The tuner is pretty interesting but very gummed up, so it needs to stay on 3.
I moved it between channel 1 and 2 then 3 and 4, with no sound coming through on any of them. I have been tripped up by adj channels before, so that was a good idea to try Wayne.
Next was to replace the 5 uf cap on the ratio detector, still no improvement. Swapping tubes in the IF and sound stages also did little. Note that measured voltages were close enough despite this set using an odd split/stack of B+ to sweep and signal sections across a 330 volt B+ supply. It does NOT use the audio output tube to drop B+ to IF strip like so many others.
Then I decided to investigate the sound take-off coil L14, which two turns of #24ga enameled coil wire, through a spaghetti sleeve to C55. Lifting the ground side did nothing, then I figured to rewind it on L14. I added a few more turns and moved A3 slug but still nothing but buzz. Note that 21.75 MC from a signal generator flowed through both sound IFs with no gain drop.
Not to be stopped at 11pm after all the time I had invested, I hit the books with Wallace TelAid's for Post-war sets 1947-53 (mfr specific book of service info like Riders) to see how Motorola, Philco and RCA did sound take-off. I figure Sparton had to come up with their own way of tapping sound carrier or had to pay a royalty to the others.
RCA and Philco both used a 3.3 pf cap from the second IF tube cathode, that cap tapped into a slug/coil to feed grid of the first sound IF tube. Motorola took sound off the video tube plate via 2.2 uf cap, with one fewer sound IF.
I took a clip lead from C55, after disconnecting L14, and clipped to a few of these locations used by others. Once I landed on L13+R35, I heard faint sound, moving to the other side of L13, on the grid of 3rd IF tube, I got much louder sound. I made the connection permanent.
A retouching of the three remaining sound coil adjustments brought it in louder and clearer, to the point that it was loud enough at 1/3 volume setting. Case closed. Thanks Tom and Wayne.
Good luck to anyone who gets a mirror-in-lid set like this, a truly odd design that makes a good picture on the 10BP4. The HV supply runs at 190 kC and makes 10 kV until its overloaded by video from switching channels or sources, requiring it be turned off then on should you lose raster. It was recapped very competently 100% and yet never had sound according to the last owner.
It went back home today
