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Old 7th Sep 2019, 12:20 pm   #70
mhennessy
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Join Date: Jul 2007
Location: Evesham, Worcestershire, UK.
Posts: 4,241
Default Re: Telequipment D75 scope.

Quote:
Originally Posted by Pinörkel View Post
Hmmm, too bad. I really hoped for some images of the units.
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.

Quote:
@mhennessy: Thank you for participating in this thread. I found the report on your website regarding the D63 repair really interesting and useful. However, I can report that the vertical position sensitivity reduction, obtained by inserting those 680 ohm resistors is barely noticeable. I compared this in my D75. The V4 in my defective D755 had this factory modification already installed.
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.

Quote:
Regarding the adjustment of reverse engineered calibration units, I assumed that the correct settings could be derived from nominal component values and the nominal voltages in the circuit diagrams of the main unit. In any case, the settings in the calibration unit should not depend on the real world values of components in the scope, since they must work for all scopes of that type. However, it is difficult to speculate on something, I have so little information on.
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.

As I've said, I fully intend to publish details about these, and have done for many years. I won't want to make promises about when I'll manage this, however. Some years ago I was publicly attacked on this forum for not publishing the circuit details of my IC1 replacement - meaning that I'll probably never bother to do that now - but detailing these calibrators is actually very high on my to-do list, providing no-one does or says anything to change that. But life is extremely busy - more so than ever - and I have to prioritise. In the meantime, I hope that the information I've provided here is better than nothing.

Mark
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