Of course, once a much sought after piece of electronic equipment is first acquired, there is some small voice in the back of one’s mind screaming urgency in getting it working again. It’s kind of like acquiring a new mystery book in an established series…and wanting badly to skip to the back pages to get a jump on the conclusion.
A word to the wise: Don’t do that. While you might think you’ve found the key to solving the mystery, there could be an unwelcome, well-hidden surprise waiting in the wings!
The hallmark of an excellent electronic equipment restoration is the combination of clean visual cosmetics, coupled with proper and reliable electrical operation. When your project was first designed, the manufacturer was no doubt proud of its workmanship. They were not anticipating any need for a ‘30-second/30-foot’ type warranty where the new owner is stuck with a true lemon. Now, as a restorer, you are taking on the role of remanufacturer-owner…..and if you’re like me, you don’t want to ‘fix’ your prize again…and again…and again. Right?
So, if you’ve been following along, we’re now ready – with confidence – to get to the nitty gritty.
Cleaning the equipment’s chassis, sockets and various connectors is the next logical step. What to use depends on the equipment itself. If the chassis is formed from aluminum sheeting, then a soft bristle brush combined with a mixture of Simple Green, a dash of ammonia and some distilled water will do the trick.
Begin with a small, easily accessible area and, while using a small brush, gently wet and work through the dirt and grime. To get into the many small crevices, try using wooden handled cotton-tipped swabs. These gems are quite inexpensive when bought in 100-count lots from eBay, etc.. Buy several hundred at a time --- you’ll go through them in short order. Quickly, dry your work with a soft, clean towel to gauge progress. Work through the entire chassis in this fashion…a small step at a time. Bear in mind, it is unlikely that the chassis will be fully cleaned to your liking after this first pass, so expect to repeat the process at least once.
Now, here’s what not to do while undertaking this first stage of cleaning: Do not wet the various tube/connector socket parts and steer clear of any rotary switch wafers or potentiometers. We have other cleaning strategies for those.
Next, examine the set’s wiring harness and gently clean any noticeable areas of accumulated dirt or oily residue. If your set is wired with cloth covered hookup wiring, try using a different cleaning mixture consisting of 40% lacquer thinner and 60% isopropyl alcohol. When applied to the wiring via a small brush, the dirt and crud will easily (surprisingly) melt away. Again, dry your work as you go with a soft, clean cloth.
Now, with the chassis thoroughly cleaned, let’s next focus on tube sockets. Here, you’ll need a squeeze bottle of CAIG DeOxit D-100 for the metal contact surfaces in tube sockets and rotary switch wafers. I prefer using the needle-dispenser container type versus either brush or spray-on versions. By using the needle dispenser it’s possible to place a tiny drop of the chemical directly onto each socket’s pin receptacles.
Once a tube socket is treated as described, the next step is to gently work the chemical into each pin and, by so doing, break up any surface corrosion. How, you ask?
A useful hack involves a trip to the local grocery. In the kitchen supply section, you’ll find bamboo shish kabob spears. These are hard wooden sticks approximately 10 inches long, 3/16” in diameter and with one end fashioned into a fine point. Place the pointed end into each tube pin receptacle, pin by pin, and gently twist the stick clockwise/counterclockwise a few times. The combination of the DeOxit and the rotary action of the pointed stick does a great job in cleaning the small contact fingers, thereby assuring a good, low resistance connection with each tube’s pins. You’d be surprised how many operational problems are caused by dirty or poorly seated tube pins.
Now, let’s move onto the rotary switch wafers. Wafer contact fingers and wipers are usually made of silver-coated beryllium copper along with some sort of light lubricant. Normal operation of rotary switches is generally sufficient to keep contact surfaces clean. Yet, after many years of non-use, the silver surfaces will usually have oxidized and are black in color. This, in itself is not a problem as silver oxide is a conductor, however, the applied switch lubricant gradually attracts dust and dirt…which translates into high-resistance contact surfaces. In a receiver, for example, this is manifested by noisy or intermittent operation, loss of sensitivity or complete receiver failure.
To do an effective switch job cleaning, start with a cotton swab soaked in isopropyl alcohol. Gently wipe the various contact surfaces on each wafer and notice the amount of residue removed. In cases of heavy dirt accumulation, a couple of swabbing attempts may be necessary.
Once the surfaces are reasonably clean, apply a small amount of DeOxit to a clean swab and gently apply it to the various contact areas. Pay particular attention to the small switch wiper(s). It is this little contact area that is the principal culprit for poor switch reliability. Make sure each is clean but be doubly sure not to damage the switch’s various contacts. Rotary switches used in low-level circuitry are rather fragile and very difficult to replace.
If you plan to ignore my advice and use the spray-on version of DeOxit, DO NOT soak any phenolic switch wafers with the stuff, thinking more is better. A saturated switch wafer can cause contact-to-contact signal leakage and is often a recipe for killing an otherwise usable switch. If your switch has been DeOxit drenched, try applying a liberal amount of lacquer thinner to the switch and gently wiping off the residue. This little hack can reverse the mistake….sometimes….
Finally, we deal with the tubes. Earlier, you closely inspected your set’s tubes and tossed the obviously broken or leak-damaged ones (where the normal silver getter has turned a visually milky-white color). Now, we need to test them electronically.
The best type of tube tester is one that tests for transconductance (Gm), or the tube’s ability to provide amplification. Simple emission tube testers only verify that the tube’s heater and cathode are functional. Yet, while a tube can have good emission, it can still have degraded transconductance due to physical changes in tube geometry (caused by localized over heating of elements) or internal part contamination by gas or other means. Your tube tester will have either a booklet or roll-chart that depicts the various element switch settings needed to test your tubes. Just match the tube type number with the info in the chart.
Often, tubes will have multiple tubes in one envelop…such as dual or triple triode sections, or triode-pentode sections, or dual pentode sections, or some combination of triode and diode elements. High-end tube testers can often test dual triode tubes having identical section configurations with the press of one button. Examples of such tubes are 12AT7, 12AU7 etc. This dual-test feature saves time when many tubes of the same type are to be tested. Should a tube’s test results indicate Gm below the listed minimum value, replace with a complaint tube.
People are often astonished that electron tubes are readily available today from numerous on-line sources. Keep in mind, many millions per month were manufactured during the receiving tube’s prime use period. WW-II was where an astronomical number of tubes were produced and, of course, the military branches bought huge numbers of tubes for its test equipment and communications gear up until around 1990. Tubes are plentiful, reasonably priced and are far easier to source than many of yesterday’s solid-state parts.
Now, I get to pick on my Ham Radio brethren a bit. It seems common practice for Hams to buy a replacement tube or two for a project….which is good….but will then place the old worn out tube into the new tube’s box and keep the dud for future use. WHY? The old tube isn’t going to magically heal itself and a new tube box has no restorative powers. Just toss it!
Once tube testing is complete, use a soft, damp cloth to gently clean the tube’s envelope of dirt, dust etc. Dirt and dust inhibit the tube’s radiant cooling process and heat is the number one enemy of tubes. Odd, isn’t it? Tubes need heated filaments to function, yet overheated tubes due to excessive plate dissipation or higher than designed filament/plate voltages silently kill tubes. The US military and FAA have each conducted many studies on tube life versus tube envelope temperature. Their findings show that excessive heat quickly kills tubes and that tubes whose bulb temperature is held below 250 degrees live relatively long lives.
These various life-test studies spawned development of specialized tubes shields that help dissipate heat and lower bulb temperature more readily. So, what can you do to lengthen tube life for your equipment? Here are some suggestions:
Monitor your location’s AC line voltage. Radios and electronic test gear made up to and including the mid-1950s was designed for 110VAC electric utility standards, yet line voltage standards have evolved where the target today is 120VAC while some areas can experience levels as high as 127VAC. Excessive line voltage correspondingly elevates tube filament voltage and rectified DC plate voltage…in combination, these elevated levels equate to far lower tube life and related circuitry failures. Consider installing a buck-boost transformer or autotransformer to lower applied line voltage to that which your equipment was designed.
Do away with shiny tube shields. The common shiny, nickel plated tube shield is a death sentence for tubes. The tube’s radiated heat is merely reflected back into the tube…slowly cooking it into oblivion. Back when tubes were in major production and their purchase costs to equipment manufacturers were mere pennies, prematurely dead tubes were no big deal. That was then...this is now. Ideally, toss these tube killers unless the circuit demands shielding (i.e., mixer stage or oscillator) to minimize signal leakage.
Install IERC tube shields. These special military-approved tube shields are internally and externally black in color and have internal contact fingers that firmly grip the tube’s envelope. By so doing, heat is not reflected back into the tube but is more efficiently conducted to the local ambient environment. Check eBay or Hamfests for IERC tubes shields. These shields are available for all common-sized tube envelopes.
Install fan cooling, where possible. Improved air flow around tubes expedites the removal of heat. Small, low-cost muffin fans are ideally suited to equipment cooling needs. Better yet, add both fan cooling and IERC shields when possible.
We’ve covered a lot of ground in this segment. And, while your project is starting to look better, we’re far from done. So where to go next? Let’s replace some bad and soon-to-be-bad parts!