2 of the 1st IF.

2.) What is the wattage rating of R12? (4700 ohms) I had to replace it as it was open.
It had a big 10watt sand coated resistor in there.
I removed it and replaced it with a 10 watt 5000 ohm resistor.
However it only drops 25 volts... so that means its only handling 5 ma which means I really could use a 1 watt resistor or less...right?

Well I just traced the phono audio in signal path with my scope. ( I think)

I put in 400hz audio from my sig gen into the C5-R8 junction.

It goes through the switch s1-1R ..going in on contact(11)and out (9) on up into the grid pin 2 of the 12At7 and out the plate pin 1.

From there it goes down to the switch s1-2F (5) and out (2) then on down to R13 then up to R11
... then it continues on up through C13 and the 2ndaries of the 1st IF transformer on up to the grid pin 1 of the 6bA6.

.. then it seems that it comes off that 6Ba6 plate pin 5 down through the primaries of the 2nd If transformer and on down to R14.

Then from R14 it seems to go back over to switch S1-1R (3) and out (2) and then down over through C23.

From C23 it then goes over into pin 1 of the tone controls socket and back into pin 6 at the bottom of the vol control and so onward to the 1st audio etc.

Does that make any sense?

Ok.. so then I still want to know what the center pin of the "phono AC socket" does?
And what should the wattage be on R12? ( why was the original big sand resistor 10 watts if it only drops 25 volts @ 5ma?)

I am not having luck finding a good djvu viewer, so I can't read your schematic. However, what you have is a Zenith Cobra phonograph circuit. The oscillator is what I believe to be a Colpitts oscillator. The phono cartridge contains a coil and a stainless steel paddle shaped vane. As the vane moves back and forth it changes the inductance of the coil. The vane is connected to the return side of the coil wiring, but, as I have found, will work just the same while being completely disconnected. The vane may also change capacitance, but I believe that it mostly changes inductance.

Lead wiring is critical. Using a coaxial cable that has too thin of insulation will render the unit inoperable. I believe that the factory supplied cable provides the right amount of resonant capacitance, with perhaps a paralleling mica capacitor across the cable jack where it enters the 'amplifier'. Changing the cable requires changing adjustments of the trimmer or paralleling cable capacitor (if any).

I've never seen the circuit described thoroughly anywhere, but I believe that it works like a frequency modulated slope detector. Changes in frequency from a preferred point create voltage fluctuations on the output side.

The key to this design's superior record playing capabilities is that only horizontal movement of the vane affects the coil's inductance. Vertical movement from the needle does not change the coil's inductance. This eliminates a lot of surface noise and scratch. Some rather heavy scratches that would normally be audible are almost completely eliminated by this reproducing system. While it is somewhat possible to isolate movements transferred to a crystal pick-up element, this method doesn't seem to be as thorough. The method of mounting the crystal flat horizontally that was used commonly back then also led to excessive pick-up of surface noise. Modern crystal stereo cartridges are well enough designed that if the two channels are wired in parallel, they can, to a great extent, eliminate a lot of vertical surface noise. The result is often near that of a Cobra pick-up, and I kind of like the fidelity of the crystal units better, though the Cobra pick-ups can sound very nice. Cobra pick-ups also have a very definite cut-off point that tends to eliminate remaining surface noise while keeping the treble fairly bright. Zenith often put a .001 MFD treble limiting cap on the phono output, but I much prefer to use the circuit without this cap. Results are very good.

If you wish to feed ordinary audio signals into the amplifier, it would be best to do so at a standard point. The volume control most likely precedes a two stage audio circuit (triode and pentode), and here is where you'd want to feed in line-level audio signals--in a conventional manner.

Since I cannot see the schematic I do not know what the wire to the 2nd IF tube grid is for. My Zenith Cobra phono is in my parents' attic, so I can't look at it, and I don't recall the model number. It doesn't have this wire, but the IF tube plays a part in the phono circuit, as a sort of pre-amplifier (evidently the output of the phono oscillator alone isn't strong enough). It seems to me that when the phono circuit is activated, plate current to the IF tube is cut out, and all audio information is instead taken from the screen grid (the phono oscillator output is fed into the IF tube through the control grid). I could be wrong about this, but I do believe that the plate circuit of the IF tube is in fact cut out. The IF tube could still somehow be performing as more than just a pre-amplifier. It might actually be a slope detector--utilizing one of the IF transformer windings (either grid or plate) as the tuned circuit. In that case the phono oscillator is probably supplying a frequency around the IF. ....But I can't remember for sure.

There was an extra wire in my phono cable that was used as a 'trip' wire. It went to a button on the front of the radio. The phono motor also served as a transformer, with a secondary winding. Either the tone arm ratchet or the 'trip' button on the front of the radio would actuate a solenoid that'd start the record automation process. ....Doesn't sound like that's what you have, though.

T.

Sorry...meant to say IF tube 2nd grid (screen).

T.

:Sorry...meant to say IF tube 2nd grid (screen).
:
:T.

From service manual Model S-14001.

"The operation of the Cobra pickup is considerably different from Crystal and Dynamic pickups. These pickups generate audio power, while the Cobra controls power generated by a radio frequency oscillator. The triode is a modulated oscillator, detector and audio amplifier. The oscillator operates at a frequency of 2.5 Mc. Modulation is accomplished by changing the engery losses in a tuned circuit...the stainless steel vane, which is in the field of the coil, is spot welded to the osmium-iridium tipped stylus."

On and on. That's the reason for the lead from phono. Peter I will send you the SAMS, ( need record changer model number) if I have the correct one, you copy and pass around if you wish, and return. Thomas same to you.

Now Peter, the schematic you're using is different from ones in SAMS 120-13 (10H20) and 151-13 (10H20)for the radio chassis. The 4700 is a probably 2 watt.
Bill

::Sorry...meant to say IF tube 2nd grid (screen).
::
::T.
:
:From service manual Model S-14001.
:
:"The operation of the Cobra pickup is considerably different from Crystal and Dynamic pickups. These pickups generate audio power, while the Cobra controls power generated by a radio frequency oscillator. The triode is a modulated oscillator, detector and audio amplifier. The oscillator operates at a frequency of 2.5 Mc. Modulation is accomplished by changing the engery losses in a tuned circuit...the stainless steel vane, which is in the field of the coil, is spot welded to the osmium-iridium tipped stylus."
:
:On and on. That's the reason for the lead from phono. Peter I will send you the SAMS, ( need record changer model number) if I have the correct one, you copy and pass around if you wish, and return. Thomas same to you.
:
:Now Peter, the schematic you're using is different from ones in SAMS 120-13 (10H20) and 151-13 (10H20)for the radio chassis. The 4700 is a probably 2 watt.
:Bill

Or maybe look up the model of your changer on this site for all the good theory on the Cobra. I didn't think.
Bill

Thanks Bill. I'd agree with the 2.5 watt rating for the resistor.

Regarding 2.5 MC, it's interesting to note that occasionally I have received Morse code, among other things, while listening to phonograph records. The particular oscillator must have been tuned close to a transmitting station. I could also, on occasion, make the oscillator heterodyne with various incoming signals. It was somewhat annoying because a slightly eccentric record would cause the heterodyne to go up and down in pitch as the record played, though it was usually quiet enough to not be too annoying.

T.

...Or 2 watt. I'm getting my brain signals crossed.

T.

Bill:
Thanks, I don't know the changer model number. I'll ask the customer next time we speak.

Thomas:
Thanks tons!
That's the most comprehensive explanation I've heard.

You are truly amazing!
The depth and breadth of your knowledge never ceases to amaze me!

BTW ..as far as a deja vu viewer goes I got mine fropm Lizardtech:
http://www.lizardtech.com/download/

I tried, but it didn't want to work. Perhaps I'll try it another time.

T.

Thanks a bunch Edd:
I also found that cobra radionics info on page 5 of the Zenith s1401

I had a devil of a time with this old radio until I found that T4 (the discriminator) was all screwed up with silver-mica disease. 200 ohm short across the 2ndary leads.
There are 4 caps in there total and they were all shot so I couldn't measure them properly for replacement..
So I measured the coils 4.5uh primary and 5.5uh 2ndary... then went off to calculate the resonant values at 10.7mhz and came up with 50pf and 40pf respectively.
I guessed at 50pf for the 4th one.
So I used two 75pf caps in series on the 2ndary that got me close enough.
When re-tweaking the primary slug had to go a bit deeper but the 2ndary dipped well with little movement.
Now this thing is sounding great.
These lousy tone switch "rocker-type" contacts are strange though.
Do you know if they are supposed to have a spring to them somehow?
Even the customer says he just pulls them out with a finger nail after pressing them inward.
But he isn't the original owner so he doesn't know if there ever was an original spring mechanism associated with them.
I only have the chassis w/o the cabinet here and the tone switches are dangling at the end of a 2 foot cable.

As far as finding an input point for AUX audio:
I tried going in on pin 2 of the 12at7 and also pin 1 of the next 6ba6. Both work very well.. but I think I've settled on connecting via a new .05uf cap to the top of the 470 ohm resistor (R8 Bietmans or R126 Sams) where it goes into the switch where it feeds the 1st IF tube 12at7 pin 2 in "phono" position only.

Perhaps it should really go in before that 470 ohm resistor as a current limiter?

It works very well with the audio out from both my mp3-thingie player as well as a transistor radio and it stays completely out of the circuit until the switch goes into "phono"
It also seems to have plenty of amplification sensitivity. So as long as the phonograph is off it's fine. Maybe I'll also add an aux switch to do it right.

Those push buttons on the Zenith never did have a spring return on them. You do have to lift them back out.
Sounds like you did a good job on the repair Peter ..

Good explanation they gave. Now it makes perfect sense to me.

T.