Sparton 4940TV sound issue

[DavGoodlin]

I have checked voltages, replaced paper caps, ohmmed out the transformers but still get just an intercarrier buzz.

If switched off-channel, the static is very loud like normal with volume at max, but tune in a channel from DTV box and its just some intercarrier buzz that is tunable but no audio detection at all from the discriminator.

How do you go about tracing the 20 mhz sound carrier?

[damen]

Are you sure your DTV box is outputting an audio signal ? I had a similar experience and found out my box had a muting feature that I could only turn off with its remote.

[Electronic M]

I would double check with a different signal source as some TVs and some consumer RF modulators don't get along well.

If that doesn't solve it I would determine the exact sound IF frequency and set up a signal generator to create it. If it's SS lab grade it should have FM modulation capability to make audible checks. If it's a tube signal generator get a digitally tuned SW reciever with continuous coverage like a Sony ICF-SW7600GR set the radio to the exact frequency and tune the generator for best signal with worst signal coupling...I've found a lot of tube signal generators have AM modulation so crude it both produces FM components, and confuses frequency counters, but a good radio is hard to trick.
Once you have a signal generator set up signal injection troubleshooting/alignment becomes viable.
Might be possible to trace with a scope too, but signal injection is often easier.

[DavGoodlin]

Originally Posted by damen

Are you sure your DTV box is outputting an audio signal ? I had a similar experience and found out my box had a muting feature that I could only turn off with its remote.

The DT box has volume and Ive done that too. It worked fine on a 52 Magnavox 17" chassis I just finished.
Its the one sold under the name Winegard and has a wall wart, no on board PS to fail

[DavGoodlin]

The sound carrier is at 21.75 MC, which my sweep generator WR69 can produce. It traces from sound take off coil at 3rd IF, through both sound IF 6AU6 tubes up to the ratio detector transformer L32.

I can't seem to resize sufficiently to upload the schematic but its here.
Sparton 4940TV.pdf
http://earlytelevision.org/pdf/spart...tv_rider_3.pdf


Still the same symptom, no audio information at full volume, but very loud waterfall when signal is removed. DT box worked fine when checked on the 70 Zenith 16" color TV nearby.

I pulled off the shield of L32 expecting to find a typical, partly-exposed and leaky silver-mica cap. It was not that at all, much more robust and serviceable. Same ceramic dog-bone capacitors as in the picture.
Attachment 205793
Resistors and coils check out individually, what about the mica and ceramic caps??? Shotgun those caps?

[old_tv_nut]

Is it possible the set is tuned to the wrong sideband? Try going one channel up or down?

[DavGoodlin]

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