75S-3 on AM

[n1eu]

A 75S-3B in okay condx found its way to me a while back and I just got around to dusting it off and firing it up.  I was surprised at how decent it sounds on AM through the IF xfmr filtering - there's no AM mech filter installed.  I experimented with moderate success at improving the rx audio by tapping the audio at various points after the product detector, but I'm admittedly no tube circuit wiz.  I've posted the AF schematic at http://n1eu.com/s3_audio.gif as well as attaching it here.

The two points I've tried are:
1. at the input to the AF output xfmr T7 (using 1uF 250V cap for coupling)
2. at the AF Gain pot R31 wiper, and then externally amplified

I'd appreciate any comments on what might be an ideal take-off point for the audio or any other info on using the 75S3 on AM.  Also, has anyone installed an AM mechanical filter of whatever flavor in the 75S-3 and how did that fare?  Through the if xmfr filtering, the audio passband has decent downward-sloping response out to 5kc on AM.

Thanks & 73,
Barry N1EU

[K5UJ]

I've never owned a S3 and make no claim to being a rx guru but I looked at the schematic and the detector/BFO/first AF stage and AF gain pot arrangement have general similarities with my 75A-3 after its envelope detector got turned into a product detector by pulling an oscillator signal off one of the BFO grids.  In my case the C.W. from the amfone experts was to tap the AF gain pot wiper.  That not being good enough for me, I proceeded to poke around with a 600 v. 0.1 uF cap with an alligator clip on one end and a tube AF amp on the other and tried 3 or 4 different pick off points and looked at all of them on a scope.  It turned out I should have listened to the experts and saved myself some time and trouble (except there was some education gained I suppose) for the AF gain pot wiper proved to be the best detector audio take-off point. 

I can't tell you much about the mech. filter but if you have a 10 Kc passband with a nice rolloff without one, then that's just about perfect for nice sounding AM.   that's what I do with the A3 out to a p.p. tube amp and it sounds great.  FYI:  I had no load on the AF transformer and it sounded unhappy about that so I pulled the single ended 2 or 3 w. AF amp tube.

You can also try putting a 10 or 15 pF cap like a 2 KV ceramic job on the plate of the last IF tube and bring that out with a second grounded audio cable to a scope for looking at modulation envelopes.  The visual display of the passband is also nice for seeing noise amplitudes which is helpful if you use any phasing/nulling boxes for RFI elimination.

Rob

[Steve - K4HX]

If I'm reading the schematic correctly, you should be able to use the CW sidetone jack (J10). Check for detected audio there with a scope. I there is, use a coupling cap (I'm guessing 0.1 uF or so and feed it to an amp with as high an input-Z as you can get/make).

[n1eu]

Thanks Rob and Steve!  Good info on all counts.  The cw sidetone jack does indeed provide the same audio as the AF Gain wiper so that's good info.  I will be trying the scope connection as well.