When purchasing a classic receiver or transmitter, unless you absolutely know assume the radio will need work. Often you can get a top-of-the-line radio needing a bit of repair or clean-up inexpensively. Don’t worry these radios were designed to be repaired by their owners and curiously, except for cosmetic parts such as cabinets and knobs, parts are much easier to find for 60-year old radios than a 20-year old imported transceiver! Chances are the radio has gone for years without use. Even if it has been recently used, don’t completely trust components that might be 60 or more years old. Don’t start by plugging in your new acquisition! To do so might damage a hard-to-replace power transformer, or cause a fire. Instead, if the radio didn’t come with its owner’s manual, get one. Armed with the manual, remove the radio from its cabinet. You very likely will find evidence of unsightly repairs, modifications, or even dangling wires. While modifications aren’t necessarily bad, they can certainly add some drama to any necessary subsequent troubleshooting. It’s up to you to reverse or remove them.
Correct any obvious problems such as dangling components. Replace the line cord with a 3-wire, grounded plug for safety. If the radio is one with a “live” chassis, you should operate it from an isolation transformer for safety. It’s also a good idea to add a fuse, if the radio doesn’t originally have one. Are we ready to give it the “smoke test”? Not so fast! “RECAPPING” Obviously, aged components deteriorate, and capacitors are particularly prone to developing leakage or short-circuits with age. There are as many opinions on capacitor replacement as there are radio collectors, but at the very least you should replace the electrolytic filter capacitors. Here’s why: they will short circuit sometime, and when they do, they’ll probably take the rectifier tube and the power transformer with them. Modern high voltage electrolytic capacitors are reliable and much smaller than their classic counterparts. You can mount the new capacitors under the chassis by mounting a new terminal strip (do not just wire them to the old capacitor terminals), you can re-stuff the old capacitor’s can with new capacitors, or you can buy a new can from places such as hamfest. In any event, follow the manufacturer’s schematic — don’t assume that the “–” (minus) end of the capacitor goes to ground, as in some radios the ground path is through a resistor so as to develop bias for the audio output stage or RF gain circuit. Observe the polarity or you’ll soon be cleaning up a stinky mess! Old paper-wax and black plastic tubular capacitors should also be replaced. Again, a short circuit in one of them could take out other components, too. Modern film capacitors of the appropriate voltage are great replacements. Opinions vary as to whether all should be replaced, but replacements are cheap and you have the radio apart now, so why not?
REMOVING AND REPLACING COMPONENTS
Replacing capacitors and/or other components isn’t difficult, unless they are buried under other components. The Hallicrafters SX-28 and SX-42 receivers are examples of receivers that have extremely difficult to reach components. Like the “re-capping” question, there are different schools of thought on the “proper” component replacement method. You can use solder wick and/or a desoldering tool to remove the solder from a terminal, unwrap the wires, and install the new component by wrapping the lead around the terminal and soldering it securely. The proponents of this method point out that this is the preferred military and commercial method. I find it often will needlessly damage other components such as tube sockets and create solder droplets inside of the radio. Back in the day, radio repairmen clipped out a component leaving a short stub of wire, made little coils in the new lead, then soldered the coiled lead to the old stub. This is a much faster, easier and neater method.
Powering Up the Equipment
Get out your volt-ohm meter and measure the resistance from the B+ line to ground. Filter capacitors will cause a initial low-resistance reading that increases as the capacitors charge. If the resistance stays low or does not increase beyond tens of kΩ, find the short circuit before you proceed. Now it’s time to plug in the radio. It’s best to use a variable transformer such as a Variac and ramp up the voltage slowly, or use a “dim bulb tester” (a 100-W light-bulb wired in series with one leg of the ac power). Turn on the radio, and watch for any sparking, flashing or a red glow from the plates of the rectifier tube, or smoke. If any of these occur, immediately remove power and correct the problem. Observe that the tube filaments should light (although you won’t see the glow from metal tubes, you should be able to feel them warm up). Again, any tubes that fail to light should be replaced before you continue. Now hook up a speaker and antenna, and test the radio. With any luck you’ll be greeted by a perfectly-performing radio. Seldom, however, is that the case. You may encounter any number of problems at this point. Dirty bandswitches and other controls manifest themselves by intermittently cutting out; they can be cleaned by DeOxit contact cleaner applied with a cotton swab (don’t spray the switch directly!). Scratchy volume or RF gain controls can be cleaned with some DeOxit; in some cases you might need to remove the control and uncrimp the cover to reveal the carbon element inside.If a receiver is totally dead at this point but the filaments and dial lights are lit, double- check to see that the “Receive-Standby” switch is in the receive position, and any battery plug or standby switch jumpers (as described in the manual) are in their correct place. Although comprehensive troubleshooting is covered elsewhere in this chapter, the next step is comparing voltages with those stated in the user manual. If the manual doesn’t have a voltage table denoting the expected voltage at each tube pin, expect between 200- 350 V at the tube plate terminals, a few volts at the cathode (unless it’s directly grounded), 70-200 V at the screen, and slightly negative voltage at the grid. If you’re faced with this situation and a newcomer to troubleshooting vintage gear, help can be found at forums that cater to boat-anchors and/or vintage radio repair and restoration.
Over the years hams have been cautioned that alignment is usually the last thing that should be attempted to repair a radio. In general this is true — but it’s also a certainty that a 50 year old radio will need alignment in order for it to perform at its best. In any case, replace the capacitors and any other faulty components before you attempt alignment it’ll never be right if it still has bad parts! You’ll need a good signal generator and a volt-ohm-meter or oscilloscope. Follow the manufacturer’s instructions, and with care you’ll be rewarded with a radio that performs as good as it did when it was new. Additional information on alignment can be found in the section “Testing Within A Stage.”