Early (1957) Ameco Code Practice Oscillator
Ameco produced several code practice oscillators, including this older version mentioned in Popular Electronics in June 1957. This early model CPS was available either as a kit or completely built and tested. As noted in the catalog ad below, it was sold either with or without the tubes. The tubes are a 35W4 rectifier and a 50C5 oscillator/ audio out.
This piece is in rough cosmetic condition, typical of a 50+ year old piece that has seen lots of use. Because of the silk-screening, I have not yet decided how to repaint it.
A hazardous circuit
This early Ameco CPS has the same problem as the Bud Codemaster model CPO-128A, putting 140 volts of B+ on the key and the terminal strip to which the key is connected. . Using it as is will place 140 volts between the key and any grounded object such as any modern 3 wire appliance, computer, or radio.
The later Ameco CPS at this link is a much safer Ameco. If you decide to use this early CPS version, I recommend rewiring it to match the later Ameco or at the very least, taping over the key terminals on the back and adding a warning.
Repair Notes
This early CPS had an open power switch. The problem was solved by application of contact cleaner and working the switch. The tone control was also cleaned. The volume control which is a simple low ohms speaker fader control, was intermittent. I had to open that control to apply contact cleaner directly inside to solve the problem. After that treatment, the oscillator worked as expected.
Modifying it for safety
The original unsafe circuit may have allowed for easier conversion to a transmitter monitor. However, in the interest of safety, I advocate making modifications, especially if they are reversible.
I removed all the components and wires to the key side of the terminal strip, connecting them together under the chassis but not to either key terminal. That makes this Ameco CPS oscillate whenever powered. Therefore, like the later CPS, I simply connect the key in series with the speaker to send code.
The volume control on this early CPS is actually a potentiometer on the speaker side. That provides a load on the output transformer at all times. If the output transformer was not loaded on key-up, I would have added a 50 or 100 ohm resistor across the speaker terminals to provide a constant minimal load.