Solartron CT 436
This is a dual beam scope designed for the UK military I would guess in the late 50s. I come from Farnborough in the UK where a Solartron facility is located so I have an interest in having an example from Solartron in the collection. This design was also built under contract by Hartley. I like its small size of 10H x 10W x 15 long and low weight of 28lb (with the AC power transformer fitted, more on this below). Below, it is showing a 100kHz square wave that is triggered off the 10μS time marker. It is both accurate and linear!
The one I have (serial number 79255) turned up on eBay, located in Canada. It is a decent but basic “almost two scopes in one box” having a bandwidth of DC to 6MHz and a sweep range of 500 mS/cm to 1 μS/cm. The time base can operate in repetitive or triggered mode and the triggering works quite well. The downside of the dual beam approach is that the Y amplifiers cannot be linked in add mode so it is of limited use in investigating difference signals. I do this a lot and so this scope, while very neat, would be of limited use to me if it where my sole scope.
When I received it, it was not quite working, apart from the usual need to lubricate switches, tube pins and pots, the focus and brilliance controls were not working, one beam being just visible. I traced that fairly easily to a very leaky unblanking coupling cap, having replaced that, it works rather well.
The unblanking technology is interesting and (was) new to me. (I subsequently came across similar technology in an earlier Dumont designed AN/USM – 32 scope.) With a triggered scope, the beam is blanked except on the forward sweep when it is unblanked by the sweep gate signal (which is switched by the trigger); the problem is that at slow speeds, DC coupling is necessary to maintain the trace brightness across the screen. Since the CRT grid is usually some 1500v or more below ground, this is a problem! John Kobbe of Tektronix solved it in his design for the 315D by providing separate windings for the CRT cathode and grid on the (HF) HV transformer and strapping the ground end of the grid supply to the unblanking driver. Interestingly, Allan Dumont did not believe that Tektronix had DC coupled unblanking until a Tektronix engineer, Frank Hood, showed him the circuit upon which he remarked “I thought of that but did not believe it would work”. Furthermore, when John Kobbe proposed his solution, Frank Hood resisted it because he too, did not think it would work!
The method used here is to ac couple the unblanking signal to a bistable that is riding on the negative end of the CRT cathode supply (in this design). A separate small 140v supply, also riding on the CRT cathode supply provides the operating voltage for the bistable. When there is no sweep gate, the bistable rests in the off state, when the leading edge of the sweep gate arrives, the bistable switches to the on state, driving the CRT cathode negative and the CRT on; then when the trailing edge of the sweep gate arrives, the bistable is reset positive to the off state, cutting the CRT off. This technique provides a constant unblanking potential, no matter what the sweep rate is without DC coupling to the sweep gate.
Another interesting feature is the provision of a 400nS delay line in each Y channel to facilitate viewing fast leading edges. Notably, this is the only design outside of Tektronix that uses ceramic strip construction. These guys meant business with this design! Note; Tektronix originated ceramic strip construction which allowed for complexity with reliability. No more “rat’s nests”!
The power supplies are mostly regulated which does make for stable calibration. Other than gas stabiliser tubes and the CRT, there are some 23 valves, 14 of them being gold pin Mullard E88CCs. (Audiophiles, hands off!) The CRT is a neat dual-gun GEC DHM9-11 3 1/2″ flat face unit operating with an accelerating potential of 1.75kv. Here it is, sternly guarded by a pair of 50 triodes.
An interesting feature is that the power transformer (below) pulls out to permit a squalid state DC-DC converter to be fitted permitting operation on 24-30 vdc. Hmm, that would date the design in the 60s rather than the late 50s. The picture on the right shows the left side of the scope and the open space on the lower left is where the transformer or DC-DC converter goes. The panel is where the line cord goes, there is another line cord location on the back of the transformer and the switch selects which location is active, quite neat and mostly useless!
Here is a picture showing the rucksack handle on the back and the alternate power cord location. I guess the design team were envisaging hauling the bloody thing across muddy fields, up hill and down dale! The front view shows the compact 10 x 10 format and 1μS markers again showing good accuracy and linearity.
Below is a top view showing the ceramic strip construction (after Tektronix) and near the bottom, the delay line for the Y1 channel. The picture on the right shows both delay lines and the cover over the CRT supplies.
The next picture shows the underside, again revealing the space for the power pack. Right under that space, you may observe a purple wire that connects to the (physically small) replacement unblanking coupling cap which is mounted on top of the leaky one. I left the original in place to preserve the look. To the right of the oil cap, the tube shield contains the unblanking bistable referred to earlier.