[Pinörkel]

Originally Posted by mhennessy

Well, as I said, I do plan to add the details to my website, and obviously that will include pictures. In the meantime, I have too many other things that must take priority. Besides, what are you hoping to see in the pictures? The units have been pretty well described in this thread.

Sorry, I did not intend to sound impatient. I just thought that knowing the looks of these units could help me recognizing one, if I happen to stumble upon one on ebay.

Originally Posted by mhennessy

The mod is to reduce the sensitivity of the DC BAL control, not the vertical position.

I haven't bothered to compare the effect of the factory-fit 680R resistors to my 1M resistors, but as I say on the website, I recommend that people do what Telequipment did - it's physically easier, and likely better. Whichever approach is "best", it is a fact that it is fairly easy to set the DC-BAL control on later units that have the Telequipment mod, so that's a worthwhile improvement on the originals.

I of course meant the sensitivity of the CD balance control. On my unit, just slightly touching the CD BAL control with a trimmer tool already lets the 5x trace jump one tick up or down and this has, according to my experience, not really improved with adding the resistor.

Originally Posted by mhennessy

You've misunderstood my last post - the most important function of the Y-calibration module is to correctly set up the high frequency performance of the Y-amps on the main frame. So DC voltages and nominal component values are of no use.

To be clear, of the 3 adjustments in this unit, two of them are trimmer capacitors that set the HF characteristics. And actually, the pre-set resistor also adjusts the HF response.

These 3 adjustments must be correct in the calibrator before you attempt to adjust the Y-amps in the main-frame. Then, having used the calibrator to adjust the main-frame, you then adjust the HF performance of the vertical plug-ins to match the Y-amps in your main frame (at 5mV/div).

So the point of the calibrator is to ensure that all main-frames have the same HF behaviour, so that plug-in units can be swapped between different main-frames.

Before I had this unit, I had no option other than to pick one of my plug-ins at random and declare that as a reference. Then I calibrated all my Y-amps (in four DM63s, and that's 2 Y-amps per 'scope) to get the best HF with that plug-in. I then calibrated all my of other V4 plug-ins to for best HF. It sort-of worked, but when I got the correct unit, I found that all my Y-amps were some way away from the optimum HF response - unfortunately I was unlucky with the plug-in I chose (Sod's Law!). As a result, I repeated the whole process, and found that the -3dB point was now 20MHz rather than 15MHz (approximately, from memory). The DM63 is sold as a 15MHz 'scope, but actually, the setup instructions state that the -3dB bandwidth should be at least 18MHz.

Of course, once you've got the HF performance right on the 5mV/div setting, you then have to adjust the HF response and input capacitance (with an input normaliser) of all of the attenuators. It takes a long time to do this.

So, back to the vertical calibrator, even if you built one, how would you calibrate it? Of course, Telequipment would have had a procedure to do this, and it would have involved a test jig to correctly load the output of the calibrator. Unless someone has access to this documentation, we're guessing. When I eventually find the time to look at this, I will attempt to reverse-engineer a calibration procedure from my existing unit, but I can't guarantee it will match what Telequipment did. Given the age of my unit, I can't even be sure it's still in reasonable calibration - though it certainly appeared to be last time I used it (10+ years back).

The calibrator is also used at LF to set the DC offset and gain of the Y-amps, and while I haven't read the whole thread in detail, I appreciate that the thrust of your endeavours has related to DC matters, but really, the DC side of things is really very simple.

If it helps, when the switch on the calibrator is in the "BALANCE" position, both inputs of the Y-amp are connected to ground via a 2k resistor instead of the amplifier in the calibrator. Each of these resistors are the black-bodied precision types - might be 1% or 0.1% (I can't see without disturbing them).

Nearly all of the other resistors are standard-grade - there's just one exception - and there are no adjustments for DC offsets or DC gain.

Ah, I got that wrong in your previous response. Until now, I focused on the trace centering parts of the calibration procedure and kind of over-read the barely readable "vertical pulse response" section in my copy of the D75 calibration instructions. That makes a calibration indeed way more complicated than I thought. Thank you for the detailed description.

@Argus25: Hmmm, I already wondered how to produce those required 1ns rise time square waves. HP or Agilent function generators with these specs are way out of my financial range. I speculated that a cheaper function generator could be pimped with something like a Schmitt-Trigger stage to get faster rise times. So, if you say that a good calibration would essentially need one of these four slot Tek power frames, equipped with a calibrated set of the modules you mentioned, a calibration of my D75 might be completely out of reach for me. Maybe I can try to isolate the aspects on my scope that are off the most and see if those can be targeted via the less complicated calibration steps in the manual.

Denis