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This radio has been in my collection for many many years, I started to restore it years ago by recapping, only to find out that the problem with the set was more than the electrolytic capacitors. This idea for restoration article is the final restoration of this radio which required circuit modification to get the set operational. If you are a purist collector, who is interested with what you see is what you get, this idea is not for you and the radio must remain inoperative. If you have a problem like this one and you want to get the radio to work then read on.
In 1964 Emerson Radio and Television produced the model 911. This was made in USA but some parts were imported. Emerson used RCA transistors. This 9 transistor radio features TRF front end, a larger deeper speaker for richer tone and output transformerless, using a specific pair of PNP transistors. Emerson utilized a unique design that features one PNP connected as an emitter follower(current amplifier) and the other PNP connected as a common emitter (voltage amplifier). Their outputs are connected to a 16 ohm speaker. The 6 volt battery supply is tapped at 3 volts to provide base bias to both transistors. The 3 volt tap allows the transistor to operate like a conventional push-pull (Class B) amplifier using a transformer. One transistor is operating while the other is off, when the signal flips the transistor reverse their operation. Using the battery tap voltage reduces components and lowers quiescent or standby current lengthening battery life and lowering parts count as well as eliminating the output transformer. The bad part is the use of specialized parts, which cannot be replaced or restored.
The radio has not output at all. The speaker checked good.
Found out that the on-off switch used in the radio was not working. The schematic closest to the right is an Emerson factory schematic. Notice that the battery has two switches to turn the battery power on or off from both sides of the battery. Emerson tapped the 6 volt supply to provide a 3 volt bias for the base drive input for the audio output transistors. In my set one switch was a dead short and the other permanently open. When I manually connect the set it works fine. The big problem here is that the dual on-off (SPDT) switch is on the volume control. It was special made part for Emerson, but try to find one.
Here is my approach to get the set working:
1. Get a transistor radio potentiometer with a SPST on-off switch, trim the shaft to fit the knob.
2. Put together a fix bias 120 ohm resistor divider connected to to the battery negative side and the other end to the switch terminal away from the battery (see schematic on the farthest right).
3. Connect divider tap to speaker and speaker jack.
Connect the wires back to the potentiometer, switch and install the resistor divider.
Radio put back together and works great. From my experience, Emerson designed a nice performing set, but at the end, the use of a special pot, tapping the battery for bias and the elimination of the output transformer and replacing it with a custom made one are basically cost savings in parts count and assembly. It is interesting to note that all the original electrolytics were imported components.
Modification and Wiring
Potentiometer was selected and trimmed to fit existing knob. Picture on LEFT. View of defective switch and volume control, specifically made for Emerson. Notice the SPDT switch on the rear. I doubt one can find a replacement.
A rotary tool like the Dremel shown here can be quite handy to trip and custom make parts. The rotary tool was used to reduce the diameter of the potentiometer shaft I chose to rebulid the set.The potentiometer on the left is a sample of what I used to replace the original. The potentiometer in the middle is the one I modified for the setThe potentiometer on the right is the original.
Had to trim the bracket to fit the replacement potentiometer in.
Put speaker wires and bias line back.
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