Tektronix model 453 Oscilloscope

images and remarks

ralphk klimek June 2009

abstract:  Defunct Model 453 Oscilloscope repaired, recovered from dumpster

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I retrieved this device from a dumpster.  Initially I was quite pleased, as normally, things are not thrown out because they are merely defective but that they are no longer bright and shiny. Alas, in this case, it was thrown out because it had ceased to function.

This was one of Tektronix'  first fully transistorised scopes, apart from a number of Nuvistor triodes that were used in the circuit where one might use a FET today.  Mine was not functional, it had no horizontal drive at all. The beam finder revealled that EHT and other major circuits, including vertical amplifier were working.  The beam finder also showed that some of the horizontal drive and deflection amplifiers were still partially alive.  This was clearly repairable !  Luckily, the BAMA archive had the service manual for a device that was closely related , sufficiently close such that there were very close similarities in the horizontal deflection circuits that permitted me to debug it.  The true Model 453 used Nuvistor Triodes in selected circuits, my 453 did not use Nuvistors ! must have been a slightly later model.

The cause of the fault was fairly clear.  The top of the instrument had some ventilation holes , small enough to keep out dust, and keep in the hot air, (!) but not small enough to prevent moisture ingress. It would appear that at some time in its long life both a mystery fluid "X" and rat urine had contaminated the horizontal deflection amplifiers.  Someone had also immediately sprayed it with WD40, the sticky residue was still apparent.  When I attempted to solder the board, the unmistakeable odour of rodent urine revealled the identity of one the the mystery contaminants.  

Rat urine , after 40 years, does bad things to electronics. Many of the components showed some corrosion and some had corroded to oblivion.  Would I persist ?   The instrument to very close to being fully functional it was... and I had a manual.  Without the circuit I would have certainly permitted this instrument to continue its journey to landfill, where it could continue its demise by adding to Melbournes' groundwater toxic heavy metal overburden, but lets not mention that!  I decided to persist.

All the instruments transistors were mounted in sockets,  an anachronistic effort, but consider that all the other Tektronix products used thermionic valves where this was the accepted practice. Active devices , including the "new-fangled" solid state types went into sockets. Some of the sockets had corroded to pieces. The plastic transistors had not survived, their leads had corroded away. I dont think the quick application of WD40 at the time of the original liquid spill had penetrated into the component sockets.  I replaced them with random equivalents.   The main horizontal deflection amplier transistors leads fell off when I tried to move them, corrosion had completely severed the leads right against their berylium oxide heatsinks.  No problem, I used some old high speed high voltage video amplifier CRT driver transistors recovered from a discarded VGA monitor.  Some circuits also used the impossible to replace Tunnel Diodes, but these were still functional.  (whatever happened to the Tunnel Diode? Tinfoil Hat anyone ?)

I had only to replace about 5 completely damaged components and the instrument springs back into life. The horizontal scan is not as linear as could be but at least it lives and is usuable as it is.  Not bad for something built in 1965, and its vertical amp was rated to 50Mhz too! I was just starting school then.
Build quality , as you would expect, from a technology pioneer like Tektronix was superb. If it were not for the possibility of liquid ingress this scope would still work in another fifty years.  The screen size is very small. Hewlett Packard had bigger and brighter screens but they had the patent for a special final mesh accelerator and focussing electrode geometry.  This scope's faceplate had a die cast mounting ring that I would imagine had been used with an specially attached camera. The original deflection transistors ran from a 75V supply rail, so the available deflection voltage was limited. Their older valve based scopes had bigger screens as they had the ability to amplify higher voltages.

I will probably take this scope to work with me, for the few times that I still need a scope for. Not bad for forty odd  years, eh ?

see also

Not dead yet,  Im alive !!!  side panel with aux controls and outputswhite ceramic (BeO?) heatsinks and my replacement deflection transistors
horizontal deflection and triggers and rat's gunkgood old linear PSUview of vertical amp and crt
vertical deflection ampunderside vertical input ampunderside vertical input amp
horizontal amps,triggers,timebaseunderside vertical input ampsideview showing cast aluminum chassis


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page created   Fri Jun 19 18:54:51 EST 2009