Sentinel aka. "Frankenstein" Restoration Project

When I prepared to take the chassis out, I noticed an octal tube base was mounted inside the cabinet next to the speaker. The wires from it went to the radio. Now, that was a very unusual sight. I proceeded to unscrew the tube base from the inside of the cabinet and take the chassis out. The speaker was mounted to the cabinet so I had to unplug the speaker plug from the chassis. Finally, with the cabinet out of the way, I looked under the chassis. This was a very unpleasant sight, obviously this was a "Frankenstein" radio.

I normally plug in a radio and find out what is preventing it from playing, but this one I refused to plug in in the first place. With a mess in the chassis, it had a good chance of catching fire and the circuitury was quite scary. It had some missing tubes and tubes with bad emissions. I replaced them with tubes that have similar characteristics such as replacing the 41 with a 42, and a 6S7 with a 6U7.

The octal tube base that was mounted on the side of the cabinet was for the rectifier tube and is not original. Crammed on the left half of the chassis where the power supply circuit is located, were several box capacitors, disconnected wires, resistors, and all the other nameless junk a simple power supply does not require. So, the best way I had to overcome this was to rip everything out of there and rebuild the power supply. Picture of the junk pile from the power supply below.

With all the junk out of the way, the hole in the chassis where the rectifier tube socket was probably supposed to be was replaced with a speaker socket. The speaker is a PM (permanent magnet) type, and the socket was wired to be used for a FC (field coil) type.

I decided to inspect the entire radio before proceeding. The radio circuitury looks original and untouched, except two controls. One control, for some reason, was replaced with a faulty one and is used to control the volume. The other control was connected to the B- and to the ground; because of this, the control would be used to reduce or increase the B+ voltage. I took this control out of the circuit and left it alone since I couldn't determine what it really was supposed to be for.

Anyway, I replaced the speaker socket with an octal tube base for my rectifier tube. I chose a 5Y3 because it can handle the output of the transformer, which fed the two plates 350V. I used a choke and one 10uF 450V filter capacitor. Unfortunately, I noticed all the capacitors in the radio except the filter capacitor are rated 200V, so this seems to be a clue that a voltage divider (resistor) was missing or the transformer is not original even though the hole in the chassis for the transformer looks original. My power supply provided 350VDC, so I pulled out two identical power resistors from junker chassises and put them across the B+ and B-, the new 175V B+ is obtained from the connection between the two resistors. Now, the hard part was to figure out where exactly the B+ went. Without a schematic and a clue what the previous person intended to do with this radio, I traced most wires from the plates and screen grids. It all ended up mostly in one place, which I hoped is where the B+ feeds into. After connecting the wire from the power supply to that terminal, I slowly fired up the radio.

Thankfully nothing exploded, but the radio made no sound. I checked several places with my oscilloscope and learned the oscillator coil was not oscillating. I decided to recap the entire radio. After that, I fired up the radio and checked the oscillator coil, which was now oscillating well. The first and second stages seemed to be functioning properly, however, the radio hummed loudly and was motorboating. However, to my surprise, this "Frankenstein" radio was actually trying to pick up some stations. I found some stray oscillations occurring in the third stage, (first audio output stage). The humming stopped, then came back slowly when I touched this certain grid on the tube. I learned that the bias battery in that circuit was bad. I replaced it with a lithium watch battery and the loud humming disappeared. Now the stations came in well, though with some hum. I tinkered with the I.F. coils and aligned the radio. Now it plays several stations, still with a slight hum, but they all were soft. I fixed the problem with some tube shields and the stations came through much better.

Now it was time to refinish the cabinet. The cabinet was also a horrible sight, the coloring on the top had faded. Paint drops can be found on some parts of the radio. Someone wrote #13 with a permanent marker on the side of the cabinet. Some corners of cabinet, seem to be chew marks possibly by rats. Well, simply I sanded and stripped the cabinet which didn't take very long. I stained the cabinet with Golden Oak stain color then varnished it after a while. As the varnish dried, I was busy removing the speaker plug and the socket. Then I put the speaker wires through a hole in the chassis and reconnected the wires. When the cabinet dried, I inserted the speaker and chassis carefully and screwed them in.