Author Topic: Renovating an early Tektronix 475 Oscilloscope: A Few Random Hints and Tips  (Read 1371 times)

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Offline tggzzz

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I recently noted one repair I'd made to a Tek 475, and some people said I should write it up and not leave it buried as post #7623 in that thread. So here's the edited highlights of the repairs to the timebase, the vertical sensitivity control, sticky switches, and a couple of other boring things; the full gory details are probably only of interest to someone that has the same problem(s).

Timebase

The principal problem was related to the microswitch which detects whether the B timebase is locked to the A timebase, or is on a faster sweep speed. The timebase knob contains a pin which rests on an asymmetric coaxial white plastic lever, which rests on the black microswitch (on the right in the pictures). The knob pushes the pin in one direction, and thereby pushes the microswitch lever down. The force in the other direction is provided by the microswitch, not by a dedicated spring – an irritatingly cheap and nasty design. That force was insufficient to reliably move the pin.

Pin by 1us/div:


Knob, Pin, Lever, Bezels:


A/B Independent:


A/B Locked:


The root cause was that the pin was sticking slightly inside the knob, for reasons unknown. There was no room to add a spring, so I considered using a drop of light machine oil, but decided it would probably be too viscous for the weak force from the microswitch. Disassembling the mechanism and repeatedly rinsing with IPA solved that problem.

Due to the limited space, reinserting the timebase assembly is an interference fit! It is easier if you remove the assembly, pop out the small white front-panel plastic bezels, re-insert the assembly, and finally pop the bezels back in.


Vertical Sensitivity Controls

These were also a pig, but if I had realised how bits were held together, it would have been significantly easier. The principal problem was that some of the HF compensations were grossly inaccurate. Avoiding some of the blind alleys mentioned elsewhere...

Some of the compensation capacitors in the attenuators were extraordinarily sensitive to their position, in a way suggesting a poor contact. I decided to carefully re-solder the mounting pin connections inside each attenuator, since some have reported that can improve things. It did improve the sensitivity, but did not resolve the principal HF compensation problem.

The attenuator and channel switch assembly were then removed. The four 1/4 inch nuts that secure it to the front panel are a pig. There’s no way a spanner can reach them, I don’t have a 1/4 inch nut runner/spinner, and neither do any of the local shops. Fortunately I realised that extension shanks for bog-standard screwdriver bit sets are 1/4 inch; they just fitted in the space available and the nuts were removed.

1/4" Nuts Behind Capacitor, Hex Standoff Foreground Left:


After removing the attenuator and switch assembly it became clear that it would have been sufficient to remove only the 1/4 inch nuts plus the hex standoffs between the attenuator PCB and attenuator shield. The preamp board and switch assembly could have been left untouched. Oh well, live and learn.

The principal contribution, from Fabio Trevisan, was to note that adjusting the attenuators requires a “normaliser”. I had been following the service manual and using
  • a 2.5V 50ohm step generator with <1ns risetime
  • an RF attenuator to reduce the amplitude
  • a 50ohm through terminator attached directly to the scope input
That gives the correct amplitude and risetime, but the 50ohm gives invalid time constants when driving the 1Mohm attenuator sections. A normaliser mimics the time constants of a *10 probe, and consists of a 1Mohm//20pF in the signal line between two BNC sockets. The normaliser is inserted between the 50ohm through terminator and the scope’s input.

I quickly threw the relevant components together and there was a significant improvement – but it was still far from perfect. Eventually I noticed that the corner edge was better/worse depending on the pulse’s amplitude, and that was the case within a single range and on a known good scope. Now I had used a 20pF C0G capacitor, which is supposedly not voltage and temperature sensitive; either I have a counterfeit component or there is a noticeable voltage sensitivity. Replacing it with an air dielectric trimcap removed the problem.

Finally I was in a position to tweak the attenuator’s capacitors. After a few loops this was achieved, resulting in a nice 1.8ns risetime on all vertical sensitivities and with *10 probes or a direct  50ohm terminations. Nonetheless, I'm surprised that deviation from the service manual was necessary.


Sticky Switches

The electrical "suboptimalities" were tackled with Caig DeOxit.

Some of the front panel multipole latching push button switches were reluctant to mechanically pop out to their normal position without assistance.

The problem was that the small pin under the bronze spring was sticking rather than jiggling around. A small drop of light machine oil solved that, and all switches work well.

Latching switch:



Miscellaneous

Traditional remedies sufficed for noisy pots and the u/s fan. FFI: https://entertaininghacks.wordpress.com/2018/02/27/renovating-a-tektronix-475-timebase-switches-potentiometers-and-hf-response/
« Last Edit: February 27, 2018, 08:06:57 pm by tggzzz »
There are lies, damned lies, statistics - and ADC/DAC specs.
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Online bd139

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Excellent notes. Thanks for writing them up. I’m suffering from noisy vertical position pots on my 465. This is motivating enough to get me to look at it. Thank you :)
 

Offline tggzzz

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Thanks.

As noted in the full version, don't expect perfection. I'd also wait to see if there are informed comments about my procedures, before rushing in.

I'm unsure as to the real significance of the difference between DeOxit and DeOxit FaderLube. As far as I'm concerned there are far too many kitchen/bathroom/window spray-on cleaners and they are interchangeable (scouring powders excepted :) ), and I'm not sure that Caig isn't following those marketing principles. Nonetheless, it might be important to have the "for carbon tracks" variant inside Tek pots.
There are lies, damned lies, statistics - and ADC/DAC specs.
Glider pilot's aphorism: "there is no substitute for span". Retort: "There is a substitute: skill+imagination. But you can buy span".
Having fun doing more, with less
 

Online bd139

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To be fair I'll probably just use WD40 contact cleaner (not the normal WD40!) on them. I've done this with a couple of crusty Heathkit carbon track pots before and it worked a treat. The main thing that scares me is dismantling the pots in situ.
 

Offline tggzzz

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To be fair I'll probably just use WD40 contact cleaner (not the normal WD40!) on them. I've done this with a couple of crusty Heathkit carbon track pots before and it worked a treat. The main thing that scares me is dismantling the pots in situ.

Dismantling would be an overstatement of what I did!

Undo as many screws as possible, jam a jewellers screwdriver in incipient cracks between different bits of plastic/metal, twist so there's a 0.25mm crack. Get a drop of whatever on a screwdriver, place on crack and wait for it to move in via capillary action. Crude? You betcha.

I suppose you could try to resolder the pot from the board, then disassemble on the bench.
There are lies, damned lies, statistics - and ADC/DAC specs.
Glider pilot's aphorism: "there is no substitute for span". Retort: "There is a substitute: skill+imagination. But you can buy span".
Having fun doing more, with less
 

Online bd139

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I think I'd have to take the vertical amp out to get at the bottom pot. And on the 465 it is a relative bastard to get out compared to the 475 and 465B unfortunately so I will try your "open a crack method" then blast it.

If it doesn't work or makes it worse i'll have a legitimate excuse to buy another oscilloscope so win win.
 

Offline tggzzz

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I think I'd have to take the vertical amp out to get at the bottom pot. And on the 465 it is a relative bastard to get out compared to the 475 and 465B unfortunately so I will try your "open a crack method" then blast it.

If it doesn't work or makes it worse i'll have a legitimate excuse to buy another oscilloscope so win win.

If it a nice scope in other respects, it wouldn't be a win.

My 475 pots (and IIRC the 465 pots) are black and brown plastic, in several layered sections along the axis, all compressed by screws from the base. I cracked the joint between the various layers.

I've just had a look in a 475A+DM44. The relevant pots are blue plastic, and are of a different construction to the 475. I'm unconvinced the crack and drip technique would work with those blue pots.

Summary: don't trust me, use your nous when you examine your pots :)
There are lies, damned lies, statistics - and ADC/DAC specs.
Glider pilot's aphorism: "there is no substitute for span". Retort: "There is a substitute: skill+imagination. But you can buy span".
Having fun doing more, with less
 

Online bd139

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That’s the ones. The blue ones are the later Bourns ones and are riveted shut. Those seem to be ok so far, touch wood. Not had a bad one yet. I did consider an attempt at finding a generic replacement but it turns out that they are custom ones with two different resistance tracks so that’s off. Bourns will do custom ones but they are £60+ a pop and you have to pretend that you’re going to order thousands of them.
 

Offline BravoV

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Though don't have the Tek 4xx scope .... "yet"  :P ..., apart from the great info, really appreciate the effort on the details, sharp close-up shots, bookmarked and thanks.  :clap:


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