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Old 25th Aug 2019, 9:24 pm   #61
Pinörkel
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Join Date: Jun 2019
Location: Dortmund, Germany
Posts: 161
Default Re: Telequipment D75 scope.

Had some time to work on the scope again. To further optimize the temperature behavior of channel 2, I ensured that all resistors in the early differential amplifier stage are mounted under equal conditions. I.e. that not one resistor of a pair touches the PCB or another component and the other one is soldered floating. If you have similar issues pay close attention to R624 which might touch the metal casing of the invert switch. This then acts as a huge heat sink.

The DC calibration now stabilizes after about 4 minutes. One difficulty with setting the variable gain balance via R754 and R645 is, however, that it makes a difference if those are set when the unit is
  1. connected via an extension lead
  2. connected directly with the module side cover removed for potentiometer access and
  3. connected directly with the module side cover in place.
This makes adjustments somewhat complicated. I am suspecting that the changed environmental conditions somewhat shift the thermal equilibrium point of the sensitive circuit.

While checking the variable gain calibration I noticed that the calibration is always off when leaving the center position. That means the trace vertically moves when operating the variable gain switch in cases where the vertical zero position has been moved above or below the center. I could observe this behavior on both channels and on both my V4s, but I am not sure if this is normal behavior.

One useful hint for working on a V4 amplifier without desoldering everything is, to remove the top or the bottom aluminum profile. Do not remove both at the same time for improved stability. For this, you have to remove the labeled part of the front bezel by removing all knobs and removing the screw-on rings that holt the vertical position potentiometers in place. I found the best way to remove those is to insert a pair of angled pliers into the notches and use it to loosen the screw-on ring. The front bezel has to be removed to access the screws that connect the bottom panel with the front panel. One of these screws is the screw-on ground connector near the invert switch rod. The handle of this rod can be screwed off for removal. Once the screws on the bottom, the back and the front of the bottom panel are removed, lift the metal lever up from the invert switch just as high, so you can pull the rod to the front without moving the actual switch. This will free the rod from its guide rail without having to bend any of the guide rails plastics which might break. Now you can access the modules PCBs from the back as shown in the attached image.

Removing and inserting resistors or other components in this state can be slightly complicated. To do this with only two hand I clipped the affected component into a DMM gripping clamp like shown in the image. Use a piece of paper in between to avoid gripping damage. Then I alternately heated the respective solder joints to remove or insert the component by successively and carefully pushing or pulling. A soldering iron like the TS100 which is only hot at its tip and a low soldering temperature (270°C worked fine for me) can help to avoid damaging any wires.


Next I will try to improve the calibration of the scope. Unfortunately, this seems to require two telequipment calibration modules, namely 067-0672-00 for vertical and 067-0673-00 for horizontal. I could not find any useful information on these units like images, a manual, or schematics on the net. Does anybody here know what exactly these modules do or how to calibrate without using these modules? I do not expect that any of these calibration fixtures are still available.

As far as I understand, the also required input normalizer is not really necessary because it only simulates a probe with a certain input capacitance. Thus, a normal oscilloscope probe can be used for calibrating the input attenuators. However, this will break calibration compatibility on attenuators not calibrated using the same probe.
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Last edited by Pinörkel; 25th Aug 2019 at 9:36 pm.
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