at Duncan Terrace, October 1964 to April 1966

My introduction to Sinclair Radionics

dunter22/jpeg It was a month before my 21st birthday when I joined Sinclair Radionics in 22 Duncan Terrace, Islington in October 1964. The offices were on the second floor. There were three rooms: in one large room was Clive Sinclair with his secretary, Maisie. Next door to him in a smaller room, used as a store room. I was to share the second large room with a young lad, Jim Westwood, who had joined Clive about a year before.


Jim was working on the X-10 amplifier - the first batch of boards had been produced as a mirror image, which made them unsaleable! But with care the boards could be used as prototypes by making the transistors cross their legs. Which had been enough to show that there were more problems to come! Not only could the amplifier deliver nowhere near the 10 watts that had been advertised, but the distortion was not as low as claimed. Jim had done enough work to convince Clive that it was pretty much a disaster! But the correct boards had arrived by the time I got there. Jim was testing them, and putting them as right as he could!

x10/jpegThe X-10 had been designed by Cambridge Consultants using an idea first propounded in Wireless World February 1963 (Link is to - World Radio History archive of the magazine. Article starts on physical page 27). It had been designed as a 2.5 watt r.m.s. amplifier. 2.5 watts r.m.s. is a peak power of 5 watts - which figure had been given to Clive. But there was a misunderstanding - Clive thought this was the r.m.s. value so doubled it again! I have full information on the Z-10 and may write it up in more detail - please ask. The technical problems are quite interesting.

Micro-6 radio

micro_6/jpegSinclair's existing products at that time were the Micro-amp, Tr750 and Micro-6 radio. All were sold as kits for home construction and inevitably some customers had problems. Most of the problems were sent by post to Sinclair's Cambridge depot, which was the Comberton Village hall. But from time-to-time a customer would turn up at Duncan Terrace with a problem - usually a Micro-6: I can't remember having to repair a Micro-amp or Tr750.

Jim was very busy with the X-20 so it fell to me to sort these problems out. It turned out that I had something of a gift for fault-finding and repairing items such as these. Some of these were very interesting: I recall one, a Micro-6, where the customer had simply melted two puddles of solder over the bottom of the board. When it had cooled he had taken it to a grinder and ground the solder puddles down to about 2mm thick. For some reason, the radio did not work!

And there was another which mysteriously didn't work: the customer's opinion of his own soldering skills were clearly very low, so he had used a non-conductive solder replacement - Solderlene, which you can still buy. Even I could not make this Micro-6 work!

Clive Sinclair was in some ways a difficult man to work with: he had a commanding presence and a very high i.q. - he was a mensa member. Certainly I always found him overwhelming and so, I believe, did Jim Westwood. After I had been at Duncan Terrace a few months, I had rigged up a radio and Jim and I used to listen to classical music - the BBC Third Program (which later changed its name to Radio 3). This was my choice in music - but Jim very quickly came to like it and we found it relaxed us and made it easier to work. Until, one day, Clive, in a bad mood, walked in and said "Turn that blasted radio off, and don't let me hear it again". So we went silent. But I found that I was tense and on-edge with the radio off. So, too, was he, admitted Jim. So I summoned up my courage, knocked on Clive's door and told him. He agreed we could listen to the radio.

Then there was the time that Jim and I disagreed with some technical decision Clive had made - I forget the details. We discussed everything we could think of that backed up our opinion or that Clive could raise as objections - for we knew Clive did not like being contradicted. We went into Clive's office and told him our argument. Immediately we had finished Clive came up with a point that we simply we had not thought of and knocked our argument out of the water. Only several days later we managed to work out that Clive's point whereby he had made us back down was in fact irrelevant!

Transistors to Hong-Kong

AT_trs/jpgThen in the summer of 1964 a large number of heavy boxes arrived which Jim and I had to carry up to the box room on the second floor. I don't know the exact size of these, but from memory they would have been something like 15" x 9" x 7" (this was before Metrication!. Imagine how many of these transistors would have been in such a box, and there were about 40 boxes

2 years earlier Associated transistors had been bought out by Mullard and these were the surplus transistors which Mullard could not use. Essentially these were reject transistors - but were perfectly usable. In fact a number of them were reject because they were too good - they had Hfe greater than 200 - very unusual for a germanium transistor, but above specification so could not be sold as standard devices!

Clive had won a contract with a Hong-Kong manufacturer to supply transistors for their radios: by using British transistors the radios could then be shipped back to the UK without any import duty being slapped on them. So I was put to testing how the transistors worked in the radios: they were fine.

Once this had been done, the van arrived to take the boxes of transistors to the shippers. Jim and I now had the job of carrying the transistors down 3 flights of steps - the ground floor was raised above a basement. After a few boxes, we decided to open a window and throw them out of the second floor. This was far easier and a lot quicker! Now Duncan Terrace is a Georgian row, with iron railings to prevent passers by falling into the basement. Our box-throwing went well unto one box landed on the railings and got impaled, spilling transistors all over the place!

But we retained a number of these transistors for future use in products such as the Z-12, including the ultra high gain transistors (Hfe over 200 if I recall - so well over their specified performance, so rejected as too good!). These were far too good to loose! They became vital to us when they were designed into the Micro-FM later in 1965.

Sinclair X-20


Because of the X-10 fiasco, Clive had Jim working on a new class-D amplifier that would work properly. This was quite successful when it was released as the X-20. But there are problems with any switch-mode amplifiers. To work as audio amplifiers the switching frequency must be well above the audible range, so well above 20KHz. So such amplifiers radiate well into the RF making any near-by a.m. radio unusable. Furthermore, you cannot use them as a stereo pair because they interfere with each other causing whistling. Class D has therefore never been used since for audio amplifiers. It is however the standard way of controlling low voltage motors where the switching frequency can be significantly lower and cross interference is not a problem. I used pwm exclusively when I traded as 4qd, manufacturing and selling motor controllers. But I digress!

The X-20 was significantly larger than the X-10, and had a lot more components. It was then more expensive to produce. It was never advertised in Wireless World, but adverts did appear in some other magazines, starting at the end of 1965 but only for about 3 months. It was soon replaced by the Z12. But for some time Jim Westwood and I were both working hard on different amplifiers: Jim on the X-10 and X-20 and I on the Z-12.

One day we noticed the ceiling above Jim, which was covered with polystyrene ceiling tiles (it was 1965!) into which were stuck the leads of many power transistors where Jim had been clipping them from X-20s and they had gone flying, to lodge in the tiles. There were also some above my space, from Z-12 power transistors. It very soon became a race to see who could get the most leads stuck into the tiles!

The X-20 seems to have sunk without trace: a search of the www has revealed no photographs but I have found a digitisation of an article in Radio-Electronics, September 1967, which gives a picture, and the circuit diagram. Contact me if you are interested,

Sinclair Z-12

z12_phys/gif Although the X-10 and X-20 were problematical, the response to them had proven to Clive that there was a huge market for a relatively cheap amplifier. So once the Hong-Kong radio transistor affair was over, Clive gave me a circuit and a rough sketch of a new amplifier - the X-12 - which I was to engineer into production. This I did, and wrote the instruction manual for it. This all went so smoothly that today, over 50 years afterwards, I cannot remember much about its development!

The Z-12 was released in 1965. It sold in large numbers until it was replaced by the Z30 in 1969. Of course, as with any product for the DIY market, there were a few problems. The main problem was that the circuit board was too thin and too brittle: it was made of SRBP - Synthetic Resin Bonded paper. Although it was sold in a bespoke box, if the GPO dropped the box on the wrong end, the component end would get bent upwards and the board would break in two as the heavier end with the power transistors and heatsink was held fairly tightly in the box by the instruction manual which had been folded up to work as cushioning. Nevertheless, we received back quite a lot of boards broken in half!

As I was adept at repairing failed Z12s, I used to run the service department, initially doing most of the repairs myself in the evening, paid piecework rates. I could even repair a smashed Z12: the SRBP material of the board bonded well using the new PVA wood glues and the tracks could be repaired with tinned copper wire.

The other problem was the power transistors: Clive sourced manufacturer's rejects. These were re-tested before shipment to us, but the test specified was quite minimal and did not always reject bad transistors. However when the Z-12 was first designed the germanium power transistors were of fairly good quality, albeit rejects from, I think, Mullard who were the main manufacturers in those days.

There is a good amount of technical information available on the Z12, which I may compile into a separate page. Contact me if interested.

Cambridge and Thetford

In February 1965 the Z12 was being geared up for production and I was sent up to Cambridge to sort out production and testing at Harvey Hall Electronics Ltd. in Thetford: they were in the old Methodist Chapel in Earls Street (photo, right, shows the premises as they are in September 2016), which is nowadays a snooker hall (and is currently for sale).
Harvey Halls factory in 2016
Thetford snooker hall and old factory.
The main electronic assembly was in the old chapel but there were adjacent buildings (in left of photo and currently vacant) where Harvey Hall Ltd. had mechanical facilities: lathes, drills, milling machines etc. They also had printed-circuit board production facilities and manufactured many of Sinclair Radionics early circuit boards. I never saw that part of their works, but Sinclair later also used these mechanical facilities.

While in Cambridge I roomed in Sawston with Glenys, one of the women who worked at Sinclair's other premises at Comberton Village hall. I forget the exact time I was there, probably a few weeks. Some days I would drive from Sawston to Harvey Hall's factory in Thetford. But a lot of the time I went to Comberton to help sort out problems there. And did they have problems!

In those days, Comberton was given as the contact address for sales and service. It turned out that the Micro-6 radios I had seen for repair were the very small tip of a very large ice-berg. There were piles of returns, unattended! One of my main tasks was to sort these out. There were letters to be answered. In truth - that period of Sinclair's history is best forgotten. Which is exactly what I seem to have done!

Sinclair Micro-FM

Micro-FM/jpeg The Micro-FM saw the light of day in 1965. This was an apparently-innovative design using pulse counting. It relied on the very high gain transistors we had saved from the Associated Transistors deal. Years later, I found that it was not at all original: it was a solid state version of a valve design in Practical Electronics in the 1960s. Searching the www divulges the circuit of this (or at least its desendants) pulse-counting FM receiver to be almost identical in detail but with valves replaced by transistors and values changed appropriately.

Contrary to Alfred Mark's comments reported on Planet Sinclair the Micro-FM worked very well. But there was a problem that was not at first apparent: the detector transistor had to be extremely low gain - around 30 if I recall correctly - a gain that is essentially unobtainable these days! Any higher and the signal would distort: tuning would broaden out and a poor audio could be heard only if the dial was off-tuned slightly. Once I had discovered this and instigated testing of suitable transistors, that problem was solved. Fortunately with Clive's policy of buying bulk reject transistors such low gain devices were not too difficult to find!

But in 1965 FM coverage was not consistently good. A hand-held portable was subject to weak and variable signal in many areas. So any portable FM receiver can be temperamental. The Micro-FM was no different in that respect! Also, being hand held, even touching it could affect the signal strength in marginal conditions.

The other problem with the Micro-FM was that it was a kit: it used resistors mounted on end, tightly packed together, and it had quite a high component count. So there was a lot of room for constructor error, and if resistors weren't bent tightly or were misoriented then their leads could touch.

Sinclair's advert boldly said It can be built by anyone. And a lot of "anyones" tried - and failed. Once again, it was I who did all the servicing. Its construction made it difficult to remove components, for you had to unsolder two or 3 joints while holding the board and pulling on the component. I remember a jig I made to hold the board. It was hinged so the horizontal board could be flipped to the vertical where it locked, or released so it could turn over 180° for soldering. Behind it, on the component side, was a rod along which could slide a spring with a hook on its other end. I would stretch the spring and hook it onto the component to be removed. Then when the solder was melted the component would be pulled free, sometimes to ping across the room.

There was great satisfaction to be had from receiving back what was essentially a pile of rubbish and returning to the customer a perfectly working Micro-FM! Many were the boards I stripped quite extensively and re-built using this jig.

There is a good amount of technical information available on the Micro-FM, which I may compile into a separate page. Contact me if interested. Already available is the circuit of the Micro-FM.

Sinclair Z-120

Z-120/jpegBecause of the ostensible performance of the X-10 and X-20, Sinclair had won a contract to design and build an amplifier capable of an output of 120 watts to drive a vibrator for the aircradt industry. Jim and I were not aware of this until February 1965 when we had to make room for a new chief engineer, Martin Wilcox. He had been employed initially to design the Z-120.

Shortly after Martin joined, Clive went to Hong-Kong, following the transistors that had been shipped: it had been suggested that I should be sent as I had been working on the radios that were to use them, but in the event Clive decided it was too important a job for a newbie. It did not take Martin long to realise that doing a 120 switchmode amplifier was impractical. So Martin went for the technology he knew best and designed a full-bridge class AB amplifier fully capable of meeting all the specifications laid down in the contract. It was a 19 inch rack-mounting design which was made on time and the contract was fulfilled. But the Z-120 saw very little other action: what market was there for a 120w amplifier in those days? Maybe Clive Sinclair was right not to push it. But there were later products which were not Clive's vision and somehow these also did not succeed as they might have.

The Z-120 was mentioned in an editorial in the March 1967 Wireless World, page 47/147 (from where the picture above came). It was a successful product but unfortunately I have no technical information on it. Certainly I never had to repair one! If you have any details I would love to hear from you.

Sinclair Stereo 25

In 1965, having got production of the Z12 rolling, it was a clear success. The instructions had shown how to fit tone controls to it but a hi-fi amplifier without a proper pre-amp was hampered. It was clear that there was an untapped market. So Clive sinclair one day came into the lab with a circuit and design ideas for a stereo preamp.

Sinclair Stereo 25 preamp
Sinclair Stereo 25 preamp
Harvey Hall Electronics in Thetford had been making the "heatsinks" for the Z12 and the stereo 25 was designed with their engineering facilities in mind. It had an aluminium chassis on which were mounted the controls. The circuit board was parallel to the chassis mounted on four pillars. A brushed-aluminium panel was supplied which was to be mounted on the front of whatever box the user installed it into. The whole was finished off with brushed aluminium turned knobs. Although a pleasing and well-engineered design all this bespoke metalwork did not come cheap, so the Stereo-25 retailed at just under £10 (£9.9.6d). The Z12 in contrast was just 89/6d - £4.6.6d - half the price of a Stereo 25.

Because of this engineering, the Stereo 25 was, in some ways, the best pre-amp Sinclair made: it had proper stereo pots - unlike the later units which used pre-set pots.

And then a power supply was needed - the PZ3. This was a transformer mounted on an L-shaped bent aluminium sheet. Unfortunately Clive had specified 1/16" (1.6mm) for this - which was far too thin. The transformer was mounted via its own bracket to the base using 2 BA (about M3.5) screws and often the weight of the transformer bent the aluminium chassis whilst in post. The circuit of the PZ3 was innovative - it used a gyrator as a smoothing choke. There is more on the PZ3.

The stay at Duncan Terrace did not last long - Sinclair out-grew it within two years and we all moved to Cambridge in April 1966.


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First published Tuesday the 30th of August, 2016
Last modified: March 01 2022 14:42:24.
Written by Richard Torrens.
© 2016 - 2024 Richard Torrens.