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Old 5th Sep 2020, 3:01 pm   #121
MrBungle
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Default Re: Telequipment D75 scope.

Might be worth replacing it with a slow blow and a PTC in series instead.
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Old 7th Sep 2020, 4:36 pm   #122
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Default Re: Telequipment D75 scope.

Fuses are not the precise thing a lot of people seem to think they are. It is not there to go open-circuit when the 'scope consumes 1001mA from the mains - it is there in case of a major failure like a shorted turn in the transformer.

With that in mind, there is no point getting worried about this - any 1A time-delay fuse that is the same physical size will do the job.

I've just taken a look at several of mine, and they all have 1A fuses from random manufacturers. No idea if any of them are original or not.

Fuses can fail for no apparent reason when they get old. I've not yet seen it with a Telequipment 'scope, but have seen it plenty of other times.

So, if a replacement "1AT" fuse fails, you've got a problem with your 'scope. If not, don't worry about it.

I don't know which suppliers you favour, but this link should show you how many choices you have from RS:

https://uk.rs-online.com/web/c/fuses...419,4294584567

I'd probably not risk eBay and Amazon Marketplace for something like this.

If you're in any doubt about the power supply, then it would be worth using a lamp limiter while you investigate.
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Old 7th Sep 2020, 7:44 pm   #123
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Default Re: Telequipment D75 scope.

Thank you for the answers. I already figured out that the response time of a fuse at a given current is specified as a quite large interval for different current values, what leads to a min and max graph specification. What got me thinking was that the interval can apparently be very different for fuses of the same type. A 1 A time-delay fuse blowing anywhere from 1 to 5 seconds for 2 A is quite a difference to one blowing anywhere from 10 to 30 minutes at the same 2 A. In many cases the response times get more similar (and much smaller) for higher current values, but I was concerned about the moderate overload situations.
The TDC11 is not a simple wound up wire like other time-delay fuses but has a three component construction. I consists of a fast blowing fuse wire rated for a higher amperage than the actual fuse to ensure fast blowing at really high currents. The second component is a metal with a low melting point and the third one a comparatively high resistance wire that forms a spring and can heat up the low melting point metal, which is then pulled apart by the spring. This allows much better control about the fuse response time at different loads. I just thought, a fuse with a comparatively short response time for moderate overloads may have been chosen for a reason. However, if you say it does not matter, I could try the fuses from my local dealer.
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Old 7th Sep 2020, 9:36 pm   #124
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Default Re: Telequipment D75 scope.

Yes, I'm familiar with the construction of your fuse.

However, all the fuses I looked at earlier (4 or 5) were of the simple construction - just a bit of wire. OK, I can't be sure if any of them are original, but based on the very limited data we have so far, it's perhaps looking more likely that yours is a replacement.

Perhaps others with these 'scopes would be willing to look at their fuses to see if anyone has the specific type that yours has?

Or perhaps it really doesn't matter
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Old 25th Oct 2020, 10:30 pm   #125
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Default Re: Telequipment D75 scope.

During some repair work on my Bradley 192 calibrator, the brightness of my D75 suddenly dropped severely in several steps over time. I immediately suspected one of the 10K carbon resistors in the HV-section and measurements revealed that R322 had gone high to 80M. While I was at it, I measured the other partially highly inaccessible carbon resistors of the same type on PC148.

R317, R318, R319, R321 -> ~10 MOhms (should be 10 MOhms)
R322 -> 80 MOhms (should be 10 MOhms)
R323 -> 5.2 MOhms (should be 3.9 MOhms)
R324 -> 4.8 MOhms (should be 4.7 MOhms)
R325 -> 5.8 MOhms (should be 4.7 MOhms)
R326 -> 7.8 MOhms (should be 5.6 MOhms)
Outch!

The HV Metal film ones 304, 305, 309 and 313 all measured just above 6.7 MOhms with an intended value of 6.8 MOhms, so they should be OK.

Since I cannot find identical resistors, I will probably replace all the carbon resistors listed above with Vishay VR37 high voltage resistors. Unfortunately, it seems to be quite a challenge to find a dealer that can supply all the values without charging a fortune for shipping.

Regarding the accessibility: does anyone know the best way to access PC148 for swapping the resistors without desoldering all the wires?


A second issue showed up on my D755. Just as I thought, I had exterminated all issues, it started behaving strange again. The issues are slight brightness changes and twitching of the trace during start-up and operation. Probably connected to that is that once the scope has warmed up for about 15 to 20 minutes the horizontal and vertical deflection of the trace suddenly gets stronger. This leads to kind of a magnification effect on the trace. The strength of magnification can be influenced by turning the brightness up and down. This can be done only after the magnification effect has appeared. Before that, changing the brightness has no effect on the scale of the trace.

At first I suspected an issue with the PSU because it had been quite troublesome with this scope in the past. However the -24 V, +24 V and 105 V rails seem to be stable and do not follow the trace changes when adjusting the trace brightness. the 30 V rail is a little low, but also seems to be stable. So probably no PSU issues there.

For further testing and trying to rule out issues with the horizontal and vertical amplifiers, I set the V4 to zero coupling, moved the trace up and down via the vertical position control and then changed the brightness. The trace seems to move always away from the center of the screen, the more pronounced the farther away it is at the beginning. In addition to that, the focus of the trace seems to change ever so slightly but somehow proportional to the trace magnification effect. So I am suspecting a general issue with either the deflection or the acceleration voltages. The fact that the effect is linked to the trace brightness while the PSU voltages are stable could be pointing to an issue on PC 148 or PC 128. Unfortunately that is exactly the area of the scope with voltages high enough to overload my DMM, so I have to be very careful there and I cannot measure the voltages at the tube. My next step will probably be to check all testing points in the circuit diagram that my DMM can handle for a proportional reaction to brightness changes. Any hints where to look would be appreciated.
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Old 26th Oct 2020, 1:04 pm   #126
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Default Re: Telequipment D75 scope.

Telequipment D75 and D83.
Those resistors in the EHT chain always fail. It was a false economy by the manufacturers (Tektronix owned by then) to use standard carbon film or metal film, which are rated at about 250v. All these resistors will go high under stress over a period of perhaps a year or more, depending how much the instrument is kept switched on.
The proper solution is to change to metal glaze, as Philips / Vishay VR37, which are rated at 2500V. Lovely light blue body, you can often find several suitable in a old TV chassis.
However, the exact value down the EHT chain is not critical, as long as the ratio each side of the focus control is about right, and the grid chain is many megohms.
You can make up a chain for these scopes with 5 of 10M for the grid chain, and 10M+5M and 5M for the focus/cathode chain. Adjust the focus chain if necessary by inserting a 1watt carbon which are rated at 750V.
After all the scope you have was made between 1974 to 1980, so to fail now after 40 years or so is not too bad. Early versions of the PCB have space for lots of resistors in the chain, later versions use just one or two resistors.

The only resistor from the results you have obtained which needs replacing, I suggest, is the 80M R322 in the grid chain. Should be 20M. Put a couple of 10M carbon film in as a short term repair, and look out for some nice light blue ones. The others on the focus chain R323-4-5 are close enough in ratio each side of the focus control and it should operate correctly.

On the D75, you can get at the back of EHT power oscillator pcb148 quite easily without unsoldering everything by removing the securing screws and undoing the oscillator transistor on its heat sink.

One other tip. Most of the PCB are earthed to chassis by the fixing screws. An oxide film can build up between the aluminium chassis and the back of a PCB, giving a most mysterious fault. So check the securing screws are firm and the chassis clean underneath, particularly for the Y Amplifier and the Bright-up.

Your other trouble could be the Bright-up circuit itself. You can check that without a High Voltage probe. The transistors there often seem to fail. The output voltage swing is 40v or so, when you switch from ExtX to TB, not triggered.

You can make a high voltage probe easily with a string of standard 10M carbon and put your DVM or Avo8 across a 1M at the bottom of the chain. Calibrate it against a existing supply, such as the X amplifier feed. Those resistors will not fail under voltage stress in the 10 minutes that you use it. Just recalibrate each time you start using it again. See previous notes by me and others. Do a search here, may be under D83.
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Old 26th Oct 2020, 10:56 pm   #127
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Default Re: Telequipment D75 scope.

Quote:
Originally Posted by WME_bill View Post
Telequipment D75 and D83.
Those resistors in the EHT chain always fail. It was a false economy by the manufacturers (Tektronix owned by then) to use standard carbon film or metal film, which are rated at about 250v. All these resistors will go high under stress over a period of perhaps a year or more, depending how much the instrument is kept switched on.
The proper solution is to change to metal glaze, as Philips / Vishay VR37, which are rated at 2500V. Lovely light blue body, you can often find several suitable in a old TV chassis.
However, the exact value down the EHT chain is not critical, as long as the ratio each side of the focus control is about right, and the grid chain is many megohms.
You can make up a chain for these scopes with 5 of 10M for the grid chain, and 10M+5M and 5M for the focus/cathode chain. Adjust the focus chain if necessary by inserting a 1watt carbon which are rated at 750V.
After all the scope you have was made between 1974 to 1980, so to fail now after 40 years or so is not too bad. Early versions of the PCB have space for lots of resistors in the chain, later versions use just one or two resistors.
Thank you for the reply. My D75 is an early version with carbon resistors, my D755 already has the fewer high voltage ones. I will try to understand how the EHT chains work based on your description.
Quote:
Originally Posted by WME_bill View Post
The only resistor from the results you have obtained which needs replacing, I suggest, is the 80M R322 in the grid chain. Should be 20M. Put a couple of 10M carbon film in as a short term repair, and look out for some nice light blue ones. The others on the focus chain R323-4-5 are close enough in ratio each side of the focus control and it should operate correctly.
I will probably try to replace all carbon resistors in the MOhms range in the EHT to ensure reduce necessity of future repairs. Unfortunately I do not have even one ~10 MOhms resistor lying around for a quick test.

Quote:
Originally Posted by WME_bill View Post
On the D75, you can get at the back of EHT power oscillator pcb148 quite easily without unsoldering everything by removing the securing screws and undoing the oscillator transistor on its heat sink.
Hmmm, I will definitely check on that. My first impression was that even after unscrewing PC 148 it cannot be moved because it is connected to short wires in nearly every direction.

Quote:
Originally Posted by WME_bill View Post
One other tip. Most of the PCB are earthed to chassis by the fixing screws. An oxide film can build up between the aluminium chassis and the back of a PCB, giving a most mysterious fault. So check the securing screws are firm and the chassis clean underneath, particularly for the Y Amplifier and the Bright-up.
Thanks, that sounds like a very valuable tip. I would definitely not have come up with that.
Quote:
Originally Posted by WME_bill View Post
Your other trouble could be the Bright-up circuit itself. You can check that without a High Voltage probe. The transistors there often seem to fail. The output voltage swing is 40v or so, when you switch from ExtX to TB, not triggered.
Not sure if I understand this correctly. I will see if I can locate that part of the circuit on the diagrams.
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Old 8th Nov 2020, 5:32 pm   #128
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Default Re: Telequipment D75 scope.

I'm still waiting for the replacement resistors for my D75 to arrive.

Debugging my D755 has gotten even more complicated, since it decided to catch yet another illness. Regarding the zooming-in issue described earlier(see first two images), I checked most of the resistors and transistors on PC128, but I did not find anything suspicious yet. TR354 and TR351 are, however, not socketed, so I will have to desolder those for testing. At the moment, I am not sure, if testing the circuit components in a non-powered state will show any errors, because the zooming issue only occurs after a 10 minute warm-up phase. Also, I still have not figured out how exactly to measure the 40V output voltage swing as described by wme_bill. According to the manual, there should be a 6V square wave.

In addition to that, the unblanking circuit now stops working after a short while of operation. When that happens, the brightness ramps up significantly (see images three and four) and cannot be turned down again via the power knob. It can, however, be turned up even further. One can clearly see that the flyback of the trace is then visible on screen. Depending on the shown signal, this can be in form of a simple line or two line segments, connected via a steep drop. Keeping in mind the Tektronix oscilloscope troubleshooting guide, I am kind of suspecting that the DC-restorer HV diode D303 at the HV-transformer may have failed. If I remember correctly, HV-diodes are simply several diodes in serial. So a normal DMM-diode test will probably not work here. Instead, I could remove the diode, apply a larger voltage over a suitable resistor(e.g. 5kohms, 5W) in series and then measure the voltage drop over the diode. At the moment, I am kind of hesitating desoldering the diode between the HV transformer and PC148, because I do not know how the transformer soldering posts react to heat and it is kind of difficult reaching that place.

Moreover, I have not been able to find a source for suitable replacement HV-diodes (6kV, 10mA) yet. Can I pick ones with larger voltage and current rating? Is it maybe important to pick one that consists of the same amount of serialized diodes, since that would have a large influence over the voltage drop. On the other hand, a few volts are small compared to the maybe 2k5? volts present here.
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Old 19th Nov 2020, 12:49 am   #129
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Default Re: Telequipment D75 scope.

Quote:
Originally Posted by WME_bill View Post
On the D75, you can get at the back of EHT power oscillator pcb148 quite easily without unsoldering everything by removing the securing screws and undoing the oscillator transistor on its heat sink.
Do you know any magic spell to actually do that? I just spend two hours trying to unscrew the four countersunk pozidriv screws that secure the hex shaped brass mounting posts of PC148 to the case without tearing everything to shreds. The screws did not move one nanometer and one already bad screw is now stripped. Applying more force will only strip additional screw heads. Heat treating the connection or freezing the screw driver did not work, neither did applying WD40. Finding a working screw extractor for a screw this small will be near to impossible.

I can get all the other screws on the right side panel of the case to move. However, I cannot remove them entirely because there is no way to ever get the part cylindrical, part hex shaped nuts on the other side in place again with PC148 still mounted to the side panel. Unscrewing the hex nuts on the top of PC148 is not possible either, because there is no way to get a hex socket or a pair of pliers in there. Looks like my D75 is screwed.
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Old 29th Nov 2020, 12:18 am   #130
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Default Re: Telequipment D75 scope.

I finally managed to loosen the screws holding PC148. The actual issue was that the screw heads had started cold welding to the chassis. Thus, the probability to be able to remove the screws only by using a screwdriver was exactly zero. The trick was to get a 6BA open ended spanner and give the four hexagonal mounting posts a 1/4 clockwise turn. Then the screws came off easily. The only mounting post that also could have been turned with a pair of pliers is the bottom right one. The other three, however, ... no way.

After that, I could replace the 10M resistors on the brightness chain with VR37 ones and the scope worked again. It still has small occasional sudden brightness changes, but nothing critical. Maybe this will be fixed, once I manage to find a way to also replace the resistors on the focus chain.

With the D75 working again, I continued to debug the D755. Unfortunately the issue seems to be kind of a heisenbug (it disappears if you try to observe it). It can run for hours, without the issue showing up. However, it seems that the issue is not in the bright-up circuit on PC128. Its output signal is exactly like expected, a square wave with the lower level at 6 V and the upper level depending on the set brightness. Also the bright-up output signal does not change when the issue occurs.

Next, I examined the low voltage side of PC148. The amplitude and the DC offset of the oscillator signals at test points 1 and 2 seems to be right. The frequency, however, is not at 28 kHz but at 40 kHz. In addition to that, the sawtooth at test point 3 has no flat top and its amplitude is too large. Unfortunately, I could not observe yet what happens to these three signals when the issue occurs. I have to find a way to trigger the issue. I does not seem to be directly heat related, since heating PC148 with a hair dryer could not trigger it.

At the moment, I am kind of expecting something on the high voltage side on PC148 to draw too much power, driving the EHT supply out of regulation. All in all the issues look a lot like the issues @Anode_to_Joy described in this thread. Unfortunately his solution was close to replacing everything on PC148.

One time, I could observe continuous arcing at the output of EHT transformer connecting to D303. It occurred on the back of the soldering post, inside the transparent, wax-like coating near the paxolin board. While it was there, the brightness of the trace was fluctuating.
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Old 29th Nov 2020, 6:45 pm   #131
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Default Re: Telequipment D75 scope.

I may have mentioned this before up thread, but a little problem that crop up is the rubber sleeving (see you second pic in last message) can go leaky. I had it a couple of times with the black rubber sleeving.
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Old 29th Nov 2020, 11:47 pm   #132
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Default Re: Telequipment D75 scope.

Quote:
Originally Posted by MotorBikeLes View Post
I may have mentioned this before up thread, but a little problem that crop up is the rubber sleeving (see you second pic in last message) can go leaky. I had it a couple of times with the black rubber sleeving.
Les.
Thank you for the hint. I will keep an eye on those. However, on my D755, all leads around those connections have additional insulation sleeves. In addition to that, a closer look on the place at the transformer I observed arcing at revealed a poorly soldered solder joint with a pointy extension. If I remember correctly, solder joints like that can be an issue in high voltage environments. Maybe I am lucky and it is just that.

It would be nice, if I had a way to determine on which winding (grid or cathode/focus side) of the transformer the arcing occurs. Seems like I have to finally get myself a high voltage probe and a bench multimeter with fast logging capability, like a Keithley DMM6500. Alternatively, a cheap differential high voltage probe like the Micsig DP20003 might also do the trick.
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Old 17th Jan 2021, 3:42 pm   #133
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Default Re: Telequipment D75 scope.

With the help of an HV-probe, I managed to narrow down the issues on my D755 to a leaking grid voltage. The most likely culprit is, unfortunately, the EHT transformer and more specifically the second pin to the right on the first attached picture. As I see it, my options are to replace the whole paxolin board with some close to 3.5mm thickness FR-4 board, or to insulate the solder posts on the HV side in the existing board. For insulation, I could try some small ceramic tubes or heat shrinking tube. While the former seems very hard to acquire, the latter might make the posts unstable. This might be an issue, since e.g. the attached cable of the quintupler is under quite a lot of tension. Thick FR-4 board, on the other hand, seems also hard to source in small quantities. However, for both solution strategies, I have to remove the solder posts from the paxolin board first. They are coated with a surprisingly hard and heat resistant, semi-transparent coating which seems to be very hard to remove without damaging the flimsy transformer wires, especially on the low voltage side. Has anybody here done this operation before and can share some hints on damage-free post removal and replacement options?
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Old 17th Jan 2021, 8:14 pm   #134
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Default Re: Telequipment D75 scope.

I have never come across this myself, but I believe it is a well known problem with similar TEK HV transformers. Sometimes they are charred I believe. You could try drilling a series of stepped holes around the connection pin / pins thereby increasing the insulated path, and reducing likelihood of further leakage. Alternatively, unsolder the wires, cut off much of the paxolin, and add another layer above it fixed to the rest of the original with some epoxy resin.
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Old 6th Feb 2021, 9:30 pm   #135
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Default Re: Telequipment D75 scope.

Quote:
Originally Posted by MotorBikeLes View Post
I have never come across this myself, but I believe it is a well known problem with similar TEK HV transformers. Sometimes they are charred I believe. You could try drilling a series of stepped holes around the connection pin / pins thereby increasing the insulated path, and reducing likelihood of further leakage. Alternatively, unsolder the wires, cut off much of the paxolin, and add another layer above it fixed to the rest of the original with some epoxy resin.
Les.
Sorry for the late answer. I didn't notice your reply until now, because the email notification did not work.

I found that the pins can be removed by carefully using a pair of pliers after completely removing all solder with solder wick. The 1.5mm diameter pins are made of bent metal sheet with a bump on the reverse side of the board. This way, they get fixed to the paxolin board as soon as the transformer wires are soldered to the other side. At the moment, I see two repair options. Option one would be to replace the board with a new paxolin (often called Pertinax here in Germany) or FR4 board. The board thickness is 3.5mm and I could get some pre-cut 4mm Pertinax or FR4 for less than 20€. This would have the disadvantage, that I would have to remove all solder posts, even the ones with the flimsy wires at the low voltage side.

The other option would make use of some sleeves, consisting most likely of silicone carbide, that I found on some power MOSFET legs in my magic part box (see attached image). They have an inner diameter of 1.5mm, an outer diameter of 3.5mm and the perfect height for the job. I could widen the original holes on the HV-side of the transformer to 3.5mm, insert and glue in the carbide sleeves and insert the transformer solder posts into the sleeves.

However, both solutions would require tools for precision drilling perpendicular holes into the boards and I do not own a drill stand, just a Dremel-like handheld multi-tool. In addition to that, both solutions would require a HV-insulating and heat resistant lacquer, to cover the posts. I think, part of the observed arcing at the solder post was due to a damaged lacquer coating. The two-component epoxy glue I have would most likely melt during soldering the wires back on. So I tried to determine what the original lacquer on the board is. At first, I suspected it to be shellac, but putting a flake of it in methylated spirits did not dissolve it, even after a week. So it is probably made of something else.

Does anyone have a hint what HV-insulating epoxy could be used here?
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Old 7th Feb 2021, 3:42 pm   #136
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Default Re: Telequipment D75 scope.

D75. Leakage across the high voltage terminals of the EHT transformer.
MotorBikeLes is quite right. There was some discussion about this fault in 2013 and also a bit earlier.
I posted the cure I have used since several times in April 2013 is to use a standoff insulator or a TV tuner ceramic feed through capacitor to make a new well insulated terminal. Reduces the risk of the wire breaking off as you try to re-insulate the paxolin panel. Search under Telequipment D83 EHT.
If you want to do it the hard way, a blob of Araldite will insulate quite well or I have also used Tensol 70, the perspex cement, even better.
You will soon be wanting to measure the high voltages. This keeps coming up. See postings of November 2011 (under D83) and more recently of October 2013 upon making a high voltage probe for EHT supplies, and is simple provided you re-calibrate each time.
wme_bill.

Last edited by WME_bill; 7th Feb 2021 at 3:52 pm.
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Old 7th Feb 2021, 5:01 pm   #137
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Default Re: Telequipment D75 scope.

Quote:
Originally Posted by WME_bill View Post
D75. Leakage across the high voltage terminals of the EHT transformer.
MotorBikeLes is quite right. There was some discussion about this fault in 2013 and also a bit earlier.
I posted the cure I have used since several times in April 2013 is to use a standoff insulator or a TV tuner ceramic feed through capacitor to make a new well insulated terminal. Reduces the risk of the wire breaking off as you try to re-insulate the paxolin panel. Search under Telequipment D83 EHT.
If you want to do it the hard way, a blob of Araldite will insulate quite well or I have also used Tensol 70, the perspex cement, even better.
You will soon be wanting to measure the high voltages. This keeps coming up. See postings of November 2011 (under D83) and more recently of October 2013 upon making a high voltage probe for EHT supplies, and is simple provided you re-calibrate each time.
wme_bill.
Thank you, nice hint. I did not think of ceramic feed through capacitors. However, I am not sure if something like this would work. At the soldering post which I observed arcing at, the coronary discharge jumped at least 1mm farther than the insulation distance of these capacitors. It may be that this was just possible due to surface creepage on the somewhat conductive surface of the old insulation lacquer. I found HV clearance computations on the web that suggest between 10mm and 15mm of clearance between 2.5kV conductors which could not be achieved just by using feed through capacitors or my ceramic sleeves. This is the reason, I am suspecting that I need some extra insulation lacquer. The issue with epoxy based two-component glues like Araldite (or something like UHU Endfest 300 here in Germany) is that I am not able to find specifications on their electrical insulation or moisture absorbance properties. This would be important, since once you put this stuff on, it cannot be removed anymore and could potentially ruin everything. The best solution would maybe an insulation lacquer that hardens into a brittle state, so it can be removed mechanically.

For measuring high voltages, I have acquired a HV-probe for 10MOhm DMMs.
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Old 7th Feb 2021, 7:50 pm   #138
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Default Re: Telequipment D75 scope.

One item confuses me. you refer to Silicon carbide insulators. Silicon carbide is conductive. Not a simple conductor, but high negative resistance with increasing temperature and I think VDR properties as well.
When I first started work, we had "Globar" kiln; the elements were silicon carbide rods. Capable of very high temperatures, but increased series resistors had to be added as the temperature increased.
Epoxy resins seem to have very high electrical resistance, but I often use a Loctite compound for holding stuff together. NOT high strength like epoxy, but it would probably be perfectly OK for your job. It is a silicone based sealant, NOT acetic acid based. Loctite 5920 as I recall.
Les.
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Old 7th Feb 2021, 8:44 pm   #139
Pinörkel
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Default Re: Telequipment D75 scope.

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Originally Posted by MotorBikeLes View Post
One item confuses me. you refer to Silicon carbide insulators. Silicon carbide is conductive. Not a simple conductor, but high negative resistance with increasing temperature and I think VDR properties as well.
When I first started work, we had "Globar" kiln; the elements were silicon carbide rods. Capable of very high temperatures, but increased series resistors had to be added as the temperature increased.
Epoxy resins seem to have very high electrical resistance, but I often use a Loctite compound for holding stuff together. NOT high strength like epoxy, but it would probably be perfectly OK for your job. It is a silicone based sealant, NOT acetic acid based. Loctite 5920 as I recall.
Les.
Hmmm, the insulating sleeves I referred to are shown in the picture in my previous post. I am not 100% sure they are made of silicon carbide, but they look like it and show the same fracturing pattern. I found them in a defective ATX power supply on the legs of some 500V rated power MOSFETs. Since they were used in a 240V environment and all of my DMMs showed an unmeasurable resistance value larger than their maximum measurable value of 100 MOhms, I assumed, they should be safe to use. Maybe I am wrong. Originally, I tried to acquire some identically sized aluminium(III) oxide sleeves. However, ones of the right size are very hard to find and usually only offered in very large quantities for industrial buyers.
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Old 8th Feb 2021, 12:02 am   #140
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Default Re: Telequipment D75 scope.

Your suspicion was right, Les. I examined the sleeves a little further and found out that they are magnetic, so maybe a kind of ferrite. Poking google about MOSFETs and ferrite, I learned that they are usually referred to as ferrite beads and that they are used to add impedance for filtering high frequency oscillations like ringing on fast switching MOSFETs. Apparently, ferrites usually have very high specific resistances and the fact that my measurement equipment cannot measure a resistance hints at it being larger than 100 MOhms. As I understand it, the HF dampening of ferrite beads gets significant at frequencies of several MHz. Since the EHT oscillator in the D75 runs at 28kHz, it could be OK, but I am not sure if even a very small impedance there could lead to unwanted side effects or even to the ferrite beads warming up.
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