Hello and welcome to the Retro Radio Shop repair bench!
Bringing your Retro, Vintage or Antique Tube Radio beauty back to life requires some specialized equipment. The following scenario details a typical job that presents itself and should provide some information on the equipment and process that will be used when we have yours on the bench.
Today, a Northern Electric 5508, or “Bullet” as collectors call it, came into the shop not functioning and needing a cabinet restoration. The cabinet is very rough with two layers of, what I’m sure is heavily lead laden, paint that has been applied over the original red hammered finish. We will get to that later but first let’s get the set electrically working strong.
While it can be tempting to dive right in and attempt diagnostics, it’s best to have a systematic approach that is applied to each repair job. Often the issue presents itself during the work flow process.
The first step was to removed the chassis from the cabinet, store the screws and knobs securely and then give the chassis a good cleaning. This one, as you can see, has many years of dust bunnies throughout. Using compressed air, an auto detailers fine brush, towelettes and Q tips, the cleaning begins.
Once the cleaning is finished, the volume potentiometer shaft gets a light lubrication and the inside gets deoxidized with a special agent. The tuning condenser receives a light lubrication and test for shorting of plates, malignment or degradation of mounting grommets. The dial string and lamps, if applicable, are examined at this time.
Next, all the tube sockets receive a deoxidization and the tubes are tested. In this set, the 12BE6 failed on the old trusty Knight tester. We have many in stock so replace it with a new one. All others tested well.
The next step is to reference the schematic from our database and replace all the wax/paper capacitors and the electrolytic filter condensers. These often degrade over time and fail. They are relatively inexpensive and all get upgraded.
Then all resistors are checked for drift or open status. Typically anything approaching 20% disparity is replaced. All of these were under 10% variance.
Solder connections and wire condition is now carefully examined. Nothing incongruous was obvious at this point.
Now it was time to light the set up and see what happens. After slowly bringing up the power using a variac, the set emits a slight low hiss with no stations. Ok, well, time to dig a bit deeper.
Using a frequency generator, a 455khz signal was injected into the sets front end. Voila, the tone made by the generator booms through the speaker. The audio amplifier section and IF transformers appear to be working. I suspect now that the local oscillator has quit running so focus on this area.
Using a signal tracer, one very weak station is heard on the grid or input of the 12BA6 Tube but nothing is heard on the plate or output. This is a strong local station but none others are heard.
It’s at this point the oscillator coil windings are checked with a dvm. Aha! One of the coils is open. After close examination, one of the very fine hair like wires had broken loose from its lug. They are so tiny it was easily missed visually. After bridging the wire, the set came back to life!
Ok, now that it’s singing again, time to finish the servicing. Using the RF signal generator the radio is aligned or “Tuned” as per the published procedure with the schematics.
This radio is to receive a Bluetooth module so a 3.5 mm aux input cable is wired into the set by audio summing resistors. A micro switch is installed to toggle from radio to Bluetooth and vice versa.
After reassembly, the radio is played for a number of hours to ensure stability. I listened to this one while stripping and prepping the cabinet for new custom finish as per request. Not the scheme I would go with but the customer is always right!