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Old 25th Aug 2019, 9:24 pm   #61
Pinörkel
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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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Old 26th Aug 2019, 12:21 am   #62
Argus25
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Default Re: Telequipment D75 scope.

Quote:
Originally Posted by Pinörkel View Post
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.
One way to set up most of the attenuator settings is initially to terminate the input with a 50R terminator on a T connector. Feed a perfectly rectangular pulse in (I use the Tek PG506 generator). On max sensitivity the attenuator is bypassed, and you can set up the frequency compensation components in the vertical amplifiers for a flat response, with at least 70 to 80% deflection on the CRT. After that, you start to increase the attenuation and increase the drive amplitude. As each section of the attenuator kicks in, you can adjust its compensation capacitors (sometimes resistors too) for a flat response until you have done them (all but one) up to the minimum sensitivity on the attenuator knob.

The effect of the input adjustment capacitor immediately on the scope's input then comes into play when the 50R terminator is removed and you plug on your scope probe. (When the 50R was there its effects are largely deactivated).

Generally the resistance there is 1Meg and the parallel capacitance around 10 to 15pF depending on the probes it was designed for. This is where a normalizer box would be used to normalize the response for some intended probe.

One quick way around this (without a normalizer box) is to just match the probe to a similar scope's input R & C values, by setting up a x10 probe's compensation adjustments (on another scope in your workshop with a similar bandwidth), then transfer the probe and adjust the scope's input compensation capacitor at the attenuator input to match that probe. Then of course that probe is ready to use on more than one scope in your workshop without having to re-tweak its compensation capacitor when you use it on different scopes, which is one advantage to not doing it with a normalizer box. So one of your scopes becomes the "normalizer"
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Old 26th Aug 2019, 5:14 pm   #63
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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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Old 26th Aug 2019, 11:32 pm   #64
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Default Re: Telequipment D75 scope.

I have both the V and H cal modules, but have never seen a circuit or manual for either one. There is very little inside them, should be easy to "reverse engineer", but then you need both the equipment and method to initially calibrate them (Old question, who calibrates the calibrators?).
Argus, I have a normaliser originally used with a very early TQ S31, and have built a copy in an "attenuator type" of box for such as the D75, but clearly I need a method of setting the parallel capacitance. Your simple method assumes the "normaliser" scope you work from is correct. But since the input R and C are usually given either on the front panel or in the manual, is there a simple method of setting my normaliser correctly? The "R" is easy, but how to set/measure the "C" value?
Les.
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Old 27th Aug 2019, 12:18 am   #65
Pinörkel
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Default Re: Telequipment D75 scope.

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Originally Posted by MotorBikeLes View Post
I have both the V and H cal modules, but have never seen a circuit or manual for either one. There is very little inside them, should be easy to "reverse engineer", but then you need both the equipment and method to initially calibrate them (Old question, who calibrates the calibrators?).
Fascinating, I did not expect that any of these still existed. Would you mind sharing a few high quality photos of the modules, maybe even of the insides? I am highly interested in reverse engineering and understanding how this kind of equipment works. Since those would then be the only photos available on the net, they could also be of interest for the Tektronix wiki at http://w140.com/tekwiki/wiki/.
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Old 27th Aug 2019, 12:20 am   #66
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Default Re: Telequipment D75 scope.

They are mostly empty AFAIK. The horizontal cal had a BC109 and pretty much naff all else in it than a bit of coax to a front panel BNC
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Old 29th Aug 2019, 5:12 pm   #67
MotorBikeLes
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Default Re: Telequipment D75 scope.

As mentioned above, I have both the vertical and the horizontal calibration units. I also have a couple of the dummy plugins used on D63/DM63 if only one vertical plugin was supplied. (these were supplied "bare" I believe, with vertical plugins specified separately)
Anyway, I will attempt to sketch out the circuits of all three versions. The dummy units could easily be adapted for the H cal unit, maybe with some difficulty for the V cal unit.
Some time ago it occurred to me that it should be possible to design a circuit enabling them to be converted into a component tester. When I have done my bit, maybe one of the brighter members will design such circuit. I did think it would need a small powered transformer, but since there are +24v, -24v and of course 0v supply lines, there may well be a way without an onboard transformer.
When it is eventually complete, I will post it as a separate thread.
Les.
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Old 29th Aug 2019, 8:20 pm   #68
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Default Re: Telequipment D75 scope.

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They are mostly empty AFAIK. The horizontal cal had a BC109 and pretty much naff all else in it than a bit of coax to a front panel BNC
The Y unit is a bit more involved, with a dual transistor and a pair of 2N2369s. There's a preset resistor and a pair of pre-set trimmers. As Les says, how do you calibrate these?

As the main point of the Y unit is to setup the HF adjustments in the main-frame Y amps, it is essential the calibrator is right. Having used the calibrator to then get the Y-amps right, you then adjust all your Y modules for best HF performance. If the Y-amps are not right, then you're in a rabbit hole with that one (been there, got the t-shirt).

If people wanted to build their own, it won't be enough to reverse-engineer the circuit (which I've done - it's not difficult), because they'd have to dream up some sort of routine to calibrate the adjustments on the calibrator. I'm sure it's doable - it might be as simple as building a test jig that resistively loads the output of the calibrator and adjusting for best square wave response - but it would involve quite a bit of effort of the part of someone like Les or myself who actually own them. It's on my to-do list, but so is everything else.

When I eventually get around to it, I'll add the details to the Telequipment secion of my website.
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Old 6th Sep 2019, 9:38 pm   #69
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Default Re: Telequipment D75 scope.

Hmmm, too bad. I really hoped for some images of the units.

@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.
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.

@MotorBikeLes: Those dummy units you mentioned sound indeed like an interesting starting point for custom modules. Unfortunately it is impossible to search for these units on ebay, or even find images of them, since they were probably mostly dumped when they were replaced with vertical plugins. On the web, there are some circuit diagrams for component testers available which are intended for use with older oscilloscopes, e.g. the Hameg HZ65. Maybe the circuits can be adapted to work with the set of available voltages in the D75.

Luckily, I discovered with my newly acquired function generator that the calibration of my D75 is not too far off. Therefore, one of my next steps will be to thoroughly check the components in the power supply to minimize any remaining voltage drifts. I addition to that, I will have a look at the trigger circuit. At the moment, the scope triggers very stable, but the trigger point gets a little bit jumpy during adjusting the trigger level, even for a clean sine wave. It is well possible that this is normal for this scope but I know that setting the trigger level on Hameg scopes of the same age is way more stable. Maybe I will also document the current state of the scope photographically, since it is now in a working, nearly 100% original state (except for the newly produced Time/Div knob).
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Old 7th Sep 2019, 12:20 pm   #70
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Default Re: Telequipment D75 scope.

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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:
Originally Posted by Pinörkel View Post
@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:
Originally Posted by Pinörkel View Post
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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Old 7th Sep 2019, 1:09 pm   #71
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Default Re: Telequipment D75 scope.

In my opinion the various calibration protocols you see for many scopes are those created around a paucity of calibrated lab equipment. Then it leads to many layers of confusion about how to calibrate a scope's vertical amplifiers and attenuators and "custom test fixtures" which assume some special properties but I think are only specific to a few scopes.

The reality is this;

To calibrate any analog scope perfectly, from any bandwidth up to 100MHZ, three calibrated instruments are required:

The PG506 amplitude and fast rise pulse generator.

The TG501 time marker generator.

The SG503 leveled sine wave generator.

Fortunately, the SG503 is only required for a final bandwidth test, you can just get away without it at a pinch.

The TG501, could be replaced with other frequency references, but the TG501 is hard to beat to set up a scope's time base correctly as its pulses suit the CRT's graticule well and it steps in frequencies like a scope's timebase.

The PG506 is the essential tool. It produces perfectly rectangular fast rise output pulses (Heaviside function on the leading edge) over a range of frequencies with a controlled amplitude. You cannot cobble this together in a workshop or use any other general purpose signal generators, the amplitude affects the quality of the output wave, but in the PG506 it does not.

The PG506 is all that is required for correctly setting up the DC gain and the multiple compensation adjustments and attenuator in ANY scope's vertical amplifier...across all of the attenuator settings without resulting in a corrupted calibration.

The PG506 gives you control of the amplitude and frequency of the test signal while maintaining a perfectly rectangular form. The various square wave frequencies are required to ensure a flat response across the scopes bandwidth, one test frequency is not nearly enough.

In the process of setting up any oscilloscopes vertical amplifier, there is no escaping the fact that this is the type of controlled & calibrated signal required, able to maintain its perfect rectangular form over a huge range of output levels and frequencies. There are other equivalent scope calibrators to the PG506 out there, but the reality is Tek really aced the design with this one and every scope in the world can be better off for it.

When the job is done you can then check the bandwidth with the leveled sine wave generator and make an assessment of the bandwidth, but it will be very close to correct if the adjustments are optimized with the PG506. The leveled sinewave generator is a useless tool for the actual calibration of the vertical amplifiers themselves, because the observer will not notice the effects of any adjustments at multiple other frequency points than the one they are observing.

(For scopes over 100MHz bandwidth its useful to have the TD pulser, and the SG504 generator that goes to 1GHz, I use these additional items to calibrate my 400MHz bandwidth 2465B scopes)

Last edited by Argus25; 7th Sep 2019 at 1:23 pm.
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Old 7th Sep 2019, 4:38 pm   #72
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Default Re: Telequipment D75 scope.

Quote:
Originally Posted by mhennessy View Post
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.

Quote:
Originally Posted by mhennessy View Post
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.
My bad, 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.

Quote:
Originally Posted by mhennessy View Post
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
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Old 7th Sep 2019, 5:14 pm   #73
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Default Re: Telequipment D75 scope.

Quote:
Originally Posted by Pinörkel View Post
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.
Denis
I would suggest the bare minimum then, a single slot TM501 power unit with just the PG506. That is quite compact assembly too and doesn't occupy much bench space and keeps the cost to the minimum. Also you will end up using it on every scope you repair, so in the end it pays for itself many times over. TM501's are multi mains input voltages selected by a jumper so they are fine in the UK or Europe.

There is a way to build a generator like this, but it is a very large amount of work. It has a unique output stage quite unlike any common generator, its principles could be copied, it is very well documented in their manual. Though I think it would be better to use their unit, even the broken ones I have bought on ebay are easily fixed as Tek's manual on it is excellent.
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Old 7th Sep 2019, 8:30 pm   #74
MotorBikeLes
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Default Re: Telequipment D75 scope.

Argus, I would like to get some of the modern TEK cal units, but since 'scopes are only an occasional hobby thing now, I can't really justify unless some land on me cheaply. i still rely on a 191 for frequency checks, but have a TQ calibrator (not the valved CT1, I moved on from that 35 years ago) -- a CT3 I think, which has amp cal, fast rise etc.
Regarding the Cal modules and dummies, only the vertical remains incomplete. A spell of migraine plus too many other jobs have halted it for a few days, maybe this week.
Mark, when finished, I thought I would let you have them off list first since you already made a start, to see if you can spot any silly errors. Easily done!
Les.
C1 and C3, not CT1 and CT3

Last edited by MotorBikeLes; 7th Sep 2019 at 8:31 pm. Reason: Memory clicked, corrn. added.
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Old 7th Sep 2019, 9:55 pm   #75
Pinörkel
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Default Re: Telequipment D75 scope.

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Originally Posted by MotorBikeLes View Post
... since 'scopes are only an occasional hobby thing now, I can't really justify unless some land on me cheaply.
Sadly so true. Of course I could buy expensive test equipment like a R&S scope, an HP spectrum analyzer and the like but unfortunately that would be like throwing pearls before swine. I am not nearly adept enough to make full use of equipment like this, nor do I have the time to use it frequently enough to justify its purchase. It would mostly sit in its box, only for a fraction of its functions to be used every now and then. I don't even have something I could call an electronics bench aside from occasionally transforming the table in my living room into one. However, repairing stuff like this D75 always puts a smile in my face, especially when I learned something interesting about old equipment or when I can celebrate one of those small victories against the planned obsolescence.

So, buying a PG506 for 500€ to 1500€ only for repairing one scope seems unfortunately like a tinzy winzy bit of overkill.
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Old 8th Sep 2019, 7:59 am   #76
Argus25
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Default Re: Telequipment D75 scope.

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So, buying a PG506 for 500€ to 1500€ only for repairing one scope seems unfortunately like a tinzy winzy bit of overkill.
That is a nutty price for a PG506. I bought most of mine for $150 USD each. They come up from time to time on ebay. Some lunatics put them up on ebay for $1000's of dollars, they never sell them.

You just need a little patience and one will come up at a reasonable price. Should be $200 USD or less.
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Old 8th Sep 2019, 12:00 pm   #77
Pinörkel
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Default Re: Telequipment D75 scope.

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Originally Posted by Argus25 View Post
That is a nutty price for a PG506. I bought most of mine for $150 USD each. They come up from time to time on ebay. Some lunatics put them up on ebay for $1000's of dollars, they never sell them.

You just need a little patience and one will come up at a reasonable price. Should be $200 USD or less.
Hmmm, seems like you are right. I just checked on some completed ebay listings. However, most units sold for that price are offered as defective or are in horrible visual condition. In addition to that, most listings from the US call for incredibly high shipping costs. Nonetheless, I will try to keep an eye open.
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Old 12th Sep 2019, 4:07 pm   #78
MrBungle
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Default Re: Telequipment D75 scope.

Joining this thread again. Picking up a D75 tomorrow that is broken!

Earlier this year I said I'd get down to one scope. I succeeded but now suddenly have 5 again
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Old 12th Sep 2019, 6:53 pm   #79
Pinörkel
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Default Re: Telequipment D75 scope.

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Joining this thread again. Picking up a D75 tomorrow that is broken!
Nice, good luck with getting only repairable issues.
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Originally Posted by MrBungle View Post
Earlier this year I said I'd get down to one scope. I succeeded but now suddenly have 5 again
Be careful, old scopes tend to multiply like tribbles. My D75 also got a D755 brother in no time.
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Old 13th Sep 2019, 1:13 pm   #80
MrBungle
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Default Re: Telequipment D75 scope.

Unfortunately this one is beyond repair. The capacitors leaked in it. The seller did a respectable job of replacing them but the capacitor juice had wicked its way up the transformer leads and the have disintegrated.

Thus if anyone needs any parts, let me know. All parts FOC - just postage cost.
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