Nightime story of a neglected Tektronix 475 with traces playing up below 20mV/Div.

Thread Starter

James55

Joined May 29, 2016
39
Hi all.


I am writing this in the hope of saving a little time as Tektronix manuals take time to digest.

Problem is;

Both channels go horribly out of focus below 20 mV/Div when upwards from 0.1 ms/Div


As I write this, I've been working my way through the Tektronix 'Troubleshooting Scopes' and '475 Service' manuals but was hoping someone with more experience might recognise the problem(s).


The story starts a couple or three years back, when I traded a old, battered Nikon binocular microscope for a dirty, neglected and 'smoking' Tektronix 475 here in Rio, Brazil.
The microscope diopter was way out, and beyond my optical skills but I thought that perhaps the Tek scope could/should be saved the from the scrap yard? At the very least it might give some valuable experience in scope repair.

After a two hour drive, my wife and I were welcomed at the door by a man whose first question was, "tell me, what do you think of Adolf Hitler?" Without waiting for an answer he ushered us inside with the declaration that he "loved him!" before telling us exactly why...
His living room was filled with generic hoarder junk and guns, with the beautiful Tek 475 on the dirty floor.
And yes, when turned on, it started to smoke immediately. Anyway, as I hadn't gone to discuss Nazi's nor guns, the exchange was done and off we went.

The cause of the smoke turned out to be a TO-92 JFET inserted in the place of an PNP (or was it the other way around?), which once replaced gave another old-timer Tektronix scope a new lease of life.

Success!!


Fast-forward to today.

Due to my not using the scope much, along with having acquired a Tektronix 465 and a blue-tube edition of a 475A, I put the scope up for sale. Obviously it is in need of a calibration but until today, had been giving nice clear traces other than the Channel 2 Volt/Div switch being a little dirty/noisy.

After a look in the manual and checking out different 4xx series repair videos I decided to check the low-voltage rail voltages and clean the Channel 2 switch contacts with IPA.

Other than the 50v unregulated TP which read 68.8v, the LV measured voltages were half respectable (assuming my Chinese DMM was telling the truth). The 50v read 52.1v for example. Once R1430 was adjusted and the 50v set, all the positive and minus voltages were correct other than the unregulated 50v which has remained around 68 volts.



On to the Volt/Div switch contacts.

As per other repairs, I removed the aluminium switch cover and lifted out the four attenuator modules before dipping some lint-free paper into isopropyl alcohol and pulling it through the closed switch contacts a few times.

It looked as though it would be a simple repair to remove the switch noise.

The 20 or 100Mhz pull-out switch contacts also received the same treatment.


So, why, when turned back on was the Channel 2 trace all over the place and fat and out of focus?

Poor quality alcohol? Alcohol hadn't evaporated?

So some contact cleaner was sprayed onto the paper as the process was repeated.
(Then I left the scope alone to have a good think about it's behaviour).

That helped.

And yes I know contact cleaner is wrong, however the scope is good again.


Back to the ongoing problem.
It would be nice to sort out this focus issue when below 20mV/Div.

Does anyone have a a good idea of where I should be looking? Or do I really have to read both manuals?


I'm off to sleep as it's late. Back in the morning.

Thanks in advance.


James
 

thedoc8

Joined Nov 28, 2012
148
You originally said it went up in smoke, but said you replaced the transistor and all was well. But what did the smoking...?
 
Last edited by a moderator:

AlbertHall

Joined Jun 4, 2014
10,380
If you have the schematic, check what is switched below 20mV. Some 'scopes have an attenuator right at the front which reduces any input down to the smallest signal the scope can see and then the rest of the Y-amplifier is operated at constant gain.

Some 'scopes have the input attenuator but it also has sections which change the gain of at least one part of the Y-amplifier. In this case the changing gain of the amplifier may lead to some instability or oscillation so the apparent lack of focus may be nothing of the sort. Does the 475 change the amplifier gain?
 

AlbertHall

Joined Jun 4, 2014
10,380
Does the poor focus affect both channels?
With no input, input selector to GND, how thick is the trace?
Is it thickest on 5mV and then getting thinner as you switch up to and beyond 20mV?
Now play with the time/div switch and variable and see if you can see any signal (so is it focus or Y oscillation causing the thickness)?
 

LesJones

Joined Jan 8, 2017
2,696
I am wondering if it is not really a focus problem. I think it may be noise getting on Y amplifier. It would be interesting to know if you set the scope to X Y mode (So there is just a spot on the screen if the spot is a blurred dot or just elongated in the vertical (Y) direction.

Les.
 

AlbertHall

Joined Jun 4, 2014
10,380
I am wondering if it is not really a focus problem. I think it may be noise getting on Y amplifier. It would be interesting to know if you set the scope to X Y mode (So there is just a spot on the screen if the spot is a blurred dot or just elongated in the vertical (Y) direction.

Les.
Definitely worth a try, but if it is something like power supply ripple it may afflict both X and Y.
 

LesJones

Joined Jan 8, 2017
2,696
Hi Albert,
Good point. If the spot is a blurred blob then if the Y gain is turned down (Set to a higher volts per division setting.) then if the spot is just elongated in the horizontal direction then it would point to a noise problem effecting both axis.

Les.
 

Thread Starter

James55

Joined May 29, 2016
39
Hi all and thanks for the input.

I have been told that the scope is behaving normally.

Looking at my original post, it wasn't mentioned that the distorted 5mV/DIV was with the probes connected to the calibrator, and that the scope was designed to give less of a clear signal when connected to 'calibrator' and in this range.

Also there is no ground connection where we are (Rio de Janeiro).

Apart from needing a more complete calibration than I have the equipment to carry out here , the scope is now working fine. :)
 
Top