Repairing the Advance Electronics SG62B signal generator
Posted by aonomus on June 5, 2010
… also known as repairing vintage equipment made before you were born.
A friend approached me to take a look at his old RF signal generator, good for 150kHz-220MHz. According to him the RF output frequency ‘drifted’. Armed with the sliver of information, and with no service manual in hand I set about trying to repair it
The first test was to just power it up and test it with a frequency counter, and I made up a small spreadsheet with the recorded values at the min and max frequencies at each range.
From first examinations it seemed as if the signal generator at lower frequencies was fine, but at the higher range settings it fluctuated and was not stable. I proceeded to open the case and extract the several-dozen screws holding the RF-generator portion of the device closed.
Device construction from this era was interesting. Large enclosures, mechanical linkages, and other workarounds to squeeze every bit of performance out of a machine. Computers and electronics were big, slow, and expensive; it was worth the increased labor costs to get their performance/dollar ratio maxed out.
You’ll notice the mechanical linkages from the back of the case to the front knobs, these allowed linkages to transmit rotation to the rear of the case without having to worry about precise alignment. They featured spring-steel shims which acted like crude universal joints, or other methods to prevent strain on rotating components.
Also the voltage attenuator is a large chunky sealed unit without even standard RF connectors on the body, requiring soldering to the case.
Opening the rear compartment, I saw a very simple circuit that involved only one vacuum tube, several coils, and a large tuning capacitor. The only components that really could go wrong were resistor/capacitor values, or component damage due to a short/overload. Since the signal generator was still working, I proceeded to measure resistor values; none were too far out of line. The silver band indicates a +/-10% tolerance which all the resistors were within. Capacitor values were not all marked, so not all could be measured.
Having seen no problem, I checked with a friend who had worked on vacuum tube projects before, and he noted that I left the power switch in the ‘mod’ position instead of CW. Oops. The AM modulation of the output signal was causing the frequency counter to misinterpret the signal. Switching to CW gave the proper signals in the correct ranges, however I had introduced a new problem: the range switch was becoming more troublesome with each range change, the dirt that built up over the last few decades was finally starting to become a problem.
I cleaned the contacts using isopropyl alcohol and cotton swabs and powered up for a test. Nothing but a faint 60Hz hum. Wait, smoke? @#$%.
After fumbling with the unit for some time I finally found where the short had occured, between the plates of the tuning capacitor, after clearing the short (a wire had come in contact with the case during cleaning), the unit functioned just fine. Strange that it still works even after a hard short from within the oscillator.
After reassembling and testing, the unit barely drifts even after warming up. Although the tuning knob no longer indicated the correct frequency, this was likely due to many years of age on the capacitors and resistors from within the case. I could replace the old components with new ones, but without a service manual, retuning would be essentially blind guessing.
That ends my adventure with technology older than me, and even though I didn’t do very much to fix the unit at least I cleaned some of the interior and cleared other faults.
One last thing of interest, back in this era, twisted wire capacitors were used. Thats right, those little stubs of thick copper wire with thin copper wire twisted on and partially un-twisted at the ends are actually capacitors. The enamel on both wires acts as the dielectric, and wrapping/unwrapping the wire actually tunes the capacitance. While small variable capacitors were available, they likely could not handle the RF current flowing through them, or were just too expensive. This is a neat trick from a time long ago that I’ll remember for sure.
Steve: your RF signal generator is ready for pickup.