Bud CPO-120 code practice oscillator

1939 vintage Bud CPO-120 code practice oscillator

If you used one of these when it was new, you practiced Morse code prior to the US entry into World War II. Introduced in 1939, the Bud CPO-120 uses just one tube. That tube, a 117M7GT combines a half wave rectifier and audio output in a single tube envelope. The tube filament, at 117 volts, can be directly fed from the power line. Two code keys can be connected to the oscillator. One key jack is on the front panel and another on the back. Pin jacks are located on the back for an extension speaker. With an extension speaker and two keys, two individuals can practice sending and receiving Morse code together.

Bud CPO-120 code practice oscillator showing added ALCI power plug
Bud CPO-120 code oscillator

The oscillator can run on AC or DC. One side of the power line is switched directly to the exterior of the two phone jacks used for the keys. That makes the device rather hazardous to someone not familiar with hot AC-DC circuits. In its original design, the key jack (and the code key itself) is connected to the hot side of the power line either when the device is turned on or when turned off by way of the cold filament. With so many modern appliances, including ham gear, coming with a three wire grounded cord, this piece cries to be modified for safety since safety grounded devices are in fact connected to the neutral side of the power line at the breaker panel. Touching one of those devices and the code key at the same time will result in a severe shock. Short of a total redesign, there is no easy way to make the CPO-120 totally safe but some simple modifications can make it less hazardous.
Bud CPO-120 code oscillator ad from Radio-News January 1940
Bud CPO-120 code practice oscillator

The CPO-120 has no volume control. An SPST switch is used for On-Off and another for switching in a 0.01 MFD cap for lowering the tone. Not being able to find a schematic, I sketched out the relatively simple circuit first. The resistors were in good condition but the three feedback caps needed to be replaced as well as the electrolytic. I left the orignal electrolytic in place to retain its vintage look. An intermittent contact problem for both the power switch and the tone switch was solved by application of Deoxit by way of the switch bat handles. Application was with the device on its back with the bat handles pointing up.

Bud CPO-120 code practice oscillator chassis showing its single tube
Bud CPO-120 code oscillator chassis

safety improvements
I strongly suggest using an isolation transformer with the CPO-120. However, I also decided to replace the power cord with a two-button ALCI plug and cord harvested from a hair dryer. An ALCI (Appliance Leakage Current Interrupter) is a two wire version of a Ground Fault Interrupter. The ALCI plug is polarized. I rewired the power switch to switch the hot side of the line only. The wire from the wide neutral blade was directly connected to the B- side of the circuit which is also, as already noted, directly connected to the outside of the two phone jacks intended for Morse code keys. Any inadvertent leakage path over about 4 to 6 milliamperes will cause the ALCI to shut off power.

More code practice oscillators

Some other vacuum tube CPOs that I have repaired can be found at this link .

N4MW specializes in collecting code practice oscillators.


A Magnecord PT6-J audio amp was the previous project on the bench.

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