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KL-7 PSU
Power Supply Unit · CE 87066

When used in a static environment, such as an office or a command centre, the TSEC/KL-7 cipher machine was usually powered directly from the AC mains, using the external power supply unit (PSU) shown below. It is suitable for 100-125 V AC and 200-250 V AC, and delivers ~ 24V DC.

The unit is officially labelled as National Security Agency A.C. Power Converter. It measures 270 x 125 x 100 mm and weights 3125 g. It consists of a metal base to which all internal parts are mounted. The base has threaded holes at the four corners, allowing it to be bolted to a table, a maintenance frame or an equipment rack.

The unit is painted in the same grey colour as the KL-7 itself. It is believed that the units that were used by the US Navy were (re)painted in the typical naval light blue (mint) colour. The device shown here was originally painted in that colour.
  
KL-7 power supply unit

In a mobile environment, the PSU was sometimes supplied in a glass fibre transit case – similar to the one in which the KL-7 was stowed – together with a maintenance kit, a spare stepping unit and a selection of supplies (paper, ink ribbons, etc.). The device has two fixed cables: one for connection to the mains network, and one for connection to the short power cable of the KL-7.

Note that the device does not have a power switch. The desired mains voltage should be selected with the slide switch at the top, prior to connecting it to the mains. It is secured with a bolt. At the rear are two fuses. The left one is for the primary (mains) circuit. The other one is a spare.

KL-7 power supply unit
KL-7 power supply unit
KL-7 power supply unit
Rear view with mains fuse (and spare fuse)
Top panel with ID tag and voltage selector
Voltage selector and (red) stamps on the top panel
Cable inlets with strain relief
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KL-7 power supply unit
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KL-7 power supply unit
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KL-7 power supply unit
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Rear view with mains fuse (and spare fuse)
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Top panel with ID tag and voltage selector
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Voltage selector and (red) stamps on the top panel
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Cable inlets with strain relief

Circuit diagram
Below is the circuit diagram of the power supply unit, which consists of a conventional mains transformer and a full rectifier. The transformer has two identical primary windings, that are connected in parallel when configured for the 100-125V AC mains, and in series for the 200-250V AC network. The voltage selector is connected at the centre of the primary windings.


The secondary side of the transformer delivers 17-24 V AC, which is rectified by the two stacked full-wave selenium cells at the right. In practice, these selenium cells are mounted at either side of the transformer. Although there are two fuse holders at the rear of the device, only one is used in the primary circuit. The other one holds a spare fuse. The mains ground wire is not present, but was added by us for safety reasons. It is connected to the chassis of the device (see below).


Restoration
The KL-7 power supply unit featured on this page, was found on eBay in late 2020, and was in unknown state when it arrived. The exterior was painted in a light blue (near mint) colour, similar to the colour that was used by the US Navy. There were several self-adhesive labels on the body, that had clearly been added by a later owner. The base of the unit still showed the original paint.

The unit had probably been stored in a dusty moisturous place for many years, as the text on the name tag and the voltage selector was barely readable. A first superficial cleaning revealed that the text on the selector was still legible.

When removing the unwanted self-adhesives from the top surface, they took some of the paint with them, as shown in the image on the right. Although this was initially regarded as unfortunate damage, we later discovered the original red stamp in the bottom right corner and concluded that this was the original paint.
  
Top with serial number tag and voltage selector

Apparantly, the exterior had been repainted at some stage – by the US Navy or by a later owner – without properly cleaning the surface first. As a result, chips of the new paint easily came off. We then decided that it would be best to remove all of the later paint and restore the original colours.

That was easier said than done. Although the paint easily came off from the top surface, there were other places where it was more persistent. This was especially the case at places where the original paint had been damaged in the past.

First, the loose chips were removed by putting wide cellotape over it, and then pulling it off. The remaining paint was removed manually, using the finger nails and several special tools. The result is visible in the image on the right. Even the red eagle and the red Moisture and Fungus Proof stamp (MFP) are still fully intact.
  
Voltage selector and (red) stamps on the top panel

The original rubber power cables had become stiff and brittle over the years, and were no longer considered safe for connection to the mains network. They were both removed and were replaced by modern neoprane cables, from which the printed yellow text had been removed with acetone.

Along with the wiring, the strain reliefs – that had meanwhile completely disintegrated – were replaced by nearly identical new plastic ones. For safety reasons a ground wire was added between the mains ground and the chassis — some­thing that was not present in the original design.

After fitting a new mains plug, the pins were checked for continuity and the primary and secondary sides of the transformer were checked for resistance. Once we were certain that the device would not short out or cause a ground leak, it was connected to the 230V AC mains.
  
Interior

It worked straight away and approx. 26V DC was available on the unloaded output termials. Next, the KL-7 was connected to it, and its MODE-selector was set to plaintext (P). After warming up, everything worked as expected. In this situation the voltage had dropped to 24V DC as expected.

The following was restored:

  • Mains AC power cable replaced
  • DC output cable replaced
  • Both strain reliefs replaced
  • Voltage selector cleaned
  • Several wiring junctions resoldered
  • Rubber feed added to the threaded mounting holes
  • Light blue paint removed and original paint restored
  • Power connector replaced
  • Tested with KL-7
KL-7 power supply unit
KL-7 power supply unit - rear view
KL-7 power supply unit, with power cables stowed
KL-7 power supply unit with power cables
Top with serial number tag and voltage selector
Interior
Interior
Selenium rectifier bridge and wiring detail
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KL-7 power supply unit
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KL-7 power supply unit - rear view
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KL-7 power supply unit, with power cables stowed
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KL-7 power supply unit with power cables
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Top with serial number tag and voltage selector
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Interior
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Interior
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Selenium rectifier bridge and wiring detail

Connections
Power
The KL-7 must be powered by a 24V DC source, that should be connected to the short piece of cable to the right of the keyboard. It has a 2-pin Amphenol 97-series (male) plug at the end, that mates with the 2-pin (female) socket on the power cable (stowed in the case lid) and on the external PSU. The required (female) socket has the following part number: Amphenol 97-series MS3101A12S-3S (shell: AN3057-4). Below is the pinout of the (female) socket on the PSU:

  1. +24V
  2. 0V
Further information
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Crypto Museum. Created: Sunday 18 April 2021. Last changed: Wednesday, 21 April 2021 - 05:42 CET.
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