SX-42 - a case history

So I picked up a SX-42 cheap on fleabay, and in doing the electrical restoration I came across so many problems (many typical of this radio and its sister, the SX-62), I thought I'd share them in the hopes that someone going thru one of these pretty and outstanding-sound receivers might benefit.

The SX-42 and -62 are AM BCB thru 110 MHz receivers. On the upper two bands, 30-60 and 60-110 MHz they use a 10.7 MHz IF, and are capable of receiving wideband FM on these bands. THey use a pair of 6V6s in shove-yank for the audio output, with a stout output trannny to boot. Common troubles include burnt bandswitch contacts and fried output tranny due to a shorted cap in the tone control network.

This particular radio was sans cabinet, and the front panel is pretty much beyond saving as it has a real rusty acne problem. The chassis had some surface rust, and the S-meter window was pushed in. I began, before even powering it up, by replacing all the wax bypass capacitors and checking to make sure the primary of the output transformer was intact. This is only the second SX-42/62 out of perhaps a dozen that's had an output tranny with both sides of the primary intact. So far so good.

Power on, the radio is totally dead. Very weak hum when I touched the center of the vol control. Replaced all the resistors in the phase splitter, which was struggling along with 15 volts on the plates. I recalled in other SX-42s that EVERY 220K resistor was bad. Why the 220K resistors? Maybe the red stripes soak up more moisture? Who knows. OK Fine, have some audio but no stations.

Problem: dead oscillator. The osc/mixer is a 7F8 loctal. After some years the pins just don't make good contact with the socket. A bit of deoxit on the pins brings it aliver, and cleaning the osc section of the band switch clears up most of the intermittent opeation.

Problem: 455 IF almost dead. I inject a 455 KHz signal at the plate of the mixer and it takes 15 dBm to get anything at all. Scoping the first IF xformer shows it ain't passing much, and the adjustments don't do a thing. Replaced all 6 silver mica caps inside the can.

Problem: 455 IF *still* almost dead. The first IF can is sorta working but subsequnt cans are dead. I take apart all of the cans and replace all the silver micas. Oh, I also tried shooting 10.7 thru the other side and that was dead also. *Every* silver mica inside these cans was bad.

Problem: 455 KHz IF alive but weak. Replaced several plate/screen dropping reisistors, including another 220K(!). Better, but still weak.

Problem: 455 KHz IF weak, and trimmer cap in scondary circuit won't align. I loosened the screw as much as I could andran a piece of paper between the plate and the mica. Had some metallic fuzz in there. NOW the IF works well and aligns fine.

Problem: IF working fine but stations REALLY weak. Check tubes in the front end and find some idiot replaced a 6AG5 with a 6AU6, even had a yellow sticker on it that said "6AG5"!. Can't find a 6AG5 amongst my thousands of toobs, so I sub a 6AK5 for the time being.

Problem: AM BCB stations STILL really weak. Look at second RF stage (first RF stage is bypassed on AM BCB) OK, plenty of plate voltage at the band switch, take out the toob extender and there's plate voltage at the plate terminal but no cathode voltage which indicates NO plate current. I resolder as many terminals on the tube socket as I can get to. Now plenty of cathode voltage.

Problem AM BCB stations STILL really weak, but if I inject at the grid of the 2nd RF toob it's strong and steady. Resolder terminals on all RF transformers, now OK FINE.

Do a quick alignment and it seems to be working fairly well, on the lower bands anyway.

Problem: BFO warbly (REALLY warbly) and weak. Keeping in mind the problem with silver micas in the IF cans I take the BFO can apart and replace the 3 silver micas inside.

Problem: BFO still a bit warbly and wants to lock onto strong carriers. I've seen this before! Problem is the BFO coupling cap is leaking. Another silver mica. At least this time I don't have to take apart a can to get to it.

Now the AM/shortwave part of the radio is working pretty well. The FM is still very weak, so I take apart the discriminator can and replace the 4 silver micas in there. Now it aligns well and sounds pretty good.

Problem: First RF tube oscillating at various points on the FM bad. Replace tube shield, makes it better but not 100%. I suspect replacing the radio into its cabinet will make it even better.

Problem: intermittent oscillator on the top band. Clean the brass variable cap wipers with a burnishing tool wetted with deoxit.

I'm almost to the point of putting it back together now. There were also a bunch of nitnoid problems like a malfunctioning clutch on the bandspread knob, bearings so gunked up they would hardly turn, more crunchy tube sockets on some of the other loctal tubes, etc. Also the S-meter hairspring is screwed up and that will take some work.

Anyway, this is what you might be in for if you're unlucky. I've spent at LEAST 40 hours on this radio and the cosmetic resotration will take some more. But it's fun and relaxing (except taking apart those stupid IF cans).

73 John