I've been asked to move this question from electrical mechanical restoration section to this section. I've been told there are a lot of SX-28 veterans here.

I re-capped the power supply and brought the old SX-28 back to life. Everything was working while radio was on its side. But, as soon as I sat it flat and switched it back on after a few hours, lo and behold, no audio.
Plenty of lights, tube filaments, etc.

All this time I have been feeding 110 volts AC via a variac (with voltage and current meters). When radio played, it was pulling about .800 amps. Now it's only pulling .400 amps.

I suspected the grounds for one of the electrolytics (C41 and C44) might have become ungrounded. Sure enough, one did and I jumpered it to its own ground. Still no sound.

1-Checked the B+ supply on the filter caps. About 340 volts at the input filter cap and a bit down from that after the choke and at the output filter cap.

2-Checked the 6V6 tubes on a tube checker. They passed. Measured correct voltages at the 6V6 tubes--300 plus on the plates. Checked all electrolytic grounds again. Don't see anything shorting to ground, etc.

Any ideas about why my radio is playing games with me? Would would be very grateful for any help I can get.

Let's start with some basics.

1. You say you replaced C-41 and C-44. Did you replace the other electrolytics: C-47, C-48 and C-49? All five electrolytics should be replaced before you power up an SX-28.

2. Did you replace the wax paper caps? After so many years, they are time bombs. Of special concern are the coupling caps to the 6V6s (C-45 and C-46). These coupling caps should be replaced before you power it up.

3. Check all the tubes. I seem to remember that a tube that went bad unexpectedly killed reception on one of my SX-28s.

4. I would strongly recommend you replace all electrolytics and the coupling caps mentioned above before you apply power to this set.

Here are two good articles showing what restoration of an SX-28 entails:

http://www.radioblvd.com/SX28Notes.html

http://www.antiqueradio.org/halli12.htm

Good luck! When you're done, you will have a first-rate receiver with audio to die for.

Joe Connor

First follow Joe's advice and take care of the other capacitors. After that if it still doesn't play, measure the cathode voltage on the output tubes. An open resistor here will bias the stage into cutoff.

The SX-28 design is one that can take out the S meter from failures in that circuit. Although it doesn't provide absolute protection a pair of diodes (one each direction) across the S meter terminals will help protect it from circuit faults. Excessive current through the movement will cause enough voltage drop to "turn on" the diodes and bypass some of the current around the fragile coil and movement.

Rodger WQ9E

Thank you Joe and Rodger for your responses.

Joe, your questions:

1. You say you replaced C-41 and C-44. Did you replace the other electrolytics: C-47, C-48 and C-49? All five electrolytics should be replaced before you power up an SX-28.

Yes, all electrolytics were replaced before power up including 47, 48, and 49.

2. Did you replace the wax paper caps? After so many years, they are time bombs. Of special concern are the coupling caps to the 6V6s (C-45 and C-46). These coupling caps should be replaced before you power it up.
Yes, all wax paper caps were replaced before power up.

3. Check all the tubes. I seem to remember that a tube that went bad unexpectedly killed reception on one of my SX-28s.

Basically checked the rectifier tube and the two 6V6s.

4. I would strongly recommend you replace all electrolytics and the coupling caps mentioned above before you apply power to this set.

Again, this was done.

When you say no audio, is there a hum or any noise at all from the speaker or is it completely dead?

What are the voltage readings for the cathode and screen grids for the 6V6 tubes?

Have you tried plugging high impedance phones into the headphone jack?

Insert and remove a plug into the phono jack a few times, a dirty shorting jack will kill the audio.

What is the S meter doing, will it still deflect when tuning across a known good signal (either your signal generator or a local broadcast station)?

Rodger WQ9E

Rodger's suggestions are good ones and you should follow them. Here are a few more.

You say that the radio died when you flipped it from its side. That involved a certain amount of movement, stress and force given the weight and size of an SX-28. Let's start by focusing on that:

1. Make sure that one of the speaker wires didn't get detached.

2. Make sure you didn't inadvertently knock the send-receive switch into send position.

3. Make sure you didn't inadvertently turn down the RF Gain or Audio Gain.

4. Look at the new caps you installed. Make sure no leads got detached.

5. Test all tubes and make sure they're firmly seated in their sockets.


Joe Connor

Ok guys. Sunday morning and I'm fresh and ready to delve into troubleshooting my SX-28.

I The first few tubes I checked indicate problems with getting power:

6V6--Measured 360 volts dc on the plates on both of these; but only .149 on the screens; and only .123 on the cathodes. Plainly I see we have a problem with getting power to these pins.


6SC7--Plate 1=.128 volts; Plate 2=.128 (yes problems here, too); Cathode= 0 volts. (ouch)


Checked all the new electrolytics again. They check out.

Appreciate your feedback.

With those symptoms, hopefully a bad connection to the power supply choke but otherwise it sounds like the choke (CH1) is open.

Well, I'm getting the correct voltage at the input electrolytic filter capacitor and the correct voltage on the output electrolytic filter capacitor. Choke 1 ties the two together. So, I my reasoning says it can't be that.

That is good, sounds like a wiring issue. Trace the wiring from your output filter capacitor to the 6V6 screen grid connection and somewhere along the path you will find the problem.

Rodger WQ9E

Ok. Will do. But, what about the other tube? the 6SC7 (--Plate 1=.128 volts; Plate 2=.128; Cathode= 0 volts.

Is tracing the next move there, as well?

I believe when you find one problem you will find them all. The same circuit feeds the 6SC7 through a resistor network.

Rodger WQ9E

You have a point there. Ok. Will do some tracing.

MISSION ACCOMPLISHED!!!!!!!!!!!!!!!!!!!!!!

Rodger. I did a continuity check between the output power filter and found no continuity between it and the screen pins on the 6V6 s.

So I hand traced the wire back to the electrolytic and wiggled its connection to capacitor. Did another continuity check between the capacitor and the screen pins and BAM!!!! continuity!!!!!!!!

Powered up slowly through the variac. Noticed the current pull was back to .800 amps and after a second or two the rush of the airwaves engulfed the room!

Looks like I need to do a better job of securing that connection at the electrolytic.

Thanks Rodger for this last leg of the repair. Learned alot. Also, thanks to everybody else for your guidance.

Happy days are here again...the stars are......

Congratulations! Glad you found the problem and especially good that it wasn't a bad filter choke. Sounds like a bad solder joint. I am not sure what your background is but if you have been used to working on solid state gear with printed circuit boards a lot of the vintage gear requires a bigger iron to properly heat the large terminals and wires.

I use a Weller WTCPT soldering station with a medium size high temperature tip for most work, a soldering gun for the rare much larger connections, and a very large American Beauty iron for really heavy stuff. Too small of an iron and a small tip without much thermal mass will make soldering much more difficult. The irons used by stained glass enthusiasts, coupled with a light dimmer or variac to control temperature, makes a versatile iron for a range of connection sizes.

With few exceptions (i.e. heavy transmitter RF tank parts) there is no reason to make a connection mechanically secure before soldering and a mechanically secure connection will serve to temporarily hide a cold solder joint and also make future repair much more difficult. There was an Air Force white paper published in the 1950s calling for an end to mechanically solid connections except for very heavy components that could tend to vibrate/move under stress.

Rodger WQ9E

Wow...really? I've been using that method for years and never knew there was an official stamp of approval for it. You just have to make the component secure enough so that it doesn't move after removing the soldering iron and while the solder connection is cooling.

Dave

Congratulations on finding the ailment. We've all had such headaches at one time or another.

You'll avoid a lot of them if you use 63/37 alloy solder. It's either pure liquid or pure solid, with no in-between "plastic" range such as other alloys (like 60/40) have.

Most cold solder joints are created by insufficient heat (heating the solder no hotter than the "plastic" state), or by disturbing a connection while the solder is in that "plastic" state.

I agree with that Air Force white paper. I've been using "lapped" joints (and 63/37 solder) for many years with good results.

Good luck with your SX-28.

Larry

I came across the Air Force paper in an old issue of Radio and Television News, if I run across it again I will scan it and post it here. As I recall it had a lot of other good information in it. The overall consensus was that most components would fail long before the stress would break a simple solder joint.

I am amazed at the amount of rosin that comes boiling out of a lot of the 1930s era solder joints, Halli gear in particular. I have a Chinese knockoff of our AN/GRC-9 which I bought NOS wax sealed in the original cartons. Along with a knockoff of a Simpson VOM (and "white elephant" brand batteries for the VOM) it also included a roll of solder and a
"soldering tool" which looks like a miniature hatchet that you heated in a fire before soldering. I can imagine the joints made with that would be pretty ugly, particularly if the radio repairman was being shelled during the repair attempt.

Rodger WQ9E

Congratulations! You engaged in logical troubleshooting and found (and fixed) the problem in good order. Now, go and enjoy your SX-28!

Joe Connor

Interesting. I thought this originated with the space program. . It looks like the space program used this recommendation. I hope someone can find and post/link this document, I would like to read it.

It was calculated that the Apollo capsules were 400 pounds lighter using this technique.

Bill