Tektronix 3T1 problem

[neazoi]

My Tektronix 3T4 (manual here https://w140.com/tekwiki/images/d/d4/070-0439-00.pdf) has problems triggering a 3S1 on a 564 mainframe scope. Mainframe works with other non-sampling plugins and the 3S1 seems to be working with non-sampling plugins.

I checked the voltages of the 3T4 plugin and they check ok.
Signal at trigger out connector of the 3T4 checks ok (like on second picture).

I try to do the calibration according to the manual.
However (first picture) even if I set R16 internally to max gain and the panel TRIGGER LEVEL to max, it still can't trigger, but only display a running waveform.

I can only see a running waveform only if I set these two levels close to the max. If set to middle there is absolutely no signal shown. There is not enough headroom to set the TRIGGER LEVEL to the center, as the manual describes, and then tune R16.

I also notice that the TIME/DIV has no effect in it's settings, but only responds to 3 ranges, ms, us and ns and not the in-between settings. And of course the Variable time/div does not do anything. I do not know if two things (trigger, TIME/DIV) relate or not

What can I do/check?

 

[danadakk]

Using internal or external triggering ?

Regards, Dana.

 

[neazoi]

Using internal or external triggering ?

Regards, Dana.

I am using internal triggering. Let me know if you need any more info. I am starting to run out of ideas.

Apart from the trigger level problem, my TIME/DIV does not seem to work, neither it's variable control.
I am not familiar with sampling plugins, but I think I should see even an untriggered waveform to change in width, or I should be able to see the bean change sweep rates at low sweep settings of the time/div switch. But I do not see such a behaviour.

 

[BradtheRad]

My experience is based on my Tektronix tube-type scope. Gradually thresholds change and the beam no longer sweeps. Then I must adjust a knob somewhere.

Free-run creates its own internal trigger and sweeps automatically. Free run is independent of your signal, but your signal still needs gain/attenuation so it appears on the scope display. Is gain applied to your signal?

When you want your signal to provide a trigger to start a sweep, there needs to be an internal voltage amplifier that detects when your signal crosses a threshold Voltage. (This threshold can be set by you turning a knob, or it can be a default level created inside the unit.)

Possible reasons a sweep fails to be triggered by your signal:

* gain becomes weak and fails to amplify your signal,

* lost contact in the switch that selects between free_run & auto & level_set.

* your signal is properly amplified but broken wiring doesn't carry it to switch.

Looking at the picture, it does not appear to have a vertical gain adjust. Instead the block diagram shows a sampling unit. My Tektronix is analog, whereas yours being digital requires a different troubleshooting approach.

 

[danadakk]

The 3T4 is the sweep portion, a 3S1, 3S4..... are the samplers that is used with it, which has gain controls.

Regarding 3T4, have you cleaned pots and sweep speed switch with a good quality cleaner ?

Checked electrolytics ?

It takes a pcb extender or hand wired cable to get it out of mainframe and onto bench for
testing ?


Regards, Dana.

 

[neazoi]

The 3T4 is the sweep portion, a 3S1, 3S4..... are the samplers that is used with it, which has gain controls.

View attachment 184512

Regarding 3T4, have you cleaned pots and sweep speed switch with a good quality cleaner ?

Checked electrolytics ?

It takes a pcb extender or hand wired cable to get it out of mainframe and onto bench for
testing ?


Regards, Dana.

UPDATE:
Here is the first sampled waveform from the scope, on a 21MHz signal.
I noticed that if I turn the TIME/DIV switch in between some positions (not in the mechanical lock states of the switch) the plugin was able to work and the scope was sampling. At that point, everything works as it should, all the knobs and functions including the variable time/div (except the switched TIME/DIV of course). This is good news initially because it seems that the precious Tunnel diodes inside this plugin are working.

However I have to check out what is going on with that multi-wafer switch as it affects more than one circuits. I can hear the internal reed switches click as I go between two positions in which the plugin scope works. Actually I can actuate two time/div relays in between two switch positions (two relay clicks).
I checked all of it's mechanical contacts and they are actuated (optically bent) when contact is being done.
Maybe the problem is not a mechanical but an electronic, I do not know.

 

[danadakk]

Dont underestimate oily air, smoke environment on contacts or oxidation. Even though
you can see female side contacts move is not the last answer. Contact cleaner,
like Deoxit, are worth every penny it costs.


Regards, Dana.

 

[neazoi]

Dont underestimate oily air, smoke environment on contacts or oxidation. Even though
you can see female side contacts move is not the last answer. Contact cleaner,
like Deoxit, are worth every penny it costs.


Regards, Dana.

Problem fixed. A slight touch to the comparator level trimmer was all that was needed. I.e. a calibration issue. It fixed all functions.

However I experience another problem, which I think relates to my vertical amplifier sampling unit https://w140.com/tekwiki/wiki/3S1 https://w140.com/tekwiki/images/c/c0/070-0632-00.pdf

The Tektronix 564 mainframe has been calibrated ok (veryfying it also with another pair of non-sampling ones). I have not yet fully calibrated the 3S1 fully nor the 3T4 sampling plugins I am using. I have this problem:

Sinewaves are suppressed a lot in the lower end of the graticule, like shown in the picture. When I position or DC-offset the waveform more to the top, the sinewave is not suppressed anymore. Both DC-offset and Position controls cause the same behaviour independently, it does not matter which one I tweak.
Suppression happens only on the lower 1.5 graticule squares.

Which part of the circuit could cause that, so I know what thing to check/calibrate?

 

[danadakk]

Looks like classic saturation of an amplifier when it runs into the supply rails
for its peaks. Offset and G interact to create this problem.

Page 3-4 discusses vert G adjust and appropriate G resistor for a specific
mainframe.

I think also the early part of Chapter 5 discussing the sampling gate, memory
operation, and amplifier apply to the issue. Assuming an "ordinary" vertical
plugin to the mainframe does not exhibit this problem.

Overall if you look at the discussion of the input stages and required adjustments
quite challenging discrete design and component aging......especially given age of
of plugin.


Regards, Dana.

 

[neazoi]

Looks like classic saturation of an amplifier when it runs into the supply rails
for its peaks. Offset and G interact to create this problem.

Page 3-4 discusses vert G adjust and appropriate G resistor for a specific
mainframe.

I think also the early part of Chapter 5 discussing the sampling gate, memory
operation, and amplifier apply to the issue. Assuming an "ordinary" vertical
plugin to the mainframe does not exhibit this problem.


Regards, Dana.

That's weird, because I have already performed this alignment. And the mainframe calibrator signal is calibrated against a modern scope. After this gain adjustment alignment, I do get the exact divisions on the mainframe as I expect. But I only get those in the upper part of the display, as in the lower 1.5 divisions or the graticule the signal is suppressed.

I haven't tried to set the gain of the internal and external resistors lower. However, this will not result in the correct amplitude signals anymore, so I am not sure if I have tro try this.

 

[danadakk]

Do you have another scope you can travel the signal path within the offset/gain stages ?

When you do calibrator adjust keep in mind squarewave already has the appearance of
clipping. Not an expert here but I would think about calibrator adjustments using sine as
source to see if cal is not actually linear over whole range.


Regards, Dana.