Hallicrafters SX-101A Receiver
After my first rescue mission was complete, I was able to bring the 101A home for an overhaul. The receiver looks like it had not been used in many, many a year. Field mice had made it a family residence for generations.
After removing all the material that had been moved into the once nice and shinny chassis, I was able to determine that all the required parts were still there.
Now the work was about to begin. I actually had to locate my old test equipment that had not been used in about 30 years. After some component replacements in my test equipment, I was ready to start operations. No one ever said that overhauling vintage radio gear was easy, but the more I looked the more I found wrong.
Tracking down paperwork and schematic diagrams for my SX-101A was the easiest part of this rebuild. The resistance chart within the manual was the best find I could imagine. I could now examine the circuits and determine where things went wrong without worrying about powering up the unit.
Circuit values were close, but not where they belong. A few component disconnects here and there were enough to let me see where some resistors had changed values over the years. Replacing many resistors and all electrolytic capacitors to the nearest currently available values brought my measurements more in line with the original specs.
This part of the restore had now taken about 2 months and I have not yet powered up the receiver.
Powering up the receiver meant finding my old trusty variac which would bring up the input line voltage very slowly.
After testing the variac unit, I plugged in the SX-101A's power cord and thought that 30 volts AC input for several hours would be a good start. Nothing seemed to be happening. I measured with my trusty old EICO VTVM and found the voltages were proper. So I let it sit there for about 4 hours. Now to raise the input voltage to 50 VAC and check for smoke. All clear at this point. I decided to let it stay there for 4 more hours.
While waiting and watching, I thought about at what voltage I should see the filaments begin to glow. I figured at about 70 volts should be sufficient. Since I had no signs of problems up to now, I raised the voltage directly to 70 volts. At last, a very dim glow was starting to appear in some of the tubes. I let it sit there for 2 more hours and found that only 4 of the 18 tubes were warm to the touch. I thought that maybe they need a little more. I set the variac to 75 volts and all tubes started to glow. The real test is now to come.
Bringing the voltage up slowly is the only way to go. I gradually increase the variac to 117 volts over a period of several days. With this method I had no problems with tubes or other components failing. I was just lucky!
I now attached an antenna and speaker on the receiver and gave it a test run. I had some signals, but not much.
Now that I obtained a tube tester from a friend, I checked all the tubes and found several that were very weak.
I decided to replace as many as I could locate and the old SX-101A came to life.
Now was the time to check the voltages from the receivers tube voltage chart and see how far off things were. Not too bad. Now comes the hard part....the alignment. Following the procedure in the Hallicrafters manual, I proceeded to align the unit according to the factory procedure.
I wanted to use my scope to help with the alignment, but the B&K scope decided it was not in working order. After a short while of troubleshooting the scope, I found a shorted bias line in the scope, replaced a filter capacitor and was now back in business. My Beckman function generator supplied the input signal and the scope did its job and showed my accomplishments. Now the SX-101A is back to factory specs and operating like it was almost new. Except for the tarnished chassis from being used as a "Hotel" for years, the exterior looks to be very presentable.
As far as the receiver goes, some prior owner did make one modification to it. The on-off switch that was originally incorporated into the function switch must have failed and an on-off switch was added on the front panel next to the "standby-receive" switch.
Except for this modification, the rest of the SX-101A receiver is in original condition and works flawlessly.
After removing all the material that had been moved into the once nice and shinny chassis, I was able to determine that all the required parts were still there.
Now the work was about to begin. I actually had to locate my old test equipment that had not been used in about 30 years. After some component replacements in my test equipment, I was ready to start operations. No one ever said that overhauling vintage radio gear was easy, but the more I looked the more I found wrong.
Tracking down paperwork and schematic diagrams for my SX-101A was the easiest part of this rebuild. The resistance chart within the manual was the best find I could imagine. I could now examine the circuits and determine where things went wrong without worrying about powering up the unit.
Circuit values were close, but not where they belong. A few component disconnects here and there were enough to let me see where some resistors had changed values over the years. Replacing many resistors and all electrolytic capacitors to the nearest currently available values brought my measurements more in line with the original specs.
This part of the restore had now taken about 2 months and I have not yet powered up the receiver.
Powering up the receiver meant finding my old trusty variac which would bring up the input line voltage very slowly.
After testing the variac unit, I plugged in the SX-101A's power cord and thought that 30 volts AC input for several hours would be a good start. Nothing seemed to be happening. I measured with my trusty old EICO VTVM and found the voltages were proper. So I let it sit there for about 4 hours. Now to raise the input voltage to 50 VAC and check for smoke. All clear at this point. I decided to let it stay there for 4 more hours.
While waiting and watching, I thought about at what voltage I should see the filaments begin to glow. I figured at about 70 volts should be sufficient. Since I had no signs of problems up to now, I raised the voltage directly to 70 volts. At last, a very dim glow was starting to appear in some of the tubes. I let it sit there for 2 more hours and found that only 4 of the 18 tubes were warm to the touch. I thought that maybe they need a little more. I set the variac to 75 volts and all tubes started to glow. The real test is now to come.
Bringing the voltage up slowly is the only way to go. I gradually increase the variac to 117 volts over a period of several days. With this method I had no problems with tubes or other components failing. I was just lucky!
I now attached an antenna and speaker on the receiver and gave it a test run. I had some signals, but not much.
Now that I obtained a tube tester from a friend, I checked all the tubes and found several that were very weak.
I decided to replace as many as I could locate and the old SX-101A came to life.
Now was the time to check the voltages from the receivers tube voltage chart and see how far off things were. Not too bad. Now comes the hard part....the alignment. Following the procedure in the Hallicrafters manual, I proceeded to align the unit according to the factory procedure.
I wanted to use my scope to help with the alignment, but the B&K scope decided it was not in working order. After a short while of troubleshooting the scope, I found a shorted bias line in the scope, replaced a filter capacitor and was now back in business. My Beckman function generator supplied the input signal and the scope did its job and showed my accomplishments. Now the SX-101A is back to factory specs and operating like it was almost new. Except for the tarnished chassis from being used as a "Hotel" for years, the exterior looks to be very presentable.
As far as the receiver goes, some prior owner did make one modification to it. The on-off switch that was originally incorporated into the function switch must have failed and an on-off switch was added on the front panel next to the "standby-receive" switch.
Except for this modification, the rest of the SX-101A receiver is in original condition and works flawlessly.