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Old 26th Aug 2019, 5:14 pm   #63
Pinörkel
Pentode
 
Join Date: Jun 2019
Location: Dortmund, Germany
Posts: 161
Default Re: Telequipment D75 scope.

@Argus25: Thank you very much for summarizing this. I read a tutorial on this several days ago but was not sure how to correctly map this onto the calibration procedure in the D75 manual.

What I noticed is that the attenuator of the V4 modules seems to use kind of a three stage approach. On each Volts/Div setting containing the number 5, the signal noise in 5x mode is comparatively low. The noise increases for settings containing the number 2 and again on settings containing the number 1. I figured that this could be because in these settings the resistance of 1, 2 and 3 resistors is put in series, each introducing a certain amount of noise which then adds up.

Unfortunately I do not own a signal generator. Thus my tests until now were based on using the 300mV and 30mV cal signal from the scope and some custom 10kHz and 1kHz square waves that I generated using an Arduino. Using this, I had the impression that the attenuator on my D75 is still pretty well calibrated but the timebase in the S2A needs some attention. Maybe I should finally buy a decent low cost signal generator like the Siglent SDG1032X. I do not really trust those cheap 80€ FY6800 like Chinese ones. They seem to have quickly degenerating square wave quality and a lot of sampling issues.

As for the vertical calibration, setting stable values for the 1mV and 5mV gain is complicated because of the wiggle sensitivity of the vertical pots. Just touching them or the snapping-back of a nearby push button can cause them to jump one to two ticks. Since I thoroughly cleaned these potentiometers I am quite unsure what causes this. Nonetheless I was able to achieve a semi-precise calibration that matches the 300mV cal signal and my 5V signals from the Arduino in the respective Volts/Div settings.

A strange phenomenon happens when I touch the set screw of the 5x variable gain knob of channel 2 with my hand while rotating the knob. This sometimes couples a strange signal into the scope, even when channel 2 is coupled to ground. The only explanation I can think of is that my body acts as an antenna for some EM signals which then somehow get coupled into the amplifier circuit. So far, I have not been able to reproduce this with channel 1. Wiggling the knob horizontally reproducibly switches this behavior on and off, so the rod might be touching something it shouldn't.

Regarding the Telequipment vertical and horizontal calibration fixtures, I mentioned earlier: They are used at the beginning of the calibration to set the horizontal/vertical balance and sensitivity. To me this seems to serve the purpose of centering the electron beam vertically and horizontally and configuring it for a reasonable and equal amount of deflection in both directions with respect to the horizontal/vertical position controls. In the manual, it seems that the horizontal calibration unit has no controls whatsoever and just produces a dot. This dot must then be centered via a pot while applying a 5V DC signal to input. Maybe this just emulates something similar what is happening when using a S2A amplifier in EXT X mode. In that mode my D75 produces no sweep resulting in a vertically centered dot at about 1 division right of the horizontal center. The vertical calibration unit, however, seems to have buttons to put it in either BAL mode or CAL mode. While the BAL mode is just used to center the trace, the CAL mode is used to set correct deflection of some defined input signals and also to to optimize the pulse response to the square waves. Thus, I am not suspecting the calibration units to contain complex circuitry. However, it could be possible that they also contain simple signal generators for the signals required in these calibration steps.
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