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Old 8th Feb 2021, 7:20 pm   #141
MotorBikeLes
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Default Re: Telequipment D75 scope.

Ferrite beads, i understand. Have you searched for porcelain fish beads, or Alumina fish beads? That should find some if still available. They used to be fitted on things like leads inside electric irons, toasters etc.
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Old 8th Feb 2021, 7:36 pm   #142
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Originally Posted by MotorBikeLes View Post
Ferrite beads, i understand. Have you searched for porcelain fish beads, or Alumina fish beads? That should find some if still available. They used to be fitted on things like leads inside electric irons, toasters etc.
Les.
Yes, that was actually the first thing I tried to find. I have a large supply of those aluminum(III) oxide beads, harvested from the heating spiral of an old oven. However they are far too big and smaller ones of the right size are apparently hard to get and in most cases very expensive, since one has to order 1000 pieces minimum. I also did not find a source of aluminum(III) oxide tubes, which could be split into small sleeves using a micro grinder. I am sure they exist, but they are hard to get for a non-industrial customer.
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Old 24th Feb 2021, 9:27 pm   #143
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Default Re: Telequipment D75 scope.

Since I couldn't find any usable sleeves, I gave up the search and built a completely new board for the transformer instead. Initially, I wanted to use a Paxolin/Pertinax board to keep the original look, but the manufacturers insulation ratings given for those did not sound trustworthy. So, I tested a cotton based PCB material instead which looked really ugly and was too soft. In the end, since my D755 was already uglified by the previous owner with blue paint for the case, I went with 4mm FR4 material. This has the additional advantage, that any arcing can be seen immediately through the translucent FR4 material. It took quite some time to drill the holes by hand, but the result looks kind of nice. The biggest issues were the damage-free removal of the old Paxolin board and the removal and installation of the transformer in the narrow HV section of the D755.

Since I did not have any conformal coating for the board and did not want to destroy everything by putting epoxy everywhere, I only degreased and cleaned the board thoroughly and used gloves during the installation to avoid any conductive contamination. After some safety checks, I powered the scope on and everything seemed to work. However, after approximately two hours of operation, the nasty fault with the sudden brightness boost appeared again. This time, it was not accompanied by any arcing and I have not been able to reproduce it ever since. My current theory is that maybe the diode D303 has developed an intermittent fault caused by the current overload due to the arcing on the old board.

Until now, the diode withstood all of my attempts to prove its faulty condition. In an external low voltage diode test configuration, it showed a voltage drop of 9.x volts and appeared to work as it should, even when exposed to heat. Maybe the fault only shows up with several kilovolts applied. Searching for a replacement diode, I only found HV diodes with a similar voltage rating and 5 mA instead of the needed 10 mA current rating and on the other side diodes with a higher voltage rating of 8 kV to 12 kV, resulting in a much higher diode voltage drop. I suspect that this could have a negative impact on the required grid/cathode voltage offset.
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Old 24th Feb 2021, 9:48 pm   #144
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Default Re: Telequipment D75 scope.

Glad to see this thread is still going. I am impressed by your persistence!
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Old 3rd Mar 2021, 3:19 pm   #145
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Default Re: Telequipment D75 scope.

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My calibration fixture had covers.

Honesty after that last D75 I never want to see another vintage scope
Well I went back on my own words here. I think the trauma subsided finally. I just bought a D755. Will inevitably refer to this thread when it inevitably doesn't work properly!
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Old 3rd Mar 2021, 8:15 pm   #146
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Default Re: Telequipment D75 scope.

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Well I went back on my own words here. I think the trauma subsided finally. I just bought a D755. Will inevitably refer to this thread when it inevitably doesn't work properly!
Ah, a latent case of vintage scopeitis extrema vulgaris. Rumor has it, there is no known cure.
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Old 3rd Mar 2021, 10:15 pm   #147
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Default Re: Telequipment D75 scope.

Yes tell me about it. This is actually my 51st oscilloscope now.
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Old 3rd Oct 2021, 10:11 pm   #148
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Default Re: Telequipment D75 scope.

Thread reopened at OP's request.
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Old 4th Oct 2021, 12:03 am   #149
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Default Re: Telequipment D75 scope.

Hi!

Quote:
Yes, tell me about it. This is actually my 51st oscilloscope now.
I've got two O.U. Generatorscopes (MK1 and MK2) coming that'll take my collection into the 50s as well!

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Old 4th Oct 2021, 12:16 am   #150
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Default Re: Telequipment D75 scope.

Reopening this thread, because my is D755 still is not fully fixed and my D75 just literally caught "Corona".

Earlier this day I checked on my scopes and my D755 seems to have caught kind of a focussing issue. The trace is kind of blurry and the blurryness of the trace no longer matches its blurryness when the trace locate button is pressed. Since this could be related to the potentially defective high voltage diodes, I left it that way until I finally manage to find suitable replacements for the diodes.

Next, I turned on my until now quite reliable D75 and it took longer to warm up, but finally stabilized. Suddenly, it began arcing like a tesla coil brightly from somewhere near the tube socket. I immediately pulled the plug, waited for the scope to discharge and tried to locate the issue. However I could not find any traces of coronary discharges near the tube socket. So I switched on the scope again for just one second which was sufficient to pinpoint the source. As can be seen in the attached image, the arcing occurred between the solder joint connecting the D305 130V Zener diode with R330 and the green isolation of the grid supply voltage cable. I think, someone here warned about that could eventually happen. So the arcing probably shorted the grid supply voltage to ground through the defective wire insulation. As a preliminary fix, I just put a silicone insulation sleeve on the wire between the diode and the resistor. Unfortunately, this only solved the arcing but not the consequences. Now, the scope only operates at full brightness with little to no control possibilities via the intensity knob. At the moment, I am suspecting that the high current caused by the arcing may have fried the high voltage diode D303 that is responsible for the grid voltage and maybe also damaged the HV diode of the cathode supply.

This is very unfortunate, since I cannot find suitable replacements for these diodes which also need to be replaced on my D755. The diodes need to be rated at least for 6kV, be able to provide 10 mA and have a forward voltage drop of around 8 V to 9 V. Most available HV diodes have either only 5 mA or values far bigger than 10 mA. To my understanding, the latter can cause the supply to provide dangerously high amounts of current which could potentially cause damage irreplaceable parts like the tube. Also, most of the diodes I have found have a much larger forward voltage drop, the consequences of which I am not yet certain of. Does anybody have a recommendation for suitable HV diodes with matching parameters?
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Old 4th Oct 2021, 4:07 pm   #151
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Default Re: Telequipment D75 scope.

Telequipment D75 / D77 / D83.
I think you quoting the current rating 10ma of the EHT diodes used.
But the current used by the CRT is about 10ua,so total in the circuit will be 100ua or so. The current drop is irrelevant at these output voltages.
So almost any TV EHT diode will do. I have used BY182, BY710, BY409, BY509. All rated 10Kv upwards, 2.5 ma or greater. Raid the nearest scrapTV or Monitor with a tube.
The stuck brilliance sounds rather as though the protection diode D305 between grid and cathode has short-circuited. Put your ohmmeter across it (after switching off and leaving it for a coffee time to fully discharge the capacitors). This is a high voltage zener to limit the grid-cathode voltage during warmup or under fault conditions. Quite uncritical. Older scopes before the days of zeners used a little indicator neon. Philips liked the ZA1001-2-4 and Telequipment the NE2 glow neon., all 60 to 120V. Check the diode, and see what happens if you substitute a stray neon or zener.
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Old 4th Oct 2021, 7:46 pm   #152
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Telequipment D75 / D77 / D83.
I think you quoting the current rating 10ma of the EHT diodes used.
But the current used by the CRT is about 10ua,so total in the circuit will be 100ua or so. The current drop is irrelevant at these output voltages.
So almost any TV EHT diode will do. I have used BY182, BY710, BY409, BY509. All rated 10Kv upwards, 2.5 ma or greater. Raid the nearest scrapTV or Monitor with a tube.
Thank you very much for your helpful answer. If the forward voltage drop is not really important, it should be much easier to find a replacement. There are many diodes available for 6 kV to 12 kV rated with 5 mA. The only one with a low forward voltage drop is the HVCA DP6, which is only available at the manufacturer for commercial buyers. I initially thought that using diodes with a higher forward voltage drop would result in more calibration issues.

Quote:
Originally Posted by WME_bill View Post
The stuck brilliance sounds rather as though the protection diode D305 between grid and cathode has short-circuited. Put your ohmmeter across it (after switching off and leaving it for a coffee time to fully discharge the capacitors). This is a high voltage zener to limit the grid-cathode voltage during warmup or under fault conditions. Quite uncritical. Older scopes before the days of zeners used a little indicator neon. Philips liked the ZA1001-2-4 and Telequipment the NE2 glow neon., all 60 to 120V. Check the diode, and see what happens if you substitute a stray neon or zener.
wme_bill
Direct hit! I measured the zener yesterday and discovered nothing suspicious. However, I took your advice and measured it again white also applying various amounts of tension. At some point the diode suddenly measured only 22 Ω which is near to a dead short. Since I have no matching zeners or neons at hand, I have to order some replacements.
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Old 4th Oct 2021, 8:10 pm   #153
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Default Re: Telequipment D75 scope.

Yes, I warned about the rubber sleeves on the screened leads in the EHT area.
Am I following this correctly? It seems more likely that your corona flash has taken out the zener on the tube base, bringing the grid and cathode together instead of perhaps 70v neg on grid wrt cathode.
Those EHT diodes have been reported to fail, but it is not that common. If you do replace one, replace both with the same types, as you don't want one heating up quicker than the other. Hopefully if they follow each other, you should get less drift in brightness. Like Bill, I have a range of EHT diodes, but use BY182 for these if I change any.
Les.
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Old 4th Oct 2021, 9:23 pm   #154
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Default Re: Telequipment D75 scope.

Hi!

Will these help for spacing resistors, etc., off boards, mentioned in post no. 142?

https://www.ebay.co.uk/itm/Ceramic-I...-127635-2958-0

They have a 1.5 mm hole and about 3.3 mm dia by 3 mm long and offered in a pack of 50!

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Old 4th Oct 2021, 11:02 pm   #155
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Hi!

Will these help for spacing resistors, etc., off boards, mentioned in post no. 142?

https://www.ebay.co.uk/itm/Ceramic-I...-127635-2958-0

They have a 1.5 mm hole and about 3.3 mm dia by 3 mm long and offered in a pack of 50!

Chris Williams
Thank you for the link, but I initially tried to use those beads as an insulation insert for the solder posts in an original EHT transformer board. Since I could not find suitable ones, I eventually constructed a new board (see post 143).

Denis
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Old 7th Oct 2021, 10:22 pm   #156
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Default Re: Telequipment D75 scope.

Just got my hand on some replacement zener diodes and this indeed fixed the brightness issue on the D75. At first I thought it did not seem to work but then I remembered to readjust the grid cut-off voltage at R315 and brightness control is up and working again. Also, adjusting the grid cut-off voltage now looks more consistent than before the issue occurred. Turning the pot now has a sharp transition from a range where the whole trace, including flyback is visible in full brightness and the working range, where only the trace is visible at an adjustable brightness.

While playing with the controls, I discovered a minor issue with the TRACE LOCATE button: sometimes after pressing TRACE LOCATE, the trace remains kind of blurry. Usually, it can be brought back to normal, by pressing TRACE LOCATE again, a few times. I am not sure whether this is a sign of a dirty switch or some other effect.

In addition to that, the VAR/VOLTS DIV pots R641 and especially R642 need a cleanup. Unfortunately, it is very difficult to get to them and I am not a big friend of just drowning pots in Deoxit.
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Old 5th Dec 2021, 2:36 pm   #157
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Today, I wanted to make some measurements with my D75, and it came up much more stable after the zener diode change. However, something about the traces looked strange. Upon closer inspection, the amplitude of both channels had dropped to about 70% and the traces were a little bit blurry. I was not sure if the horizontal scale was also affected. Adjusting the focus knob was only partially successful. The focus on my unit had always been very fiddly to get right. Even minimal movements from the optimal position resulted in significant focus drifts over several seconds. Then, suddenly, the traces were back to their intended height, the focus got super sharp, and, the hyper-sensitivity of the focus knob was gone. Now, the focus is super sharp over a wide range of the knob adjustment range and almost behaves like there is an internal focus auto-lock-in. Turning the knob all the way counter clock wise brings only a small loss of focus, turning it all the way clock wise results in a complete loss of focus with a 1.5cm wide trace.

So my questions to other D75 owner are now: Is the focus knob of the D75 usually very sensitive or insensitive? Is this off centered defocussing behavior normal? I cannot compare it with my D755 at the moment, because I have to fix some other issues there fist.

At the moment, I am suspecting that a resistor near the focussing potentiometer, maybe R323, R324, R325, the pot itself my have gotten high. Or maybe the is an issue with the 150V zener diode D305. However, that does not explain the observed drop in vertical deflection.
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Old 8th Dec 2021, 8:04 pm   #158
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Meanwhile, I measured all the remaining carbon resistors in the focus chain. Most were moderately off-valued, but most important: their value changed significantly in comparison to the last time I measured them. Luckily, the last time I bought VR37 high voltage resistors, I already ordered a suitable set of these. Swapping could be done without removing the board, however, a long DMM test lead clamp was needed to insert the new ones at the back of the board underneath the large HV capacitors there. Now, the focus adjustment is very consistent. However, the centering issue did not vanish and I did not discover any clues regarding the amplitude drop issue. I am starting to suspect the focus pot being responsible for the off centered focus adjustment. All resistors in the focus and HV chains have been changed and are perfectly in spec(so the ratio of the chains should be spot-on) but the focus pot has gone moderately high. Unfortunately, its cyan colored plastic case does not look, like it can be opened without breaking something. The amplitude drop issue has not reappeared, so it's difficult to get by.
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Old 1st Jun 2022, 11:02 pm   #159
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Today I dug out the TQ calibration fixtures 067-0672-00 and 067-0673-00 again and tried to come up withe a way to verify their calibration. Like mhennessy already explained earlier, the part of both units that is used to set the horizontal and vertical center offsets and the respective gains/voltage ranges is comparatively simple. It should be in cal when all caps, resistors and transistors meet their respective specs, which is easy to test. The complicated part is the factory calibrated HF response circuit in the vertical plugin. As I see it, the vertical plugin just contains a one channel as-basic-as-possible version of the vertical circuits from the amplifier plugins without anything in the way to negatively influence an incoming signal. Then, at the emitters of the long tailed pair differential amplifier there is the HF response tuning circuit, that is usually responsible for calibrating the HF response of the amplifiers in their last stage (see attached image of PC152 of the V4 amplifier). This circuit acts as a standardized replacement during the mainframe calibration, to ensure all mainframes are calibrated with respect to the same reference. Then calibrated plugins can later be swapped between mainframes without getting another HF response.

As far as I understand it, a good HF response targets to do as little harm as possible to a very fast rising edge of a square wave, namely an as fast as possible rise time and as little overshoot and ringing as possible. Now I wonder, if it is possible to detect kind of a meaningful response optimum that is independent of the mainframe, by adjusting the two variable caps and the variable resistor in the HF circuit. For example, when taking only the plugin and supplying it with proper 24V and -24V and a fast square wave, while monitoring its signal output pins 5 and 13 of the plugin with a suitably fast and calibrated scope. For something like this it was suggested earlier that the calibrator output must be loaded correctly. Of course in this case, "correctly" could mean that it was established purely "by definition" of some arbitrary reference. But maybe there is some way to derive it from a detectable optimum, e.g. by using a purely resistive load?

One approach I tried to come up with an idea for this was to find something similar. For the Tek 7000 series there are some signal standardizer plugins (e.g. the 067-0587-01) that are used during the calibration in a similar way. However, the respective circuits there do not seem to contain trimmer capacitors for frequency response, only variable resistors.
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Old 26th Jun 2022, 4:45 pm   #160
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Default Re: Telequipment D75 scope.

Further investigations to tackle the calibration plugin calibration issue have been made.
After doing some basic component checking to ensure the vertical 067-0672-00 plugin would not short the scope, I had a look at some actual waveforms on my D755. The setup consisted of my Bradley 192 feeding a 1MHz fast-rise signal to the scope via a 50Ohms terminated BNC cable. The attached images show in order:
  1. The signal through the 067-0672-00 with proper termination at the scope side @ 0.1µS/div
  2. The signal through the 067-0672-00 with proper termination at the scope side and 10x horizontal magnification
  3. The signal through the 067-0672-00 with termination at the 192 side and 10x horizontal magnification
  4. The signal through the V4 plugin with proper termination at the scope side and 10x horizontal magnification
  5. A signal comparison between the 067-0672-00(red line) and the V4(blue line)
So, my takeaways from this one are:
  1. Always terminate at the scope side
  2. The calibration plugin seems to work, but at this point it is unclear whether it is in cal
  3. The V4 shows a slower rise time than the calibration plugin
  4. Using a cheap 50Ω china feed through terminator vs a quality one on a BNC t-connector made no visible difference
  5. My D755 with the calibration plugin has a bandwidth of approximately 58 Mhz, with the V4 only around 44 Mhz, so that could most likely be improved in a proper calibration

As a next step, I tried to pimp my knowledge about scope amplifiers. So I tried to take a bite out of Teks 062-1145-00 Vertical Amplifier Circuits from the Tek concept series. Unfortunately, I seem to lack quite some basis in electronics knowledge to fully understand many of the details there. My main focus was to understand, how HF compensations works and if and how I could use this knowledge to derive a calibration strategy for the vertical calibration plugin. As far as I understood it, most of it comes down to keeping the ratio of phase shift constant across the frequency range of an amplifier.

Some of the transistor based amplifier circuits are quite similar to the one in the calibration plugin, especially regarding the HF compensations circuits, which are called HF peaking circuits there (see. Fig. 6-54 on page 338).

Since I could not figure out, which adjustment has which effect on the response curve, I hacked the 067-0672-00 circuit into Micro-Cap 12(once over 4K€, now free) and tried to simulate the plugin in terms of a transient analysis. To estimate a proper resistive load for the circuit, I measured the resistance between the plugin signal out connectors at the scope side(~0.75KΩ) and their respective resistances(~1.5KΩ) to ground without an inserted plugin. Loading the circuit with this, I had the impression that the circuit is overloaded and only little effects of the HF peaking controls could be observed. Loading the circuit with much higher resistances, like 1MΩ produced signal responses which could be influenced via the HF peaking controls in a reproducible way. However, the behavior was always that reducing the HF compensation capacitances to zero gave the best result, which seemed illogical. Also the response curves showed no ringing, which lead me to the suspicion, that my simulation still lacks several important aspects. Does anyone here have experience with HF simulation of amplifiers in LTSpice or Micro-Cap?

Denis
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