John Eng's Dead Tech Rescue - Restoration Idea 5: Sentinel 243-T Farm Radio Restoration

Restoration Idea 5

Sentinel 243-T Farm Radio Restoration

Here are some pictures of various stages and comments in restoring a Sentinel model 243-T large table Farm Radio (1941-42) done by me.
Restored 2011Basic Specifications of Sentinel 243-TType: Battery Operated AM/SW Superheterodyne Receiver Frequency Range: AM: 540-1750 KHz SW: 5.75-18.3 MHzPower: 90 VDC @ 12.5 mA B Battery and 1.5 VDC @ 30 mA A Battery Using an AB Combo Radio Battery Tubes: 1A7GT Osc/Mod, 2 X 1N5GT IF Amp, 1H5GT DET/AVC/AF Amp and 3Q5GT Power AmpFeatures: Tone Control, Battery Saver Mode*, Independent Dial Light Circuit and On-Off IndicatorSpeaker: 6 inch Permanent Magnet * Drops B Battery use to 7.5 mA a 40% savings (but lower volume). Some may ask that the biggest power drain comes from the filament of the tubes which use an A Battery so why not do that. The reason that in this radio if you cut the filament voltage say 10%, the 1A7GT oscillator/modulator tube will not be able to work at shortwave frequencies. I tried this and the SW section barely works.

NOTE some steps omitted for space:

1. Remove chassis from radio cabinet to inspect condition. I removed all paper capacitors. They are the main reason other than tubes why vintage radio fails because over time the wax used to keep moisture out, dry out and crack. I also check if the resistors are still in tolerance. Closeup of paper capacitors and vintageresistors. I try to save the best ones for nostalgia and may sell some of the paper capacitors because there are collectors who like to take out they old "guts" and install modern parts inside but it looks old. I do this only for the larger electrolytic capacitors and any part that is easily seen.

2. Inspect coils for continuity and check tubes. The photo on the left is the antenna coil, I found that one of the windings was broken due to a previous repair job. I had to remove two turns of wire and carefully soldered the broken winding. Look carefully at the next picture on the right. If you look at the darker coil toward the bottom, you'll see a small shiny spot in the center, that's the soldered joint. The loss of a couple of turns reduces the signal coupling to the converter tube but it's still a lot better than no coupling.

3. Replacing bad tubes below. The chassis has some rust. When everything is done I will try to remove it with NAVAL rust remover. I also replaced any frayed or damage wires. In older radios they used cotton or pure rubber wires which decompose with age or eaten by insects and rats. This radio had a lot of damaged wires.

4. Repairing Speaker:
After I check the speaker electrically (try using a 1.5 V battery and place it across the voice coil and hear a click sound), I check for cracks and tears because the paper used often gets brittle with age. School glue and construction paper makes a good patch. If you can't match the color, lightly spray paint the cone. I add a preservative like "Armor-All" to keep the paper from drying out and stop the decay.

5. Lubricate controls and clean controls. Make sure you use the right product for the application. You don't use WD-40 on a switch contact for example.

6. Turn the radio on and check for for problems and do alignment. This is a farm radio and runs on 90 VDC and 1.5 VDC so I need a power supply or battery eliminator. Years ago I built one and I bring up the voltage slowly to prevent damage or stress to old parts. Alignment is like a radio tune up, I can do it by ear for initial check (requires experience because you can really mess up ,if you don't know what you are doing) but better results require equipment like a signal generator and oscilloscope and set of instructions.

7. Cabinet Parts: Dial Escutcheon and Glass
Many times a plastic escutcheon or frame is used around the dial glass, deforms over time as the plastic looses its form (out gassing). I use a heat gun or hair dryer and carefully heat the plastic to reform it. BE CAREFUL and BE PATIENT, if you put too much heat you will permanently deform it. The dial glass in many older sets were reversed painted, MAKE SURE you do not use glass cleaner or any grease remover as this may remove the paint from the glass. Handle this with extreme care, I have ruin a dial glass in the pass and was lucky to get a replacement.

8. The Cabinet. Wood cabinets have one advantage over plastic, they can be repaired and refinished. However I can still mess up and make it worse than it was before I started to work on it. This cabinet was in pretty good shape. Some vernier was missing so I try to fit and match it (you can buy this at better hardware stores), Usually the old finish needs to be stripped off. You can use most paint or varnish removers found at the "do it yourself" store. It is important to use the proper sandpaper / steel wool product to prep the cabinet. If you sand too hard you can ruin the veneers (as I have done).

As you see above, you may notice that have stained part of the cabinet with a dark stain. In the past cabinet makers often used dyed lacquer or tint to accent parts of the cabinet. Such items are rare or no longer available. I learned from trial and error to find the appropriate stain or spray paint to try and duplicate what was done in 1941. Below is the radio being prep for spray finish.

9. In 1941 the final finish was sprayed on with a lacquer or shellac. I decided to brush on an acrylic finish on the top for a smooth tough finish and spray the front and sides with a acrylic satin finish for a soft look. Here is a view with the spray finish done and cabinet parts installed.Below are some examples of cabinet parts installed:

10. Wanted to make the radio run as a farm set so I took an old battery similar to what was used and made it into a battery case with replaceable alkaline batteries.
Step by step transformation below.

Reversed Painted Glass Dial with only the dial lights on. Brings back memories!!!!

11. Restored Sentinel 243-T Radio

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