DST100 MkIII* Communications Receiver

 
 Back in 1962 my pal Dave Yates, G3PGQ, from Nelson introduced me to his neighbour in Barnoldswick to show me a DST100 and ever since I've vaguely thought of owning one. Recently I saw one advertised and I collected it a few days ago. It's extremely weighty at a little under a hundredweight and looks complete. A quick look under a chassis revealed modern plastic capacitors so it will probably work once power is applied.

 
 The dial is cleverly designed. That outer log scale rotates and provides a 24:1 flywheel action slow motion drive to the tuning condenser. The metal knob, bottom right, provides extra slow motion for the dial and it looks a little wonky so will need looking at. This control was designed to be disabled by moving the knob to the right. In the picture below you can see the set has two side-by-side detachable units. one carrying the RF front end which includes a large drum carrying a set of coils (much like the R206 receiver) and the other, the remainder of the set's circuitry. The drum is rotated with the knob at the top left. This technique also referred to as a "turret tuner" was used in most TV sets in the days when Bands I and III were in use.

 

 

 Below you can see the rear of the chassis carrying the majority of the valves listed here. Note the "modern" Radiospares electrolytic on the right.

 

 

 

 Above is a rear view of the drum and below the label on the rear panel showing the aerial connections.

 

Click the circuit to see a larger version 

The circuit shows a fairly complicated set-up because it's essentially a double superhet for the higher frequency bands and a single superhet for low frequency bands and there's no shortage of valves (I counted 13 in the above circuit diagram). Below is a sheet carrying more detail. Some say this receiver was designed by McMichael and others by R.A.P. This example was made by R.A.P. (Radio Acoustic Products), whose engineers I understand developed the receiver before WW2, although they were also built in several other manufacturers factories. The set used an external power unit, called "Rectifier No.8" (now as rare as hen's teeth) which can be seen adjacent to the picture of the receiver below, although it appears there's probably sufficient space inside the cabinet to house a custom made one. The receiver is now very rare because of its enormous size and weight which made it an unattractive proposition in the days when other more manageable surplus sets were available. Given enough sets and a spot of detective work, maybe we might be able to figure out who made the earliest examples and therefore pin down its origin? Below, the official document refers to both the Army and the Royal Navy but the parts references include items such as ARP34 which whose code letters mean "Army Receiving Pentode". Valve cover tops are labelled "10D2379" which sounds like an Air Ministry code.

Before WW2 radio signals were monitored by an organisation called the Illicit Wireless Intercept Organisation (IWIO), later renamed the Radio Security Service, RSS then given the cover name MI8(c). The Army ran an organisation called "Y-Service" operated by soldiers, many of whom (like my late father-in-law G3AQY) recruited from Radio Hams. The Navy had an independent radio interception group (Room 40 then NID25) which was merged eventually into MI1(b) then the Government Code and Cypher School, GC & CS which became GCHQ after WW2 but it seems that offshoots of the Navy group were still operating covertly at their shore wireless stations (see under "Purpose" below).. After lots of interdepartmental rivalry three security groups resulted viz MI5, MI6 and GCHQ. It's possible that the vague origins and use of the DST100 might be linked to secrecy in the late 1930s.

 

 Click the label to see large circuit diagram with parts list

Click the picture (not a VP41) to see the VP41 spec

The CV21 was a strange but seemingly logical choice for the DST100 RF Amplifier

 

Click the picture to read the Practical Wireless article from 1988/89.. large PDF

 

 

more to follow....

see its refurbishment

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