Restoring the Freshman Masterpiece TRF Radio

I mentioned finding two old 20’s-era broadcast receivers in an earlier post. Some months had passed, and restoring the Freshman Masterpiece “5F5” seemed like a fun project and a quick win. The radio was designed to run on two batteries: an “A” or 6-volt battery for the tube filaments and a “B” or 90-volt battery for the plate voltage. These batteries, especially the “B,” haven’t been made in decades and it was my intention to use two modern DC-DC converters and 12-volt gel cell to power the radio. I’ll talk more about that in a separate post, but for the time being it was simpler to use a 6-volt gel cell and ten 9-volt batteries stacked together to get the radio running.

Initially, the radio looked mostly complete, with a couple of crucial omissions: one of the five tubes was missing, the knobs for the filament rheostats were gone, and the “power switch” was incomplete. I got on eBay and found many UX-200A tubes available and soon had two in good used  condition on their way.

The knobs posed a little different problem however. They needed to fit 3/16-inch shafts — uncommon in a world that standardized on 1/4-inch shafts a long time ago. I have a pretty well stocked junk box, and a rummage through it produced two matching knobs intended for 3/16-inch shafts. What are the odds? They were in a bag marked 75 cents — I must have picked them up in a hamfest  years ago.

The “switch” was a different matter. I wasn’t able to find a picture that showed the original part, but the intent was clear: a brass rod through a 3/16-inch fitting in the front panel with a bulge on one end and a knob on the other. When pulled out, the bulge would short two springy metal contacts in the radio’s interior to connect the “A” battery and the tube filaments. I managed to fabricate a reasonable substitute using a brass #8 bolt, two acorn nuts, a regular nut and a grommet. The grommet and regular nut were threaded on the #8 bolt and pushed through the fitting in the front panel. I then drilled a hole through the closed end of the one of the acorn nuts, soldered it facing the other acorn nut and tightened them on the other end of the bolt with a little Loctite. Not original, but it will suffice until I can have something  turned on a lathe.

Radios of this era use “bayonet” tubes with sockets that press on the bottom of the tube pins rather than grip them. The “bayonet” pin on the side holds the tube  in place. They don’t always make a positive contact, and any corrosion is a problem so I spent some time buffing the socket contacts. Instead of  enclosed volume controls, the radio uses open rheostats to control volume by changing the filament voltage. The rheostats were cleaned with DeoxIT. Cleaning the top to remove dusty dirt on the chassis and variable tuning capacitors was next. I brushed the capacitors  gently with a brass bristle brush to make sure that nothing could short the plates.

One other unique component of these radios is the “grid-leak” resistor. Usually accessible on top of the chassis to facilitate replacement, these resistors often had a glass barrel (like a modern fuse) and often as not had an adjustment — made by moving an element back and forth from one end of the glass barrel. It is possible to find replacements for these fragile resistors, but I was pleased to see that the one in this radio, although cracked, was still functional. The radio won’t work without it!

The underside of the thick bakelite chassis has relatively few components, but I tested the two capacitors and two interstage audio transformers as well. To my relief, the transformers were both good. Great news since they are difficult to find. One of the capacitors had failed however, reading only a fraction of its marked value. To preserve the “originality” of the radio, I simply added a new capacitor across it. Ohmmeter tests didn’t reveal any other problems.

I had a total of eight tubes. The four originals, two I had obtained from a friend, and two others found on eBay. Surprisingly, the tube charts in my Precision 612 tube tester didn’t list the 4-pin tubes. This triode comes in many versions and many manufacturers made them. Each used different nomenclature since tube types had not yet been standardized. I had an assortment of RCA (UX-201A), Cunningham (CX-301A), and OK (OK201-A) tubes. All are interchangeable, but some collectors favor the Cunningham in the radio’s “detector” position. With help from several Internet articles, I was finally able to make an educated guess on the correct tester settings and picked the best five tubes for use in the radio. Two others were completely flat, and the last one had normal emission, but seemed to have loose internal connections.

The case had been cleaned and refinished by the previous owner, so not much work was needed. The plastic windows that show the dial setting had yellowed and clouded though. Cutting replacements out of thin Lexan sheet wasn’t difficult. After cutting the plastic, the white protective film is removed to reveal a beautiful,  clear window.

One final problem. Radios of this era used high-impedance speakers — something on the order of 600 to 2000 ohms instead of the 4 to 8 ohm speakers we use today. I looked for a speaker on eBay, but they are expensive. The horn speakers of the era  are often used as decor items, and working ones are rare. Working paper-coned speakers from the era are also rare because they were made of wood, cloth and paper and didn’t survive the decades. A modern 8-ohm speaker can be used with a tube output transformer but they are in demand by guitar and stereo amp hobbyists and therefore expensive. Then I remembered that transformers used for 25 and 70-volt line public address systems might work. Everyone has heard these systems — they’re playing music and announcements in retail stores, airports and other public places. The transformers used in these systems are inexpensive and have multiple taps for wattage adjustment  in each speaker zone. One winding connects to the speaker while the other is labelled in wattage.  An article on distributed speaker systems lists typical transformer impedances and this 10-watt transformer from Amazon would work fine. While I was shopping, I also found a 3-inch speaker that looked promising. I have plans to build it into a period appropriate enclosure — more on that in a future post as well.

Finally time for a test. About 100-feet of wire for an aerial and a ground connection are needed. I hooked up the batteries and slowly turned up the rheostats. A beautiful orange glow appeared  in the tubes. It is a little tricky to find radio stations on a Tuned Radio Frequency (TRF) device since each of the three knobs must be tuned onto the correct frequency separately. It’s also complicated for me since we are rural dwellers and there aren’t many AM radio stations nearby. My daytime tests did produce results however, with KLBJ being clearly audible. There were some squeals, another common problem when one of more of the stages goes into oscillation. Turning the filament voltage down a little will often help. I also added a couple of additional bypass capacitors to the radio and that helped as well.

Nighttime listening is the most fun though. After sundown, the ionized layers of the earth’s atmosphere rise and more distant stations can be heard. That first night, I heard stations in New Orleans, Denver, Tulsa and others that I couldn’t identify. I don’t listen to the AM band very often, but there is something about hearing distant stations on a nearly 100-year-old radio late at night that re-kindles the excitement I once felt as a child experimenter. Magic!