Thanks for the voltage info.

I checked the voltages and they all seem to be reasonable. I substituted a 6SG7 that I had on hand for the 6SK7 at the V3 (1st IF Amp) with improvement in sensitivity on Band 1. Then moved the 6SG7 to the V4 slot (2nd IF Amp) with similar improvement.

I then moved it to V1 (6SK7 RF Amp) hoping it would make a big improvement. But it not not make any difference at all. Not sure what to make of that little experiment.

I would love to use the oscilloscope to pin point where I am losing (or not gaining) sensitivity... but not sure how to do that.

I replaced R1 (33 ohm to the RF Amp) It was within about 9% of tolerance so as expected it did not make a difference.

It seems next to impossible to inspect SW7 thoroughly with out tearing the whole thing apart -- especially not knowing exactly what I am looking for -- broken wire? cold solder joint ? ??

Thanks again for the voltage info -- crazy how similar the circuit of S40A is to the S-20R

John w.

N7ZMG

I think you've proved the RF tube is ok.

SW7 does the band switching, so I thought it may be appropriate to look for a wire that may have been disturbed. The original soldering should be ok unless proven otherwise. Keep looking; revisit your work in that section. It's probably something simple.

I noticed in the photo that you previously removed the main tuning capacitor. Verify the connections by direct tracing. If there's a wiring mistake or bad joint on RF or antenna sections would affect receiving sensitivity.

I would say S-40 is just a slightly revised S-20R in a different package

Can you say if AVC and NB switches make any difference in reception?

I traced every wire and checked for continuity on all of the coils in the antenna portion of the SW7 this morning. I will do the same for the RF portion later today with fresh eyes.

I think I will also take the shield off of T3 and try and determine why the trimmer cap has to be screwed all of the way out. The signal seems to get louder as I adjust that cap further and further. Then it is all the way out (minimum capacitance).

The ANL and AVC seem to have little effect on anything.

But I do agree it must be something common to all of the bands because the sensitivity is so poor across everything. Could it still be one or more of the tubes?

Thanks for your continued help.

John w.

N7ZMG

The AVC switch gave a clue. With an antenna hooked up and RF gain set at max, even with just static, volume from the speaker should become louder when you turn off AVC. If not, check that circuit.

Since you're able to get relatively loud tone with the IF alignment procedure, that part of the radio is likely ok. I don't think it matters where in the range of trimmer setting the "peak" occurs, as long as you're able to peak.

Based on the descriptions so far, I still think the issue is at or before the converter stage. Also based on your recent posts, I think the RF and IF tubes are fine.

It's not difficult to check converter operation with a nearby radio. Also, you should be able to inject signals directly into 6K8 for each band. If converter is working, it should turn it into IF and amplify the tone.

If you don't already have it, this site's archives section has a copy of "Elements of Radios Servicing." The book has a "quick check" procedure at the beginning of each chapter. I think you can use that to isolate which section of your S-20R has the potential problem.

Checking the 6K8 tube, if you still have your S-19R, you can try swapping out tubes.

I have not done it because I don't have a oscilloscope. Perhaps someone can suggest a test using your scope.

Thanks for the info AJJ.

I pulled out my S-19R. It is way more sensitive than the S-20R... and it does not even have a RF Amplifier.

I also experimented with the antenna connections...for the higher bands I hooked up my dipole (with 50 ohm coax) to A1 & A2 with much better performance than with the long wire. The long wire works better on Band 1. So that all seems to be just what is expected.

I was able to get WWV at 10 Mhz. But here is the curious part. I had my digital shortwave nearby and heard the LO very loud at 10.905 Mhz and next to nothing at the expected 10.455 Mhz. If I understand correctly that would make the IF frequency 905 Khz. About double what it should be.

So does that mean I have the IF transformers peaked for 900 Mhz rather than 455? I have feed a 455 Khz signal to the 6K8 with the grid cap removed -- perhaps some how I tweaked them for a harmonic.

Your thoughts would be appreciated.

Thanks for the info on the "Elements of Radio Servicing"

I will re-do the IF transformers and see if somehow the are set at 900 Khz.

John W.

N7ZMG

It is possible one or more (not all) of the IF adjustments is on an harmonic instead of 455khz. Try performing the IF alignment procedure again. Peak the last one first then work your way to the front.

May I suggest another way to do this: Set your frequency generator for 455khz. Connect it directly to the last IF stage (T3) and peak by adjusting only C17B and C17A. Then repeat with T2 and T1 in that order.

Hey AJJ

We did it!

I reset the IF transformers so they were peaked at 455 Khz (not 900Khz). The trimmers were not all the way out any more. It was not easy to do -- I tried a second time without the scope and got the same results. Then I hooked the scope up after each transformer and adjusted for a peak while making sure the frequency I was peaking was 455 not a harmonic..

Then I re-aligned each band -- what do you know they all came alive

Not much on Band 4 though. I set the LO 455 below frequency on that band. (Somewhere I read that it was supposed to be below on that band but I haven't been able to find where I read that). The LO works well on that band so maybe not much to be heard. I will try at a different time of the day.

Will also investigate the ANL and AVC. I did not try them after the massive reset.

It took a little over an hour to reset the IF's and re-align. Why of why did I not do it this way to begin with.

I do appreciate all of your help... I learned a great deal in this exercise -- but feel bad that I wasted all of your mental energy for this crazy mistake.

Again thanks ever so much for sticking with me -- I am pretty sure I would have given up.

John w.

N7ZMG

This is all part of the fun. Enjoy your radio.

There isn't much on band 4. I heard some passing truckers talking on CB once.

Ed Romney's book has a few pages on his S-20R restoration. You may find this interesting: