I took this one in order to get a scope-mobile which are hard to acquire since people understandably don’t want to ship them. The original model label had been removed (perhaps in shame), note the replacement label above the CRT, it sets the context for this post!
The US government issued a bid request for oscilloscopes based on the Tektronix 535 and 545 as a result of (as I understand things) a complaint from industry that Tektronix had monopolised the market. Well of course they had, their instruments were at the time, the standard bearer for oscilloscope function and quality! As a result, Hickok, Lavoie Labs and Jetronix Industries produced a large number of instruments that were blatant copies of the Tektronix instruments. It is said that imitation is the best form of flattery but these were pale copies based on what I have read and my experience of the subject Hickok 1805A. I note that not many of these copies seem to have survived, perhaps proportionally more went to the landfill than did Tektronix units, who knows?
Anyway, here it is working and in calibration after about a month on my bench. It was quite a trial, considerably more resistant to my ministrations than the genuine machines. It is now working properly and in calibration, here it is displaying a 1 mS marker and the calibrator waveform:
I read a comment regarding the Hickok 1805A on one of the forums where the author said that he thought it was subsequently produced under the Tektronix brand! That is putting the cart before the horse to put it mildly; I find it surprising that anyone who is interested in vintage electronic instruments would be unaware of the ground that Howard Vollum and his company broke. And this is not a myth, Tektronix really did develop and produce the first high-performance, reliable, triggered oscilloscope available.
This 545A copy has the Hickok 1823 Plug-In which is a copy of the Tektronix type 53/54C dual channel model which has a bandwidth of DC to 24 MHz and sensitivity from 50 mV/cm to 50 V/cm. Operating modes are A, B, A & B chopped and A & B alternate. (The later Tek type CA was the same but with the addition of A+B and A-B modes which I find to be very useful. Incidentally, the ability to sum and subtract the two channels is a major advantage of a two channel beam switch scope over a two beam scope.) Here are pictures of the 1823:
The primary patent issues were the ground breaking sweep circuit (due to Dick Ropietquet) and the distributed wide-band vertical deflection amplifier. Hickok did not contest the two issues arising from the distributed vertical amplifier patents and so the litigation focussed on six issues arising from the horizontal circuits.
Tektronix filed suit in February 1961 and the action was not settled (in their favour) until May, 1970, the final award not being settled until early 1979 at just over $4 million. This suit set a precedent whereby the US government could not longer flangrantly disregard patent ownership and that is most likely why it took so long to settle.
Here is the rather neat summary of the Tektronix horizontal circuits in question (sweep generator), cited in the litigation proceedings:
“Generally and somewhat oversimplified, the horizontal circuits in question have four components: a sweep generator, a multivibrator, a signal trigger, and a delay (or hold-off) system. The sweep generator is the heart of the circuit. Its function is to generate a linearly rising voltage which, applied to deflection plates in the cathode-ray tube, sweeps the electron beam horizontally across the tube. The multivibrator controls the sweep generator. In effect, it turns the sweep generator on and off by supplying to it one or the other of two control voltages. The signal trigger supplies to the multivibrator a pulse, by which the multivibrator is actuated. The delay or hold-off system serves to prevent the multivibrator from being triggered out of time by an incoming signal pulse and, particularly, until the conclusion of a sweep cycle.”
You may read (if you have the stamina) chapter and verse on the technical issues at http://openjurist.org/445/f2d/323/tektronix-inc-v-united-states-hickok-electrical-instrument-co
I am not going to post a lot of pictures this time, it is sufficiently similar to the 545A I posted on previously, just not as well put together. To give you some idea of what was involved, here are before and after pictures of the HV supply:
I soon discovered that it would not work properly when a dual channel plug-in was set to the Alternate mode. In this mode, the timebase sends a negative pulse at the end of each sweep to change the state of the channel switch multi, thereby turning the Y channel just displayed off and turning the alternate channel on and so on. This signal originates as a positive going pulse at the screen grid of the second stage (right hand side if you will) of the timebase gate multi (which reverts to the positive state to end the sweep) which is amplified and inverted. I fooled around with this for a long time, on and off. The pulse seemed ok but in fact was of excessive amplitude and poor shape. It finally (in the way of these things) sank in that the screen grid choke may be open. The SG was still energised via the core resistor. Well, the choke was open so the SG signal was developed across the core resistor hence the poor shape and excessive amplitude. So, I got to glaring at the outside of this choke using a magnifying glass and despite absolutely no signs of abrasion or impact, spotted one end of the break. (Such breaks usually occur on the outer layer, fortunately.) After much further glaring, having located one end, I found the other and was able to tease each end out from the criss-cross winding sufficiently to be able to tin the ends and place a single strand of wire to bridge the break. It worked and Alternate action was now available. I placed a little varnish over the repair to fix it in place. I seem to be developing some expertise in repairing broken thin-as-a-hair choke and transformer windings! It comes with the territory I guess.
Oh, and I made a copy of the Tektronix 535A/545A operators booklet that should live in the little compartment on the top: