Hints and Tips for Renovating an early Tektronix 475: Timebase, Switches, Potentiometers, and HF Response

I recently picked up an early Tek 475 scope plus service manual plus probes at a hamfest. I asked what was wrong, and the seller indicated the timebase was unreliable. Fair enough and, given the low price, at worst it would be a parts mule.

It was a bit mucky on the outside, as would be expected of something that had been in storage. A quick look the scope showed it was cleaner inside than I expected, and playing with it made me feel it was worth resuscitating. If nothing else, the puzzles involved would be more interesting and worthwhile than crosswords and soduku.

Overall the renovation involved some traditional problems and solutions, plus a couple that are less well-known – and a few diversions down blind alleys. Maybe the hints and tricks outlined below will save other people some time.

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What could kids do in the summer holidays?

For no good reason other than avoid the >30C heat, I browsed some of the old usenet postings I’ve saved over the years. I’ve reproduced one below to illustrate how things have changed, maybe for the better, maybe not. I have a couple of such books, and can confirm that they do have chapters on building and using your own X-Ray machine, including such gems as noting that if your skin reddens you are probably using it too much.

Now I must get back to doing something more up-to-date, such as using the rather interesting XMOS xCORE processors to implement a 12.5MHz ratiometric reciprocal frequency counter purely in software. That’s possible since the have the best hard realtime architecture I’ve seen in decades. Highlights – which enable deterministic timings calculated by the IDE tools before execution – include:

  • many 100MIPS cores
  • no caches/interrupts
  • switching fabric for i/o ports plus inter-core and inter-chip comms
  • programming in xC, i.e. C with parallel extensions based on CSP with timing
  • FPGA-like i/o ports: SERDES, strobed and master/slave ports,  timed input and timed output (4ns resolution), multiple programmable clocks

Great fun, and I don’t know of any other processor with those attributes.

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A Weston standard cell – an introduction to voltnuttery?

Weston standard cells are pretty, but can they be more than a broken antique? And yes, that is liquid mercury visible at the bottom of the left leg.

One Cell

One Cell

I remember using Weston cells at school during physics lessons to measure voltages more accurately that could be done with a standard analogue meter (DMMs being years in the future). The measurements are based on using a 1m ruler plus resistance wire plus an uncalibrated NiFe cell as a bridge, and use a Weston cell to calibrate the bridge. That technique (which is a good example of using simple materials, understanding and imagination – rather than merely throwing money at the problem) is outlined below.

Anyway, I have vaguely wanted a Weston cell ever since, a little more so since I got hold of an HP3468 and have wanted to check whether it is still “accurate”. I’ve wondered about buying one on ebay, but they are unjustifiably expensive and sufficiently fragile that I doubt they would survive shipping.

Unfortunately I’ve now found a cheap one, and I fear it might lead me down the expensive path of voltnuttery. Memo to self: do not become a voltnut.
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Renovating a Tektronix P6013A 12kV 100kHz Scope Probe

I’ve recently had to debug several scope’s 2kV-3kV HV supply and the CRT’s Z-axis waveforms at 2.5kV. So far I’ve got away with using a homemade 1000:1 voltage divider and a multimeter. Since that is crude and not particularly safe, I’m not going to mention the details in order to avoid someone apeing me and hurting themselves.

Then, at a recent auction I managed to pick up:

  • a 40kV meter for measuring 17kV anode voltages, but which barely registers 2kV
  • a Tek P6013A 12kV 1000:1 100kHz scope probe

The probe was functional but missing part of the handle. While not strictly necessary, I wanted to have a little fun fabricating the missing part…

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A Traditional TDR Cable Tester With <2cm Resolution

What can you see, test and measure with a traditional time domain reflectometer (TDR)? The answer is “more than you might expect”:

  • measure impedance variations in connectors/filters/antennas/PCBs
  • locate short/open circuits and damage in cables
  • locate intermittent faults in cables and connectors
  • locate connectors in cables

and can resolve discontinuities around 2cm apart. That resolution is at least 10 times better than can be obtained with the typical homebrew logic pulse + oscilloscope combination.

I recently bought a couple of cheap 1970s Tektronix 1502s in the hope that I could make a single working frankenmachine.  My initial assessment was depressing: one had a cracked and broken case (so I assumed the CRT was also broken), the other’s electrolytic caps had spewed acid across the PSU and had a faulty 2kV PSU, and both had defective NiCd batteries – and it won’t even start without a working battery. But eventually I managed to get both working: I recapped the PSUs, rewelded the case with methylene chloride, used my “new” 12kV scope probe and 40kV meter to repair the HV PSU, created a “NiCd emulator”, and the CRT wasn’t damaged after all. Later reading of a TekScope magazine indicates it isn’t surprising the CRT survived: it is mechanically completely isolated from the chassis to protect against up to 26 12″ drops.

So I am now the proud possessor of two nice little portable waterproof instruments, literally designed for field use – one of the service manuals indicates they were used with Patriot missile defence systems.

Tektronix 1502 TDR Cable Tester

Tektronix 1502 TDR Cable Tester

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Rescuing a Broken Tektronix 465 CRT

A non-functional Tektronix 465 oscilloscope with a dented case and several serious faults was recently donated to my local HackSpace. Since they didn’t want another broken scope, I took it to see what was wrong. Eventually, after learning about CRT theory, with the help of very knowledgable people on the TekScopes forum, and gingerly measuring waveforms at -2450V, I came to the conclusion the CRT’s grid was faulty. Reluctantly removing the CRT, I found that it could not be repaired. Since parts of the CRT are rather beautiful, I decided to salvage those parts for display. This note outlines the original tube, the fault, and the process of turning it into something fit for a display cabinet.

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Images of Late-70s Burr-Brown Thick-film Hybrid ICs

In the late 70s Burr-Brown made some of the most advanced analogue ICs on the open market. Technology limitations prevented the full integration of many components, so the “thick-film hybrid” IC was used instead. As a publicity gimmick Burr-Brown distributed a calendar of the some of the ICs internals.

For no reason other than the pictures are pretty in themselves, I kept them. Now I’ve finally got round to crudely digitising them. Here they are…

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Vetinari Digital Clock

I wanted to resurrect the attractive digital clock I made in the early 1970s, but it’s been a very long time since it was interesting enough to bother hanging it on a wall. Mutating it into a “Vetinari Clock” has made it, once again, into a talking point.

You can buy modules that convert conventional crystal-controlled analogue clocks into Vetinari clocks, but they have disadvantages. Firstly they require more than minimal conceptual changes to my clock, e.g. timing being derived from a crystal rather than by counting the 50Hz mains cycles. More importantly it simply doesn’t look right: digits ticking over instantaneously have much less visual impact than a second-hand moving slower or faster. Hence I made my own module, and it turned out to be a short and fun hack. Continue reading

A 40 Year Old Hack, Disinterred

Digital clocks don’t look very exciting nowadays, but things were very different 40 years ago. Since it was the first LED display and first digital clock that friends and neighbours had ever seen, it became a talking point. One person even actively disliked it – because it showed her life ticking away. Presumably none of her mechanical clocks had a second hand.


Size: 8″ by 8″, with 0.7″ LEDs

I’ve a certain fondness for the clock because it is my earliest surviving hack, looks reasonably pretty, and is a memento of time spent building things with my father. I’d wondered about putting it back on the wall, but didn’t get around to it because – even if it worked – it would be boring and quaint rather than a talking point.  Then I realised I could turn it into yet another Vetinari Clock, so that maybe it would once again be a talking point amongst normal mundanes. When I dug it out it still worked, slightly surprisingly.

Then as now, it was difficult to find cases that are æsthetically pleasing. The solution was to find a very 1970s style decorative ceramic tile, and to cut out a slot for the display. The case sides were made from widely available painted wooden moldings. Suitable bezels were unobtanium, so smoky perspex was cut to size, rounded off, and simply glued over the slot. Looks fine from normal viewing distance

So, what did I find inside? Continue reading