Enderby's Mill. October 1970 to April 1975

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The service department

The service department had outgrown Fitzroy Street in Cambridge so it department moved to the Enderby's Mill, on the bank of the Ouse in St. Ives, mill in October 1970, several months before the rest of the business. When AIM had found the place it was derelict. AIM had not restored the main mill building (above) but had done a lot of work on the factory buildings beside it (visible on the picture below). We moved in there, next to the main building. I had an office overlooking the area. AIM's maintenance man had built up about a dozen benches to my design. I let the staff, both old and new, get on with laying things out to suit themselves.

Enderby's Mill
Enderby's Mill
We ended up with a rabbit-warren. Yes, benches were in short rows, about 4 rows, each with 3 benches. They had then put various shelves to make caves! Also the rows were so close together that the person next to the wall had to disturb two others to get to or to leave their bench. Fortunately this didn't last very long - the area we were in was destined to become stores and packing. When an appropriate floor of the mill was ready after refurbishment we were to move to the main building. But the arrangement did show me a lot: don't arrange benches so that people need to turn around to talk to each other. But they need to talk to each other sometimes. So when we moved, the benches - which had uprights at the back to take shelves - were laid out in 4s. Two facing north backed onto two facing south. Then repeat this. The layout worked well.

Enderby's Mill

Enderby's Mill
Enderby's Mill
After a time several flors on the Mill had been prepared. New flooring, partition walls and decorating. The service department moved to the first floor of the mill. My office was in the corner overlooking the river and the car-park. It's on the right in the photo. On the ground floor was reception and the general office.

The very top floor was Research and Development, and Clive Sinclair's office was at top right. His office had a thick pile carpet and the whole top floor was carpeted, with offices partitioned off for the various laboratories.

The story goes that one day Clive walked out of his office into where an engineer (I believe this was Jim Westwood) was working. He looked at the shelf over the bench "Are those the two new calculator chip samples?" he asked, picking one up. "Yes."

But only one chip worked - Clive had blown the other with the charge he had picked up wading through the carpets!

The next floor to be refurbished was the second: this became the sales offices. Also, some of the stores moved to the building visible on the left in the picture to make room for a production line, run by Tony Rogers.

Sinclair's computer

It must have been about 1972 or 1973 that Sinclair invested in a computer. This was huge! Several cabinets of electronics interconnected. It needed its own large room, with air-conditioning. This was installed on the top floor of the mill. I never had a lot to do with it, but Martin Wilcox, who was Sinclair's Chief engineer at the time, did talk to me about the part-numbering system he was devising. It was descriptive system, so that if you were familiar with the system you could take a good guess from the number at what the part was.

Some weeks later Martin was very crest-fallen, for Sinclair had used an industrial consultant (Brisch Systems, if I recall) to define a part numbering system which was totally numeric and seemed to have no connection at all with the item's description. Martin left Sinclair's in 1974 - I do not know whether this incident had much to do with his decision, but Sinclair Radionics was changing and some of us old-timers were getting disillusioned.

This disillusionment even spread to Jim Westood who had been Sinclair's first employee: he left Sinclair around this time to work for Gascol Conversions Ltd., who were converting all gas appliances from town's gas to natural gas. Jim found that even less rewarding than Sinclair's and he eventually returned to Clive Sinclair when he became sinclair's left hand.

The Sinclair guarantee and service

Clive Sinclair was adamant that service was just that - a Service. It was a non-profit duty. His adverts always made big issue of the money-back guarantee as well as the service available. All returns came into the service department for unpacking. Sometimes a customer would return a parcel for refund, addressed to the managing director: if these got to Clive, a cheque would be issued immediately.

However most of them were not so addressed, so I handled them. In most cases I had the feeling that the customer really wanted his problems sorted out. So I would phone or otherwise contact the customer saying of course they could have a refund. But the problem was... etc. So we could fix it. Did he still want a refund? Usually we ended up with a very happy customer - and we retained his money! A fault on a new product isn't anything more than an opportunity to show how good a company's response to it is! But if I sensed the request was genuine, then the request went to Clive's secretary and a cheque would be issued. Eventually Clive's secretary always referred refund requests to me for action before presenting tem to Clive. So I guess we did not adhere to the advertised "If you are unsatisfied with your product, your money will be refunded at once without question", but bending this rule did result in more satisfied customers!

But one day, handling a refund request that Clive had passed through for returning to stock, I thought I smelt a rat - the requester's name seemed familiar. We checked. And yes - he had had a refund for the same item quite recently. It turned out that he was buying items at a retailer, at a discount and sending them back for a full price refund! After that, we would only refund on equipment purchased direct from the factory. After all, if they had bought from a retailer, the retailer had taken their money and only he could refund it! The advertised refund guarantee wording was changed to reflect this.

Looking back on Sinclair as it was, today it still amazed me that Sinclair Radionics was (and still is) remembered with such affection. A good part of this has to be due to the way we handled the voluminous amount of servicing that was necessary. Although Clive set the policy, it was I who controlled that service so I think I was no small part of Sinclair;s early success.

Sinclair Radionics Technical Advice

Of course, in the market Sinclair was aiming at, technical questions were aplenty: usually by letter as there was no electronic communication, and long-distance telephone calls in those days were quite expensive. At Fitzroy Street, I had acquired a secretary to type answers to letters. The answers fitted into a well-defined pattern, so I wrote a raft of "standard answers" which made things easier. All I would need to do was to read the letter than write a few numbers on the letter. Josie, my secretary, would combine the appropriate answers into a letter and one of the ladies in the office downstairs would file the correspondence. But the volume of letters continued to increase. So we bought a Roneo duplicator, and I started writing a series of technical answer sheets which would answer the queries that arose. I still have copies of some of these Technical Information sheets, as listed.

These Roneoed sheets worked very well. But with increasing correspondence, we found that filing the letters was impractical. The vast majority of customers did not need further help, and when they did, finding the old letter to which they referred was near impossible. So we started returning letters to the writer with a standard letter explaining why. This also seemed to work well - until Clive Sinclair found out and did not like it. I guess I was ahead on my time - for this is exactly what email has now evolved into!


Any DIY project is sure to have some damaged components. Often a customer would write in with a problem and we would know immediately which component needed to be replaced. So we would send out spare parts with an honesty invoice. Almost every time, payment for the invoice would come back immediately. This saved our time in raising full duplicate invoices - which we would only do if the honesty invoice was not paid and was for a significant amount. It also pleased the customers! It worked excellently - until the company's accountant found out about it, and stopped it! But by that time, Sinclair Radionics were expanding away from audio into calculators and other digital devices.

A tragic accident

Some time in spring of 1972, I was called to Clive Sinclair's office. "A Project 60 has exploded and take a customer's eye out. I want you to go up and investigate."

So I set out from St. Ives in Huntingdonshire to drive north to Callander in Scotland. some 370 mile - over 6 hours driving!

The exploding object was the smoothing capacitor of the PZ8 (or it may have been a PZ7) power supply. At that time you had to supply your own capacitor for this as Sinclair did not supply one. The customer has used one of too low a voltage rating - below the voltage the instructions specified. If you over-voltage such a capacitor, it starts to get hot and, as it is wet inside, the liquid can boil. Modern capacitors include a safety vent for such a situation. But in those days the technology was not so good. So this capacitor blew its whole end plug off with some force. Unfortunately the customer was too close to it at the time.

So all I could do was commiserate with the customer - who was very contrite as he realised what had caused the explosion - and drive slowly back to St. Ives. Slowly - for the highlands were beautiful, and I did a bit of a detour along one of the lochs and took a walk!

I reported back to Clive that the problem was not of our doing - the customer simply hadn't followed the instructions. Clive wrote a letter of sympathy, and included a cheque as a gift of sympathy.

Sinclair Radionics at the shows

Sinclair started exhibiting regularly at shows with the Audio Fair at Hotel Russell, Russell Square, London in 1968. He had, in August 1966, shown off a prototype Microvision TV at the Earls Court Radio & TV show, but at that time only had the Stereo 25 Z12 amplifiers in the audio department - which never did appear at any hi-fi show.

But by April 1968 Sinclair Radionics had the Neoteric 60, the System 200 amplifier and the Q14 loudspeaker.

The show was a great success. We had made up a switch box so that the audio could be switched between the Neoteric and the System 2000 amplifies and the loudspeakers were either a pair of Quad Electrostatics (ESL 57) or Sinclair's own Q14 loudspeakers.

Of course the Q14 was nowhere as good as the Quad ESL 57. But they cost a mere £7.19.6d (£108 in modern money) whereas the Quads at over eight times the price (around £66 then or £850 in modern money) were beyond most people's budget. Many was the comment that, although they would love a pair of Quads, they were going to buy Q14s.

After that Sinclair exhibited regularly at the London Audio Fair and the Northern Audio Fair in Harrogate, Then the Audio Fair grew and had to move to London Olympia. For the hotel shows, Clive had employed agency girls: they may have been pretty dolly-birds but they knew nothing of Sinclair products. So when I knew we were going to Olympia I suggested to Clive that, rather than employ expensive agency girls, why not use some of our own girls from the service department. They would brush-up well and would be a lot cheaper. Besides, they knew the product from the inside and the experience would be very good for their morale.

The service department girls' presence at the show was a great success. Pamela Carter - who was line manager - and Rosemary Belcher, of Over - who organised service department stores - regularly helped at the shows, with various other members of the service department. A typical conversation with a customer might be:
"Is there anyone technical who can help me?"
"What';s the question? Then I'll know who can answer it."
"It's about the Z30 amplifier.... etc"
"Oh, that's easy, you need to ... etc."

The girls were so successful that they even acquired a sort of fan club - some customers would regularly turn up time and time again at different shows to speak to the same girls as they knew their stuff well!

Project 605

Sinclair Z30 Mk 2 amplifier
Sinclair Z30 Mk 2 amplifier
Although Sinclair encouraged anyone to put it together, Project 60 did involve soldering. So a Masterlink board had been designed to which everything connected with pre-cut wires. These had (if I recall correctly) a 110 fast-on connector on the end which push into a slot in the circuit board to make connection. These can be seen in the picture (click on it to see enlarged view). The Project 605 with Masterlink was released in 1970.

Sinclair Project 605
Sinclair Project 605
Similar slots were present on all project 60 modules from now on, so the Project 605 system was fully expandable. These slots are a way of dating Project 60 modules: those without these slots are early models! The Masterlink board is at the top of the picture (click on it to see enlarged view).

I have little recollection of the Project 605: the Masterlink did not fail so did not get returned for service!

Sinclair Z50 amplifier

Sinclair Z50 amplifier
Sinclair Z50 amplifier
Also released in 1970 was a higher power amplifier - the Z50. This was a drop-in replacement for the Z30, but could give more power.

Sinclair PZ8 power supply

Sinclair PZ8
Sinclair PZ8
Of course, a higher powered amplifier required a higher power supply. So the following year (1971) the PZ8 was released. This was a simple stabilizer with reservoir capacitor and bridge rectifier: the user had to find their own mains transformer.

But there were too many returns on the PZ8: it had no current limit and therefore could blow under any fault conditions - even a momentary short-circuit on the output. So I played about with circuits and devised one that was not only current-limited, but was re-entrant.

A current limited supply, if working into a shorted output, will get extremely hot. If the limit is 8 amps and the input voltage is 50, then into a short the power is 8 times 50, or 400 Watts. Which would cause instant failure and a possible fire! If it in re-entrant, the current reduces as the output shuts down, so 50v at 100mA would only be 5watts. Still enough to get hot, but safe! I had done a lot of work - for the low-level current had to be enough at switch-on to overcome current surge as capacitors charged up. I wa happy that my circuit would work reliably.

Clive Sinclair had released the control of Research and Development to a new director - one Michael Pye. I approached him with the revised circuit.

Mike Pye did not like the circuit! The overload point at which it cut back was not sufficiently well defined in theory for him. So he vetoed the change. We had to keep repairing failed PZ8s. I think this was about the stage I started to get disillusioned with Sinclair Radionics: our products were certainly a compromise of price against performance, but was I going to have to keep putting up with unreliability that could be easily improved?

However, a re-entrant PZ8 was eventually released in December 1973.

Sinclair Executive calculator

Sinclair Executive calculator
Sinclair Executive calculator
In early summer 1972 the Sinclar Executive calculator was released. Contrary to the Wikipedia article I had nothing to do with its design - Wikipedia says I designed the case - wrong! In those days Clive's brother Iain Sinclair did most of the design.

The August issue of Wireless World was very much about calculators: there was a general editorial on the firs page and on page 8 an editorial appraisal of the new Sinclair Executive. This article was extremely negative: it criticised the on/off switch, the keyboard, and the way the design used power switching to prolong battery life. In the September issue, Clive Sinclair had published a strong rebuttal of all the criticisms of the Sinclair Executive.

However, very shortly after the release of the Sinclair Executive, the service department started to get them back by the bucketful! There were problems with the battery contacts, the on-off switch and the keyboard. The case was ultrasonically welded together so to fix such problems, the calculator's case had to be destroyed and the calculator had to be effectively re-manufactured, This was not a job for the Service department, so they all went to Tony Rogers who was in charge of the production line at that time.

The problems were, for the most part exactly the criticisms made by the Wireless World article! I do not recall that there were problems with the battery switching - but the Sinclair Executive was soon redesigned and the re-design did not include the battery power switching feature - so there was much criticism of the short battery life.

All the calculators were tested prior to casing: this did not require technical knowledge. Testing was done by calculating 123 divided by 456 times 789 which gave the result 212.82236 or 212.82233 - the two different answers came from the different calculators the Executive and the Cambridge.

I wrote the instruction manual for the early Sinclair Executive calculator: it was fairly full of playful examples. However, as far as I recall this was the last set of instructions I wrote.

Exploding Sinclair Executive

There is a story going round about a Sinclair Executive that exploded in the pocket of a Russian diplomat. This story appears to be truth stretched to become fiction! Chris Curry says:
"Often in our case Rodney Dale (whom I never much cared for) is the source of a lot of the fiction going round on the www - the exploding Executive for example was in the pocket of Lord Rothschild not a Russian diplomat and I know because I took him round a replacement.

"The reason was that it was one of the versions which had four battery cells, but one cell had gone flat. The out-gassing which caused the problem was caused by the three goodish cells reverse charging the flat one.

"The chip used in the Executive was the TMS 1802 and the original display was from Texas Instruments. Later units used a display from Bowmar and the lens was supplied by COIL. Iain Sinclair did a typical Iain sketch of the case and I had to make it all fit together.

"The keyboard was based on the prototype that I made which had three wires with bent metal tags soldered onto them for each key. The chip had no debouncing so that the contacts were finely set for non-wiping overtravel with a contact pressure so light that touch hesitation had no effect. The contacts were selectively gold plated by Cambridge Electro Plating who also etched the matrix in the end. Of course the varying clock speed had a serious effect on the keyboard bounce tendency but this only became an issue when I made the fake keys with the vac-forming on top.

 I recall that doing the printed-circuit-board layout was a serious challenge, as was the Microvision, largely because it was all through-hole-plated and components were fitted in all orientations to fill the space between the halves of the (yes) polycarbonate case that was tooled by a company in in Birmingham."

So here is an event which had been heavily embroidered and stretched to grow into an interesting story of fiction! Reverse charging of one cell in a battery pack is always a problem if all cells in the pack are not replaced by ones of the same age, or if one cell is faulty.

The fiction is told on Wikipedia, My Retro Calculators and has been fully expanded in an article in the New York Times entitled Farewell, Pocket Calculator? where it has been fully coloured-in by Alice Rawsthorn, March 4, 2012.


Clive Sinclair was always well aware of the publicity value of editorial articles in magazines so freely gave equipment to them, as of course did manufacturers of many other items. If the contact person involved needed additional help or advice on the Sinclair samples, he would be referred to myself. One such person who made contact in the early 70s was Harold King who was then a freelance editorial consultant. He had edited at least one D-I-Y magazine (I cannot recall the name) and he was the editor of The Home Handyman Encyclopedia 1975 from 1974 to 1978 (of more),

Snclair Project 60 installation
Snclair Project 60 installation
I got involved with Harold in doing a magazine article on installing a Project 60. We made friends and after that I was a frequent visitor to Croydon where he then lived.

I had bought an old house in Somersham which we were doing up. Harold King was a good source of ideas and the occasional spare editorial sample - which he could supply pretty-much on demand.

Meanwhile Clive Sinclair and I had drifted apart: the service department was now part of sales and service. I was no longer autonomous but was under the sales manager.

So when Harold King said he was helping with the launch of a UK edition of Popular Science, and that they needed authors, I started writing regular articles for them. This was in March 1975. I had resigned from running the service department - more or less in protest at being combined with the sales department under the new sales manager (but mainly because I was aware that Sinclair's returns rate was never going to get any better! I wanted to work with products that were reliable!)

Sinclair took large advertisement space in the first few issues of Popular Science and issue 1 in April 1975 had an article on "How to build a Sinclair Cambridge pocket calculator" and a two-page advert for Sinclair Project 80. Issue 2 has an advert for the IC20. Issue 3 had an article "An Intercom / Baby Alarm" using a Sinclair IC20. There is more information on these issues of Popular Science UK.

Sinclair Radionics had a large value of surplus stock at the time. Well over £1,000,000. Part of the deal was that I could sell off this stock and pay back Sinclair cost price as I sold it. So I was trying to use these components in the Popular Science kits of the projects I was writing up.

In April 1975 I was made redundant. But that was in a recession. Popular Science folded its UK edition after only 12 issues. Sinclair Radionics were also in trouble - 1974-1975 was the first year they made a loss.

Sinclair Black Watch

Other sites state that the Sinclair Black Watch was released in September 1975. However I left in April that year and I have definite memories of the Black Watch. Very soon after it started shipping, we received them back in the service department. Dead!

The Black Watch was released as a kit, or ready-built. Such things take organisation to build, so the kit version was probably the first to ship. It turned out that the watch chip used in it was extremely sensitive to static electricity. So even taking a shirt off whilst wearing the watch would quite likely blow the chip. And the chances of static blowing the chip during home assembly were quite high!

So either I'm remembering something other than the black watch - or it was available to some before it was advertised. The truth is probably that the assembled version was available in September, the kit earlier.

The Black Watch was a loss-maker. Sinclair Radionics was foundering and in 1975 the newly-setup National Enterprise Board took over. They could not save Sinclair Radionics!

Which brings me almost to the end of my recollections of "Inside Sinclair Radionics". But I'm sure there is more to add: if you want to be informed of additions, please let me know.

Part 2 Index

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First published Wednesday the 7th of September, 2016
Last modified: January 15 2022 09:39:45.
Written by Richard Torrens.
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