Racal SA535 Frequency Counter


This really is an unusual unit from Racal Electronics - makers of the highly esteemed and justly famous RA17L tube receiver.

It is a frequency counter of 1.2MHz range - but instead of using nixie tubes for its display it has 6 columns of filament lamps - 10 in each column - to illuminate the individual digits on the plastic front panel - a grand total of 60 lamps (with spares included!).

This is a solid-state unit with many circuit boards and multiple transistors and appears to date from the early 1960's. I am puzzled as to why its display technology seems anachronistic even for its time and why its range is limited, compared to other units of the time.

Recently, I was contacted by Alan Hempel, formerly of the Australian Post Office, who tells me:

" They were sold in large numbers at a cheap price to phone companies (such as the one I worked for), where the 0 to 1.2 MHz coverage was ideal for maintaining the multichannel carrier telephone gear that was common in the 1960's.  These Racal counters have a pulse-length/timing feature and input stage configuration that made them ideal for working on relay based equipment too - relay circuits being used in vast quantities in 1960's telephone exchanges.  I'm sure that they were designed with phone companies in mind. Unfortunately they had a high failure rate and soon got scrapped.  

They had a high failure rate, transistors shorted C-E being very frequent. I remember one of my earliest jobs being given several to fix, and the Senior Tech said to me something like "Don't waste time trying to be clever - just identify the correct board(s) and check all the transistors on it/them for shorts C-E.  He was right, and I replaced lots of them. Strangely, I don't recall the display lamps failing much. They also had a unique double thickness PCB for the power supply, adding
ruggedness while retaining assembly simplicity.

They superseded in the APO a counter-timer made by Dutch Philips with the same frequency range and facilities.  The Philips unit was late 1950's vintage based on gas discharge tubes that on each input pulse stepped the gas glow from one cathode to another.  You read off the frequency or elapsed time by directly viewing the glows.  While I have rubbished the Racal's reliability, which was pretty poor compared to brands like Hewlett-Packard, the Racal was a useability and reliability advance over the older Philips product."

Return To Test Gear

Last updated 30th March 2013