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Old 22nd Sep 2023, 7:14 pm   #1
high_vacuum_house
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Smile Tektronix 545A oscilloscope completed

Good afternoon,

I have completed the restoration of a Tektronix 545A oscilloscope which now works well.

The fan was seized, the rubber mountings had rotted away meaning fan blades were fouling the cowling. Replaced with 4BA screws and nuts and adjusted until central. Fan spins ok now.

General checkover of the instrument and clean with a hoover and paintbrush showed all was present with no valves going to air. Burn marks spotted on the CRT's phosphor on L/H side. Looks like some dimwit left a spot wobbling around with full brilliance and no timebase running!!

Delay time multiplier dial would not go all of the way round. Removed and dismantled. Found a tiny piece of swarf jammed in the gearing. Reassembled and now operating as it should.

Plenty of "bumblebee" wax paper capacitors in the low voltage power supply regulator circuits. Horrible things that make even Hunts capacitors desirable!! All replaced and all voltage supplies now correct. I was then able to correctly set up the -150V DC regulated supply which supplies the reference for the other regulated supplies in the oscilloscope.

The internal calibrator was then set up. This needs to output 100V DC on the Cal test point when the calibrator selector is in the off position. Using another oscilloscope showed it was producing a nice square wave at 1KHz.

Pilot light would not light. Found an unusual high resistance between the bulbholder and the rear of the front panel. Removed, cleaned and reassembled and now lights!

Not a dot on the screen. No EHT at all and EHT rectifier heaters hardly lit. The 6AU5 output valve was probably gassy as the getter patch had a white ring around it. This was replaced along with a couple more capacitors in the EHT oscillator circuit. All Sprauge capacitors in the EHT tripler were leaky and getting warm. Odd capacitance values. Had to get replacements from China as I couldn't find any supplier who had any in the UK. Once they turned up, I fitted them all which was not easy as I had to unsolder the EHT rectifiers as the capacitors are hidden underneath, and the wire ended valves are very delicate. I then had uncontrollable brilliance!!

More Sprauge HV capacitors in the cathode and focus circuits which were getting warm. Replacements bought and brilliance and focus now controllable. I was then able to set up the EHT supply, the CRT cathode is the test point which is -1250V with a 1000:1 multimeter probe.

Half of the timebase A positions were not running. All in the slower speeds. This was caused by more bumblebee type capacitors mounted on the timebase rotary switch. These were all in the holdoff circuit. Almost all are un accessible due to its location in the middle of the control panel. The only way to get access is to unsolder many wires and remove the complete switch assembly.

I had to buy some 1/16" allen keys to get the knobs off and after an evening, managed to disconnect but not fully remove the timebase switch. All of the bumblebee capacitors were replaced and the switch refitted and connected up.

Timebase A now working except for the 6 slowest ranges. Fault finding located a snapped off wire to the 1uF capacitor in the 3 precision timing capacitor unit. Reconnecting sorted this out.

I found that the UNCAL lamp for Timebase A was now inoperative due to a wire which had snapped from the UNCAL lampholder when removing the timebase switch. It was a tricky repair due to the accessibility, but I managed to replace the wire without having to remove the switch assembly again.

The A trigger was rather flaky due to another bumblebee capacitor in the A trigger circuit and the trigger selector rotary switch needed cleaning. This brought about a stable trigger.

The B timebase was not working. This was a gassy 6AU6 valve which was easy to spot as the getter patch has almost disappeared and was glowing a nice shade of blue!! Once replaced, the timebase worked as it should.

The CA plugin module worked in a fashion except for the B input which was definitely not displaying a square wave!!. Removal of all of the valves and cleaning the bases and rotary switches sorted this out completely.

Now the oscilloscope was working correctly, All that was left was to remove and clean all the knobs as well as bash out a dent in the oscilloscopes right hand cover. The little lift up flap in the top still has the oscilloscopes operating manual in.

Christopher Capener
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Old 22nd Sep 2023, 7:20 pm   #2
high_vacuum_house
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Default Re: Tektronix 545A oscilloscope completed

It is a lovely oscilloscope to work with although huge by todays standards. The sight of lots of valves running in the dark make quite a spectacle as well as needing half a kilowatt of energy to run it!!

The one unlit valve in the Y distribution amplifier just needed a poke to wake it up!!

Christopher Capener
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Old 22nd Sep 2023, 9:14 pm   #3
Joe_Lorenz
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Default Re: Tektronix 545A oscilloscope completed

Christopher,
all thumbs up! Very good job! After doing some most similar operations on a 585A I can tell it needs a lot of patience and passion, but the outcome is rewarding!
I can second that some high voltage capacitors are unobtainable in Europe, and when I ordered some in the US they sent me "Made in prc".
So you have got a wonderful scope now with a bright display!
Joe
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Old 22nd Sep 2023, 9:15 pm   #4
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Default Re: Tektronix 545A oscilloscope completed

A colleague referred to them as 'Seasonal Oscilloscopes' I suppose that nowadays it would be 'Combined heat and measurement'

David
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Old 22nd Sep 2023, 9:58 pm   #5
kalee20
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Default Re: Tektronix 545A oscilloscope completed

It's a very good 'scope, highly spec'd, unusually with Sync option on timebase A as well as trigger option. Internal umpteen-section LC delay line so that the timebase can get started before the signal hits the 'Y' plates.

What does let it down is the pathetically small screen area - 4cm vertically - beyond which the electron beam disappears due to it being captured by the 'Y' plates. It must severely limit the usefulness of the CA dual-trace plug-in!

Congratulations on a job well done. (Do you want a Tek-compatible camera, with bolt-on screen mount, by any chance?)

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Originally Posted by Radio Wrangler View Post
A colleague referred to them as 'Seasonal Oscilloscopes' I suppose that nowadays it would be 'Combined heat and measurement'
Love it!
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Old 22nd Sep 2023, 10:05 pm   #6
Jez1234
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Default Re: Tektronix 545A oscilloscope completed

Very nice. Good work in restoring it also!
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