The RA3700 Series

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This post is part of the series 'The Racal 3700 Series':
  1. The RA3700 Series
  2. The RA3701 HF Receiver
  3. The RA3712 Dual HF Receiver
  4. The MA3752 Dual HF Drive Unit
  5. The RA3702 Dual HF Receiver
  6. The TA3762 250W Linear Amplifier
  7. The MA3772 1KW HF Combiner

June 2017

Back in February of this year I started writing a ‘piece’ on the various valve-based Sideband adapters that Racal manufactured. Well, that  'piece' took a little longer to complete due to a car load of 3700-series modules and chassis which came my way. The owner asked if I were up to re-building about eight RA3701s … to which I said “Sure!” However, when I picked up the ‘loot’, they turned out not to be solely 3701s but a selection of 3700-series kit in various stages of disassembly.

As with my experience with the RA1792s, I set about finding out just what I had filled my car with. To start with, there appeared to be ten chassis assemblies, three RA3701 Front Panels, four RA3712 front panels, two MA3700 Controller Front Panels and one MA3751 Transmitter Drive Unit Front Panel. Some of the chassis were populated or part-populated with modules whilst a couple of big blue plastic crates were filled with the remaining modules.
Racal very cleverly standardised on the basic chassis for the Receivers and HF Drive Units. In doing so, the PSU and back-plane are common. The only difference between relevent types appears to be in how the slots are labelled. There is one exception to this however: There are two types of 5MHz frequency standard employed. The 3701 is fitted with a type 9442 and the 3702 and 3712 are fitted with a type 9420.

So, I began to sort through the modules … marrying them to the appropriate chassis, being careful to check the drawing numbers since the Processor modules differ in terms of Firmware and although the Front-End modules all looked identical, there is a difference between the 3701 and 3712.
Since the chassis actually carries the overall type and serial numbers, that was my starting point. Out of the ten chassis, four were labelled RA3701, four were labelled RA3712, one was labelled MA3700 Controller and the last one labelled MA3751.
I already have an RA3701, so I already knew a bit about it. I wasn’t too excited about the Controller units since as the name suggests, these are for controlling receivers remotely. The RA3712 Dual HF Receiver sounded interesting and the prospect of an MA3751 TX Drive-Unit was indeed tantalizing.
However I was more than a few modules short. I did not have enough of the right modules to populate all the chassis and the Controllers and TX Drive-Unit were non-starters … both lacking significant modules … which was a shame.

At the end of the day … and after some repairs, I ended up with three working RA3701s, three working RA3712s and a spare Front Panel.

If the Achillies-heel of the RA1792 was the use of the dreaded 'blue-tants' then it might be said that where the RA3700 series is concerned, the actual IEC-Mains-Power connector used was perhaps a bad choice. With the RA1792 it was exploding tanatalum capacitors ... with the the RA3700 series it is exploding Schaffner Mains Filters. This only applies to units fitted with the combined Filter-Voltage-selector connector. I have had my RA3701 for ten years now and the filter has been fine, but in that time I have seen no less than three filters explode in other receivers ... and when they go, they go in style, filling the room with foul-smelling brown smoke. I am told that Racal were aware of an issue with these filters and that it was this unwanted 'feature' which probably actually put some potential customers off. Clearly word had got around about the exploding filters ... hmmm?
The power-supply unit is contained within a finned enclosure forming the left side of the common chassis. Access to this module is achived by removing two screws on the top and two on the side.
The design is kept simple by the use of monolithic voltage regulators. The PSU does not interface directly with the back-plane but via a series of cables.
The mains transformer and reservoir capacitors are located in the compartment immediately behind the front panel assembly. The 5MHz freqency reference is also situated here.
Whereas the RA1792 front panel assembly consists of two boards, this has been reduced to a single board in the 3700 series. Note the three display modules. The mains-switch is the same type as used in the RA1792. I know of at least two such switches that have failed (in 1792s). So far I have not seen any failures in the 3700 series ... but probably worth keeping an eye on.
Here is the rear of the Front Panel assembly PEC on my RA3702. Note the use of ribbon cables for each of the three displays. This may appear odd but it does make it easy to remove/replace the individual modules. Note also the three lighter coloured boards for the display back-lights.

Security Code

In order to prevent unauthorised changes to configuration data stored in the unit's non-volatile memory, entry of a security code is required. The security code used across the entire 3700 series is 7926.

Next post in the series:
The RA3701 HF Receiver
RA1792, Security code