From the Chinese “Peoples” “Liberation” Army….Got it…
They were used in pairs to send morse code signals over infantry field telephone wire. It required both a 45 volt and a 1.5 volt dry cell battery to operate. Shown below with a U.S. GI telegraph key, the set also needed an appropriate high impedance headset to operate.
The tube is a 1K2 pentode, a Chinese copy of the 1AJ4. There is a spare tube inside the case. Nice.
The tube shield in this NOS example has what appears to be a date code: 6912. December 1969? The tube itself has a “70” label. 1970? Seems about right. The manual has a date stamp of 1969.
The manual is of course in Chinese however there are a few Cryllic characters present on other pages. Indicating a possible Russian connection with the design.
The Form, Fit and Finish of the set is good as is the environmental protection when the cover is closed. The interior is laid out for good serviceability and it appears to me that it is made from indigenously sourced components. The interior metal work is chromate passivated however some of the metal parts may have been machined with a dull rock. Lots of tooling marks. Like the Soviets, they didn’t put any effort into anything that was not absolutely essential to the overall function.
The case has a removable cover. The Chinese characters are raised above the panel surface. The entire front panel is chrome plated with the chromed labels standing out above the green lacquer paint. Pretty – but was it essential?
The cover includes a metallic card with a clear schematic as well as a possible “QC” card behind the celluloid window. The meter needle looks to be radium-painted.
This set was designed to operate over a single conductor wire connected to the post, upper left.. The “return” path was via ground; a ground stake was included with the set. Volume control on the left, “sensitivity” adjustment beneath the meter. Key jack is on the right. There are 2 headphone jacks, left and middle.
Probably so the unit political Commissar can hear any counter-revolutionary messages.
The Function switch is OFF when fully CW; Standby in the mid-position; Operate (Transmit/Receive) when in the fully CCW position. The schematic shows it in the T/R position. Transmit 1000 cps sidetone is enabled on Transmit. The bipolar meter monitors incoming and outgoing Line current.
In the Off mode, both 1.5 and 45 Volt batteries are disconnected. In either Operate or Standby, the tube Plate is powered by the local 45 volt battery. In the Standby mode, the tube has its plate and screen voltage already present but the filament is off.
When the set is in the Operate (Transmit/Receive) mode either the local key or the remote set key supplies adjustable (volume control) screen voltage to the oscillator tube generating a tone. In the Standby position the tube filament is off to conserve power.
However, in the Standby mode, if the remote station closes its key (to alert the local operator) the relay applies power to the tube filament which causes it to rapidly come up to temperature. This generates a tone. So this Standby process alerts the local operator to switch to T/R mode to communicate thus saving battery power in Standby.
The set came in a cotton canvas bag with leather straps and reinforcements.
The kit included a manual, ground stake, ground lead and the headset plus key.
I don’t know what kind of range was possible with these sets but I would think they were adequate for the task; likely 10-20 klicks. I would assume that China supplied these sets to the North Vietnamese Army for internal communications and for their assault on South Vietnam. More secure that radio, faster than a messenger; the Ho Chi Minh trail probably had lots of wire strung alongside.