Can this FM detector even work?

[Bobby Dip]

I have some more info. I switched my generator from FM to AM, and this circuit seems to demodulate AM just as well as FM. Also, I found that the IF cable from the tuner was pinched in the sheet metal. After fixing that, I have real audio from the channel 3 output on my cable box. The audio sounds fine as long as there is no digital lettering on the screen. There is a nasty buzz when there is lettering on the screen. Our first TV, some forty years ago, was an old B&W set that did the same thing. I was never able to eliminate the buzz, and finally decided it was a design limitation of the set. Back in those days only the news shows had digital graphics, so it wasn't that big a deal. Is this 'buzz' problem common on old TVs? Is it a design limitation as I assumed?

Bob D.

[earlyfilm]

Originally Posted by Bobby Dip

The audio sounds fine as long as there is no digital lettering on the screen. There is a nasty buzz when there is lettering on the screen.
Bob D.

Bob, Congratulations on solving the sound issue!

I agree with old tv nut, but remember, alignment is the last thing you do to a TV, not the first.

I'd first check to see if the incoming signal is too strong, and/or the AGC is maladjusted, before thinking about alignment. Remember this set was designed for a auto antenna, not a relatively high level signal generator.

By the way, since your first pictures in another thread did not show a power supply, what are you using for one now?

James.

[old_tv_nut]

Originally Posted by earlyfilm

Bob, Congratulations on solving the sound issue!

...remember, alignment is the last thing you do to a TV, not the first.

Ditto! Make sure nothing else (like AGC) is out of whack first.

[Bobby Dip]

Originally Posted by earlyfilm

Bob, Congratulations on solving the sound issue!

By the way, since your first pictures in another thread did not show a power supply, what are you using for one now?

James.

Hi James, there is one large gauge wire going into the chassis that powers the tube filaments. It must be 12V, and originally came from the car battery. (The 6V tubes are wired as pairs in series.) I'm using a large switching power supply, and the TV is drawing 4.5A.

There is a single "B+" input on the TV chassis. (This must have come from a separate power supply that is missing.) There is a 300V electrolytic directly on the raw B+ line. The 6S4-A tube has only one recommended operating voltage on the data sheet, 250V. So 250V seemed like a good choice. (So far no smoke, fingers crossed.)

I just happened to have a pair of Heathkit HV power supplies model IP-17. These are rated at 100mA, 0 to 400V. It became apparent that 100mA was not enough to power the set, so I modified the pair to act a one 200mA supply. Each unit had 2 6L6s in paralled as series pass regulators. So there are now 4 6L6s in parallel.

So 12V @ 4.5A DC (could probably use AC) for the filaments. And 250V DC @200ma DC for B+. Once the set is totally restored, I plan on building a dedicated power supply for both "A" and "B+".

Bob D.