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Electronic Equipment Restoration Part 6: The Moment of Truth

At this point of your equipment’s restoration, all out of tolerance resistors, bad tubes, damaged wiring, and suspicious capacitors should have been replaced. The time has come to awaken the sleeping beauty and verify all is ready for basic stage-level testing.

Most of the amateur, military and test equipment you restore will have internal power transformers. These are needed to supply the various filament, plate/B+ and bias voltages used for the rig’s operation. Keep in mind though that some antique broadcast and beginner shortwave receivers likely omitted a power transformer due to cost concerns. While good for the manufacturer’s bottom line, not so much for the restorer during the testing phase…but we’ll get to that soon enough.

If your transformer-equipped project was wired with only a two-conductor power cable, now is the time to embrace personal safety and install a true three-wire power cord. Make sure to solidly attach the cord’s green safety ground conductor to the chassis…ideally to either a screw terminal or solder lug positioned closest to the cord’s entry port. And, if your project came absent power fuses, consider adding one in series with the cord’s black (hot) conductor. In the case of a “hot chassis” no-transformer set, install a polarized two-wire cord but be mindful of what wire goes where. The smaller of the two plug blades is the hot lead. It should be installed in series with the power switch and the downstream tube filament string.

If your restoration interest involves broadcast radios, many people start off with what is commonly referred to as the “All American Five”… meaning a set with five tubes whose filament voltage adds up to what was then the nominal line voltage (120VAC). Here, you will need a 120V-120V isolation transformer to safely test and troubleshoot your set’s circuitry, absent a potentially fatal electrical shock.

These seemingly simple radios used a single half-wave tube rectifier to develop around 150VDC plate voltage directly from the incoming utility line. As there was no power transformer and with the set’s tube filaments wired in series (powered directly off the 110V AC line), then the series-connected tube lineup must have identical filament current ratings. Yet, if one tube in the series-connected string suffered a burned open filament, the remaining tubes would go dark…like Christmas Tree lights of old.

All American Five radios began life in the 1930s as a way to produce consumer-grade broadcast radios at rock-bottom minimal cost. Early All American Five radios leveraged octal-tube lineups such as a 35Z5 rectifier, 12SA7 Converter/Mixer, 12SK7 IF Amplifier, 12SQ7 Detector/Pre-Amp, and 50L6 Audio Output. After WW-II, the tube lineup migrated to ‘modern’ miniature 7-pin units such as the 35W4, 12BE6, 12BA6, 12AT6, and 50C5. Many millions of these sets were manufactured, in all sorts of case shapes and artistic designs, with production eventually disappearing in the early 1970s.

So much for our short stroll down Radio’s Memory Lane. Let’s get back to the Moment of Truth:

Step 1: Tube up the set and identify, via the schematic, appropriate metering points for the rig’s various rectified voltages, such as B+ and bias. Typically, this will be the B+ filter capacitor.

Step 2: Locate either a digital or analog multimeter and get ready to measure rectified B+ at the metering point as identified. Make sure the meter is in the appropriate DC voltage measuring mode. Many a digital multimeter has been damaged through careless selection of ‘Ohms’ instead of ‘DC Volts’.

Step 3: With the Variac set to Zero volts and the rig’s power switch toggled “On” (odd how many times this important step is overlooked!), in small increments gradually increase line voltage while watching the line current meter (or Watts scale in some Variac units). Ideally, the indicated line current should gradually increase in step with applied line voltage and, eventually, level out to a steady state condition at around 115V. If, however, the indicated line current begins to rapidly increase…seemingly without limit…your set has an internal short circuit/defective component/wiring error. STOP! Consider a ‘divide and conquer’ troubleshooting approach to locate the problem.

Step 4: Assuming Step 3 has been accomplished, next carefully measure the various DC power supply voltages. The equipment’s service manual or schematic will usually identify typical design/measured voltages for each at rated AC live voltage. Measure, document the results, and compare as needed.

Step 5: Carefully observe the powered up unit for signs of trouble…such as very hot components or odorous smoke. Of course, parts such as power resistors (2W and higher), will operate at high heat levels but none should be red-hot or smoking! If you encounter this, power down the set and begin stage-appropriate troubleshooting. Keep in mind, equipment designers work hard to never overstress components as doing so results in poor reliability, a bad corporate reputation, and angry customers.

You see, the Moment of Truth is in clearing the important power-up hurdle. Many equipment circuitry problems reflect back in the power supply area. In my experience, it is the first stage that must be confirmed as 100% operational. A shorted capacitor, internally defective tube, or wiring error/defect buried ‘downstream’ will result in an excessive power load, indicated by lower than expected supply voltages or, in the case of tube rectifiers, plates glowing red hot. Red hot plates provide a ready, albeit worrisome, indication of excessive current draw which, if left unchecked, will result in a soon-dead tube.

In the arsenal of test equipment to accumulate, a metered Variac should be at the top of your list. These can be found used and viable on eBay or Amateur Radio Hamfests. If you enjoy homebrewing, you might consider building your own customized version as the components needed are readily available and construction is straightforward. Much information exists on YouTube on how it’s best done…besides, building electronic gear is fun!

Keep in mind a Variac autotransformer provides no electrical isolation. For projects lacking an internal power transformer, always install a true 120:120V isolation transformer between your Variac and the unit to be tested. NEVER plug in a newly restored unit, hit the power switch, and hope for a good result. Doing so risks blown fuses, at best, or the worst: exploding components, clouds of smoke, and permanently damaged ‘unobtanium’ such as transformers and filter chokes.

Next, we’ll discuss how to make your restoration project look fantastic!



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