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Old 26th Jun 2022, 9:24 pm   #161
MotorBikeLes
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Join Date: Dec 2012
Location: Kirk Michael, Isle of Man
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Default Re: Telequipment D75 scope.

Unless you have TQ's calibration info for the calibration fixture (and who has?), then surely your choices are either to throw it away, or assume it is OK.
Looking at your first pic, I would use the mainframe Y-amp adjustments to correct that HF part of the rising portion according to the D75 manual.
Remove cal module, install V4, and adjust that according to the manual.
Next compare it with a currently calibrated scope of 100MHz bandwidth or higher and draw conclusions from that.
All test equipment is either right or wrong. Make a choice in the absence of certainty.
Les.
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Old 28th Jun 2022, 9:24 pm   #162
Pinörkel
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Default Re: Telequipment D75 scope.

Hello Les, thank you for your helpful input.

Quote:
Originally Posted by MotorBikeLes View Post
Unless you have TQ's calibration info for the calibration fixture (and who has?), then surely your choices are either to throw it away, or assume it is OK.
At the moment, I'm still far less pessimistic about that. Sure, any manual of the calibration units seems to be lost in time. At least the guys at the vintageTEK museum could not find anything about them in their microfiche collection. However, the 067-0672-00 plugins were made to provide a way to get the best bandwidth out of the respective mainframes. For this, they include an hf-tuneable minimal differential amplifier, with everything out of the way that could potentially negatively influence the bandwidth. At the same time they must be able to properly prepare the signal for the main vertical amplifier.

As can be seen from my pictures, my 067-0672-00 plugin still provides a better frequency response than channel 1 of my V4 amplifier and performs better than the specified 50 MHz bandwidth of the D75, D755 and D83. This suggests, that it is still at least up to the task of providing a way to calibrate the amplifier in a mainframe to outperform its specification.

Based on this, I would tend to assume that my 067-0672-00 is working and will not fiddle with its current state. On the other hand, I would still like to try to quantify how far its performance deviates from a potentially existing optimum and learn interesting stuff on the way. If my simulation attempts will not do the trick, I could simply make a replica of the circuit (I have the most important part, the dual FET available) and use the replica to see, if there is an hf performance optimum, that is independent of the mainframe used. If there is, that would most likely be the point the TQ engineers tuned the plugin to, otherwise they would have wasted bandwidth. If there is not, there is a high probability that the working points of the 067-0672-00 plugins were set only based on an arbitrary definition, which can not be reconstructed without the original calibration information. Even in that case, there is interesting knowledge to be gained on the way finding this out, instead of just assuming it.

Quote:
Originally Posted by MotorBikeLes View Post
Looking at your first pic, I would use the mainframe Y-amp adjustments to correct that HF part of the rising portion according to the D75 manual.
Remove cal module, install V4, and adjust that according to the manual.
Next compare it with a currently calibrated scope of 100MHz bandwidth or higher and draw conclusions from that.
Yes, that will be done as part of a full calibration of my D755 according to the manual once I find the remaining faults that recently caused the trace intensity to become unstable again. Unfortunately, the D75 and D755 are my highest bandwidth scopes at the moment, so I have nothing better at hand to compare them with.

Denis
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Old 29th Jun 2022, 6:44 pm   #163
MotorBikeLes
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Default Re: Telequipment D75 scope.

When you have a plug in type scope, you will generally find the mainframe has a higher bandwidth than the plug ins.
For example, I recall measuring bandwidths of 80MHz on the Dynamco mainframes, but the best you got from the assembled scope was 50 MHz (or was it 35??). There are exceptions. The DM63, probably because it is a storage mainframe is specified to 15MHz when fitted with the 50MHz V4 amp like that of your D75.
You always get a lower overall bandwidth than either of the mainframe and plugin when measured separately.
I can't remember if there is a specified mainframe bandwidth for D75, or even if I have ever checked for it, but it would be easy to measure.
I suppose if I were to try to adjust the cal plugin for D(M)63, D75 or D83, I would fiddle with it to improve bandwidth, then again setup the fast rise trace shape. Then expect bandwidth to fall, so twiddle some more until you find the optimum.
That display of yours with the spike on the rising edge probably has a higher bandwidth now than it will when you have corrected its shape.
Les.
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