The Sinclair Slimline

Way back in 1963, when I was 5 years old, a guy by the name of Clive Sinclair marketed his first transistor radio. It was only available in kit-form and cost two pounds, nine shillings and sixpence. The most astounding thing about this little radio was not its performance, but the marketing 'spiel' that brought it to the attention of those who read the various electronics constructor magazines of the day. See the advert below ...









I will admit, the wording in the advert on the right is probably correct where it says 'You've never seen or heard a transistor set design like this before'. But there after, we are courted with phrases like 'staggeringly good quality' and 'entirely new Reflex Circuit'. The advert also makes a huge fuss about the use of 'genuine MAT TRANSISTORS' and an 'elegantly designed plastic case'.

Yes, its very small size immediately grabs your attention and very likely gives it that 'want factor'. But is the resolved audio of staggeringly good quality? Nope! And attributing this quality to the internal ferrite rod aerial? ... well, these are the words of a copy writer with no technical understanding.

Then we read that the set employs an entirely new Reflex Circuit design ... Really? Such designs were used in the first commercially available radios back in the 1920s and 30s. The Sinclair Slimline was probably the first commercially available transistorised radio using this technique/design, which is a form of TRF, or Tuned Radio Frequency.

The fuss around MAT transistors is actually quite understandable since transistor technology in the 1960s was rapidly evolving, with new manufacturing techniques resulting in higher manufacturing yields and better performance. The earliest transistors were what are known as 'grown-junction' types. Philco developed the Micro Alloy Transistor, which was relatively cheap to produce whilst at the same time yielding greater performance ... hence the enthusiastic touting of MAT transistors.











The photograph on the left shows a 'grown-junction' transistor on the left with a (dead) Philco MAT121 on the right. I remember buying packs of unmarked transistors in the early 1970s, many of which were likely Philco rejects. It is a known fact that Sinclair bought sack-loads of rejected transistors from Plessey and Texas which were then painstakingly tested and earmarked for future use. Given that the transistor in the photograph is clearly marked MAT121, I suspect this may not have been a reject.







Right: This is the PCB of the Sinclair Slimline that I was asked to 'get working'. The PCB is Paxolin. I don't know who the constructor was, but the soldering isn't great. I suspect it was done with a low wattage soldering iron like an Antex C15, or whatever it was called in the 60s. I have one somewhere. I immediately recognised the over-size blobs synonymous of underheating the solder. Oh, how I love my Weller 50W irons!

The current owner suspected that the transistors might be dead, whilst my money was on the two 1.25uF electrolytic capacitors. As it was, we were both correct, although only the MAT121 was u/s. One tester was unsure what it was, whilst the other said it had an hFE of 1! The two electrolytic capacitors were o/c, the 5nF capacitor measured 3nF, one of the resistors was significantly high and another was intermittent due to an extra-short lead not quite reaching through the board ... that was an interesting fault. Significantly, the non-earthy end of both windings on the ferrite-rod aerial were snapped.


The 'curly' black thing at the top of the board is a 'gimmick' capacitor ... a way of fabricating a very small capacitance (usually less than 1pF) by twisting two insulated wires together. The actual capacitance can then be 'trimmed' by varying the tightness or number of twists. Had this been a 1920s radio, this would have been a front-panel control labelled 'Reaction'. This is the device by which the self-oscillation of the RF stage is controlled.

Having replaced all the faulty or broken parts, I still needed to find a suitable PNP transistor to replace the MAT121. The quest for this was somewhat marred by two things. Firstly, there is precious little to pick up on Medium Wave these days, and secondly, what I did hear via the earpiece was woefully quiet. And when I connected it up to a large outside aerial, the set operated like a crystal set! Looking at the schematic that I had downloaded, I concluded that the earpiece was required to be high impedance ... a crystal earpiece. I measured it ... five thousand ohms ... that explained the low audio. In hind-sight, I now think this earpiece may have been the one that came with the Regency TR-1.

I found a crystal ear-piece and plugged it in ... much better ... more noise, but the 'gimmick' device was problematic ... it was either having very little effect, or the transistors I was substituting for the MAT121 were inappropriate. I eventually replaced the 'gimmick' with a modern 0.5pF trimmer capacitor and found that a Mullard OC45 worked rather well in place of the MAT121. I was able to tune in BBC Radio Scotland, BBC 5-Live and another station that I think is Talk-Sport.



As can bee seen from the above photograph, the OC45 needs to be bent over in order to fit in the 'elegantly styled' pill box. You can also see where the broken aerial windings were repaired. At the moment I'm leaving the modern trimmer in place of the 'gimmick' However a New Old Stock Philco MAT121 is on its way and I'm sure the owner will prefer that I 'revert to the original'.

Above: The replacement NOS (New Old Stock) MAT121 fitted and working. The 0.5pF trimmer required readjusting to account for the difference between the MAT121 and the OC45. I actually feel that the receiver was more stable with the OC45, but I do agree with the owner that fitting an original MAT121 maintains authenticity. Retaining the modern reaction trimmer was not however an issue.

Sinclair's use of what I believe are known as 'tipper' trimmers was probably as much a cost-saving solution as it was convenient. The fact that these devices are exceptionally non-linear in terms of action is glaringly obvious, resulting in the useable range occurring close to maximum compression. I don't know how many small nuts were provided for fabricating the tuning shaft, but it would appear that a minimum of two are required. In reassembling the Slimline, the brass screw (8BA?) is screwed into the plastic (or Nylon?) tuning control. With the board in place, the brass screw is then screwed into the trimmer (almost all the way). A brass washer is then added and held in place with a nut which is then screwed onto the shaft/screw. Ideally at this point in the process, the dial should be set to align with a known station. Tightening of the nut that has just been added will compress the trimmer. Assuming all goes well, and your station of choice is tuned in, a second nut should then be added to form a lock-nut. Now turning the dial ought to vary the compression and thus the tuning. Ideally another nut could be fitted behind the dial to ensure that the dial does not loosen. It should be obvious that this process might require a bit of iteration to cover the tuning range without the dial fouling on the case. Also, the reaction trimmer/gimmick will likely require to be tweaked.

Conclusion: How does the Slimline perform? ... Is it the receiver everyone I know will envy me for? Not really. Performance is mediocre at best and reaction stability is questionable. But it works. Think of it as a crystal set with rudimentary amplification and selectivity. Back in 1963 it was a 'first' ... a radio receiver smaller than most transistor radios of the day, so definitely a novelty ... AND with the right tools and knowledge, you could build it yourself.