Now, we embark on the fun stuff: component part renewals! Most folks immediately assume all a restoration takes is replacement of a few tubes, hit the Big Switch, and enjoy a wonderfully working piece of electronic gear. Yet, if you were paying close attention, I’d bet that the majority of tubes tested as described in Part #3 passed with flying colors.
I’ve restored countless pieces of tube-based test gear, amateur radio rigs and military radios…in most cases where the codes imprinted on tube envelopes suggest our gridded friends are in the 50-60 year category. Very few were found to be outright failures. Of course, if the equipment is a type that was operated 24/7/365, then you may find some that light up but have no indicated transconductance -- meaning that all of the electrons that could have been boiled off the cathode have ‘left the building’, so to speak.
So, what are the worst failure-prone component culprits in my opinion?? Electrolytic capacitors, closely followed by leaky paper-based coupling capacitors.
Electrolytic Capacitor Problems
Electrolytic capacitors fail in a number of ways. They can fail open, which for a power supply means excessive 120Hz AC hum where filtered DC is the expectation. Worse, when they fail shorted, a defective electrolytic can quickly damage and destroy very expensive power transformers -- closely followed by foul-smelling smoke, arcing or worse…fire. Then there is the in-between failure mode where the capacitor still works…sort of. As the capacitor's electrolyte paste slowly dries out, the insulating oxide layer can break down which causes the capacitor’s internal leakage current to correspondingly increase. This failure gradually loads down the power supply's B+ level and increases the temperature of the capacitor, itself. Left unchecked, thermal runaway could eventually result in an exploded capacitor.
Many vintage electronic devices utilize electrolytic filter capacitors housed in metal cans fastened to the equipment’s chassis. Often these are multiple section units, meaning that the single can actually accommodates two, three or four separate capacitive elements.
These little gems are often referred to by their trade names such as Twist-Lok or FP. Their marketing and engineering value is in compressing the physical space needed as compared to utilizing individual electrolytic capacitor units. These multi-section capacitors, of course, suffer from the same failure modes as their single-value cousins…except they can present a truly sneaky failure mode that some overlook: they can fail between sections! This is not the failure mode most folks anticipate when repairing vintage units, but it occurs often enough to be aware of the possibility.
My opinion of electrolytic capacitors, in the eye of a restorer who wants to restore a unit once and not make a career dealing with it, is to assume all elderly electrolytics should be replaced. Modern equivalent replacements exist that can survive far higher operating temperatures than what was originally installed, with 104-degree C. tolerant parts readily available at modest cost. Replacement, newly manufactured Twist-Lok/FP style capacitors are available from firms such as CE-Distribution, Hayseed Hamfest and others.
New-Old Stock (NOS) capacitors are likewise available from local TV/Radio parts houses (if they’re still hanging on), surplus electronic suppliers and, of course, eBay. If NOS capacitors have been stored in a reasonable environment, it is often possible to reform their oxide dielectric barrier and make them potentially safe to use. The reforming process involves the gradual application of DC voltage across the capacitor, while closely monitoring current.
By gradual, I mean a process that can entail hours, not minutes. For the typical filter capacitor, I will increase the applied reforming voltage in steps until no more than 10 milliamperes of current is indicated. Wait until the indicated leakage current drifts down to near-zero and then increase the applied voltage to the indicated 10ma level. Keep this ‘charge and wait’ cycle going until you have reached the component’s rated service voltage. If you’ve applied reforming voltage and see no immediate current indication, then discard the capacitor. And, if the capacitor shows the desired indicated reforming current but then suddenly starts to sink current at a high rate, likewise send it to the ‘round file’.
I have gone down the reforming route myself (when the part is truly obscure) and have had reasonable success. But, keep in mind, that any NOS capacitor is often an elder statesman. That’s why I suggested that a reformed NOS capacitor is potentially viable….it could just as easily crap out the next day or next year. We can’t peek inside the can, so to speak.
Where possible, choose to install freshly made replacement components.
Oh, one last thought about capacitor reforming. Some have suggested that electrolytic filter capacitors can be reformed, in-place, by using a variable autotransformer to slowly increase the equipment’s applied line voltage and let the power supply do the ‘reforming’ work. This process assumes the capacitor is a 100% viable reforming candidate…but what happens if your capacitor suddenly decides to stop reforming and fail as a hard short-circuit? When in doubt, don’t risk it.
Coupling & Bypass Capacitor Failures
Vintage electronic equipment often employs tubular paper, ceramic disc, and encased mica capacitors for coupling AC signals between stages and blocking DC voltages where appropriate. The early fixed-value capacitors used in broadcast radios and early amateur radio gear were often selected to provide performance at a low cost point. Here, the least expensive coupling capacitor found is the paper type…where rolled paper and foil make up the actual capacitor. The capacitor’s roll is subsequently inserted within a paper tube, wire leads protruding from either end (axial,) and with the two ends sealed with beeswax. Later tubular capacitor versions used a plastic housing in lieu of the paper tube. Both types, though, suffer from gradual moisture migration over many years and the eventual degradation of the capacitor’s dielectric media.
Whenever a leaky coupling capacitor inadvertently applies a positive DC bias voltage to the next stage’s tube grid, the result is often a tube no longer operating as an oscillator or amplifier but as an electronic switch in the saturated-current mode. Left in that state for prolonged periods, the allied tube is likely to be damaged or contaminated by gas released by its overheated plate and/or grid elements.
Capacitors are used to decouple or bypass AC (RF) energy from power supplies and, more importantly, to prevent unintended feedback into other amplifier stages within the device. A shorted or leaky bypass capacitor will often result in damage to associated components.
While ceramic or mica dielectric coupling and bypass capacitors are extremely reliable as compared to paper counterparts, they can likewise fail. Mica capacitor failures are so infrequent I will often assume they are functional and wait to weed out defective units via powered-up troubleshooting phase. Ceramic capacitors are very reliable, where defects are principally physical and can be often spotted by visual inspection.
Oil Dielectric Capacitors
Perhaps the most trouble-free filter capacitors are those using dielectric oil. These are produced in hermetically sealed metal cans. The capacitor’s two electrical connections are made via special air-tight bushings.
While these oil dielectric capacitors make for an excellent and highly reliable power supply filter (and are often found in high-powered radio transmitters), they are bulky and physically large in size. Oil capacitors made during WW-II and the 1950s utilized a dielectric oil that was later found to be a carcinogen. Since that time, oil capacitors have been manufactured using a safer alternative oil that is highly efficient and has the added bonus of not silently killing us.
Older oil capacitors, should they fail, will do so by leaking oil via the connection bushings. Then as the capacitor’s leakage current increases, so too will the heat gradient within the sealed can. Eventually, as pressure builds, the can might burst in spectacular fashion…spewing hot oil all over the gear. Which is a decided mess to clean up.
A good practice is to occasionally check the running temperature of filter capacitors. Before doing ANY high voltage filter capacitor testing, always short-circuit the capacitor’s terminals and make sure it is fully discharged. Using an infrared heat detector with the equipment de-energized (AND NOT using one’s hand as the temperature probe) is a way to safely evaluate the unit’s operating temperature. If higher than 130-degrees F, start looking for a replacement.
Some types of radio and test equipment manufactured during the 1940s through the mid-1950s could also contain what are termed ‘bathtub’ capacitors. This was a manufacturing practice that allowed for the packaging of multiple AF/RF bypass capacitors within a single rectangular metal can. These units contained a mineral dielectric oil and likewise had connections made via hermetically sealed terminations.
Bathtub capacitors were configured as single, dual, or triple units within one metal container. When used as RF bypass capacitors, each section within a can would typically be either 0.1 or 0.05uf. Single unit types were often low-voltage 20-40uf electrolytic capacitors and were usually found as bypasses in the cathode-end of Class A audio amplifiers.
New bathtub capacitors are the domain of the military market (meaning $$$) and are not generally available to the commercial market; however, many NOS types are available on the surplus scene. My approach though is to rebuild as-found bathtub capacitors by removing the can’s bottom cover (which is merely soft-soldered to the can), removing the old parts, cleaning out the oil, and then ‘re-stuffing’ using small modern axial-lead film capacitors.
Modern film dielectric parts are compact in size and can be readily installed within the old can. By so doing, you can rebuild virtually any bathtub capacitor and maintain the ‘vintage’ look of your equipment. Don’t bother reinstalling the capacitor’s bottom plate as once reinstalled in the gear no one will ever know what you have done – except maybe 50 years from now when your repaired unit is ready for its next restoration!
Let’s move on and tackle the next important set of components that comprise an electronic equipment restoration: Resistors, of course.