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Electronic Equipment Restoration Part 2: Gaining Project Familiarity

Let’s assume for the moment that you have decided to restore an amateur radio receiver such as a Collins 75S-series unit. If so, that would have been a great candidate for a host of reasons, first being the Collins amateur gear is well supported by many enthusiasts. For example, try visiting the Collins Collectors Association website (www.collinsradio.org). There you will find specifications, 'hints and kinks' and downloadable manuals for a wide range of Collins equipment and, as a member, much information via on-line forums and past issues of the Association’s Signal Magazine. Numerous suppliers of Collins spare parts exist on eBay, through the Collectors Association and, my go-to supplier: Surplus Sales of Nebraska.


The next step, once your project is on the workbench, is to remove the case and begin a detailed examination. And, have your camera ready. First, look for parts that are obviously missing…meaning empty tube sockets, wires dangling in space, likely spots where a part/parts used to reside, missing Twist-Lok style electrolytic capacitors, and cosmetic parts such as knobs, escutcheons etc.


Photo these examination findings and make notes. Keep in mind that a restoration project takes time to complete. You may not (and likely won’t) have replacement parts on-hand to complete the project in a few days. Most restorations are measured in weeks and, for more complex projects, months or longer. No matter, you won’t remember the fine details (any of which will conspire to derail efforts) and your normal forgetfulness will ignite blood pressure rise and consternation. Photos have saved many a restorer’s sanity during the important project re-assembly phase.


Next, flip the radio over to expose its under-chassis wiring and compare what you see to the equipment’s instruction manual and schematic diagrams. The image you see leading into this Part #2 session is the underside of a recently restored and somewhat rare Collins 75S-2 receiver. While it may look complicated at first blush, it is a very straightforward design and easy to restore. And, when restored, did I mention it sounds and works great?


I usually start work with a visual inspection in the gear’s power supply section as this is where the biggest heat load and dreaded electrolytic filter capacitors reside. I next work my way, stage-by-stage, to the end which, in the case of a shortwave receiver, is its antenna jack. Here again, look for the obvious:


· Cracked or over-heated resistors. Carbon-style resistors are known to change value when substantially over heated. These resistors are normally a rich brown in color. But if there is an area in the middle that had turned far more black than brown then suspect overheating. If broken in half, well---that is indicative of a downstream component failure such as a shorted capacitor or internally defective tube.


· Damaged electrolytic capacitors. These components have a pressure release ‘valve’ that vents if the internal pressure within the part becomes excessive. How, you ask? As the oxide forming electrolytic paste within the capacitor ages and dries out, leakage current between the plates increases. This leakage current generates heat, which hastens the dry out of the electrolytic paste. Soon, a capacitive death spiral begins and eventually a load of nasty smelling ‘goop’ escapes through the pressure release vent. Normally this aging process happens slowly over time (years unfortunately yet if one was to power up a long-dormant unit, the result can be somewhat explosive. A story about that comes later in the series!


· Burned or charred wiring. Again, an indication of a problem needing resolution. If this burned wiring involves a power transformer, tears will soon follow as a costly replacement is in the cards.


· Broken rotary switch wafers. This can indicate a few things: perhaps the unit was carelessly handled; a prior technician could have botched a repair; or the switch simply bound up (lack of lubrication) and broke. These problems can sometimes be repaired using parts from ‘donor’ switches of the same manufacturer/type. This is where a good junk box or network of like-minded collectors/restorers can come to your aid.


· Damaged or Missing Tubes. Since tubes are plugged into individual sockets, one missing is rather obvious to spot. But tubes can be damaged internally and those are sorted out later in the tube-testing phase. What you should be on the lookout for, as part of the visual inspection, is any tubes that have a cloudy white internal appearance.


When tubes are manufactured, within the glass envelope is included a material called a ‘getter’ whose job is to absorb errant gas emitted by hot tube elements. The getter is activated in the tube's manufacturing process, after its glass envelope has been evacuated. Getters have a bright chrome-like finish visible either at the top or a side of the envelope. While the getter’s principal purpose is to keep the tube clear of gas impurities as emitted from heated elements, it also works to resolve minor vacuum leaks. A tube that has major air leaks and is unusable can be quickly spotted by its getter’s milky white versus chrome-like appearance. If you see a milky white tube, it has become a 'Silent Key'. Toss it and make a note to replace.


Just a reminder…..don’t start diving in and “fixing stuff” just yet. The objective here is to gain a sense for how the manufacturer designed and built the device. As you are doing this familiarization, begin to associate component parts with the schematic diagram. If you were driving a car in an unfamiliar city, wouldn’t you check out a map first? Same applies with equipment restoration. (Oh, I know someone will point out that maps are obsolete now that we have GPS. Well, pal, there is no GPS for what your about to undertake…stick to map er schematic reading.)


Keep a running tab of what components are visually suspect and plan to electrically test them later. Pay particular attention to how parts are installed, how component lead and switch/socket wiring is dressed, and, most importantly, what a correct solder joint is supposed to look like. High-end military equipment manufacturers such as Collins Radio, the Technical Materiel Corporation, and others built first-class gear. Their soldering workmanship is gold-star excellent. Your repair workmanship should rise to that level.

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Let’s take a side trip and discuss the art of soldering. Most amateurs in my era learned basic soldering skills while assembling Heathkit products. Although Heathkit as we knew it then is no longer around, their published instructions on soldering and examples of sound soldering techniques live on thanks to the Internet. Simply do a Google search and you’ll find detailed courses and illustrations by Heathkit on soldering. The illustrations are particularly important as they depict both good and bad solder joints and provide tips on becoming a pro. If you want to learn about soldering printed circuit boards and precision connectors to the Nth degree, then check out the various NASA publications on the subject.


To get good soldering results you need good tools. For printed circuit and point-to-point component wiring, I use temperature controlled Weller WTCPS pencil-type solder stations. For heavy duty soldering, such as for soldering FP/Twist-Lok capacitor grounding tabs to a chassis or installing PL-259 connectors onto LMR-400 coaxial cable, I use a Weller 45-watt soldering iron with a 900-degree chisel tip. Plus, you’ll need some good quality 60-40 rosin-core solder. I suggest Kester solder although others are fine. Oh, skip the water-soluble resin/lead-free stuff. It really isn’t great for what we do.


Of course, before doing any replacement part soldering you first have to get the old parts out. Here, you’ll need an assortment of desoldering tools. First on the list is solder-wick, which is a rosin-infused copper braid product used to remove solder from wire terminals, sockets, etc. It comes in a variety of sizes but get one that is at least 0.1” in width as that seems to work best for vintage gear. You’ll also need a solder vacuum pump…termed a variety of names such as Solder-Pull-it, Solder Sucker, etc. Make sure to buy a couple of replacement tips as they take a beating over time.

My all-time favorite solder removing tool is the Hakko FR-301. It is fantastic for removing through-hole parts in circuit boards. Pull the trigger once and the subject solder joint seemingly disappears. It’s an amazing tool.


While one can read about how to solder/desolder components, it remains a two-handed skill that must be acquired through actual practice. No one was ever born with a soldering iron in-hand. Yet, when I had an equipment manufacturing company years ago, we'd have teenagers become soldering whizzes in short order. You can, too. Don’t be discouraged if your first few solder joints look a bit rough, they’ll improve with practice, coupled with a critical eye. Think about why the joints you make aren’t in the same league as the Heathkit or NASA illustrations and make changes to your approach.

An important aspect about soldering is to briefly pre-heat the joint with the iron and then, after a second or so, gently apply the solder. Apply just enough solder to wet the joint…it isn’t necessary to flood it and create a solder blob. Blobs tend to become what are termed “cold” solder joints or ones that have high internal resistance. If the joint is grainy in appearance (a characteristic of a cold joint), try reheating it and allow the solder to reflow.


Once these initial steps are completed, you should have a good general understanding of the project’s general layout, knowledge of what components are highly suspect and will likely need replacing and will have developed the beginning of a replacement parts list. Let’s next move on and discuss how to safely clean your project/radio, electrically test its tubes, and get the unit ready for component replacements.


73 Nick




CONTACT

Let’s Work Together

75757 Highway 1082

Covington, LA 70435

nick.tusa@ce-multiphase.com

Tel: 504-400-8873

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