Monday, August 5, 2013

Hallicrafters S-40A alignment from hell

Tuning up that old S-40A while staying sane


 As I work here in the shack I am presented with many opportunities to learn about old tube radio receivers. I have to say that so far the all time favorite that I have worked on is an old Hallicrafters S-40A from the late 1940's. 
 The Hallicrafters S-40A is a single conversion general coverage receiver with tuned band-spread for the Ham radio band 80M 40M 20M 15M and 10M. It receives AM and SSB/CW, and with a little bit of patience can make a fine receiver to any boat anchor transmitter you may have. It does have a pretty wide receive so if the band you are on is busy you hear everything within a 15khz bandwidth.
 Lately I have been working on this fine old receiver and have learned more working on this radio than any other I currently have. I have other radios here that I am working on here in the shack including a National NC-98, a Hallicrafters SX-25 super defiant and a Hallicrafters S-38 MKII all american five. But still the S-40A is my favorite.

 Ok, back to the alignment procedure. According to the manual for the S-40A it mentions tuning the dial to the top end of each band, in this case Band #4 at 36 MHZ. Somehow I had missed that you are supposed to put the DIAL at the 36 MHZ setting and then tune the radio using C19/C1 and C12 to put it on 36 MHZ. I was also tuning the tuning capacitor to 36 MHZ by putting my Frequency counter on the tuning gang and adjusting to 35.545 + the I.F. frequency of .455 to acheive this. OK, next problem.
 For some odd reason my LSG signal generator would not go up to 36 MHZ. Gotta get that on the bench next. Once I got the radio to 36 MHZ I was loosening the dial screw and setting the dial at 36MHZ. Wonder why it was so far off. Ended up using my MFJ 259B to get the 36 MHZ signal into the antenna connector through a small resistor so I didn't overdrive the front end..fun fun fun

 Ok, so I got my radio tuned up at 36 MHZ, and down to 18 MHZ, go through the same procedure, only this time with my signal generator. Got it all lined up and real close to where it should be. And of course now the TOP end is off again. So back and forth I go several times when I realize that I am supposed to have a modulated signal from the Signal generator. ooops.
 Third time is a charm I guess, so I go back to the top end, align it all up and then drop down to 18 MHZ and line that end all up. I am pretty stoked at this point as I am listening to some CW on 17M. The radio seems nice and sensitive, as well as can be expected anyway from a 60 year old tube receiver. Go back up to the top end and nothing is lined up. The dial is almost 5 megahertz off again at the top end, where 36 MHZ should be the dial is reading 31. WTH is going on with this senile old radio ???
 As it turns out, you have to retune several times to get the entire band to track properly, which is not mentioned in the S40A manual, at least not the one I have here.

 So, as I understand it, the first thing you do loosen the dial screw with the tuning capacitor fully flush at the bottom end of the band and align the lines on the dial with the center line on the glass. THEN you go to the top end of the band and set the dial on 36 MHZ. Dont worry that the frequency is off just yet, you will adjust that when you tune C19. Once you get that dialed in as close as you can, you adjust C1 and C12 for maximum noise. You may have to adjust several times, reducing the output from your signal source. Once you get that all lined up, drop down to 18 MHZ and adjust slugs 8, 5 and 6. These are right next to caps that you adjusted for the top end. Put the dial on 18 MHZ and tune #8 until you can hear your signal, reducing the output from your signal source as you go. Having fun yet ? NOW go back up to 36 MHZ and readjust so that when you hear your signal source at 36 MHZ the dial also reads 36. You will have to adjust this several times each as you go back and forth. I adjusted the frequency first then went back and adjusted for maximum signal. It just seemed to make more sense. 
 Ok you now have the top end and bottom end aligned so that it tracks fairly close on band #4. No drop down to band #3 and do this again. Once you get the routine down it will become easier to do, and wont take any time at all.