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Old 26th Jul 2019, 1:13 pm   #41
Alistair D
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Default Re: Telequipment D75 scope.

I have 3xV4s here. These are my results for the D604 readings.

V4/1 V4/2 V4/3
0 mins 10.473 10.429 10.430
5 mins 10.657 10.581 10.589
10 mins 10.678 10.600 10.612

On all 3 units the voltage rose quite rapidly during the first 30 seconds or so.

Regarding the other measurements you want, can you give me the exact circuit references just to be sure I am measuring the exact point you require. I assume that these readings are to be taken after 10 minutes.

Al
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Old 26th Jul 2019, 3:47 pm   #42
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Default Re: Telequipment D75 scope.

With regards to the 10V zener, it is expected to have a positive tempco, meaning the voltage across the diode will increase with heating. That could be expected to slightly alter the balance but most of the drift on that power rail should be largely ignored , but it could be worth trying a new one and as suggested adding a series diode to help cancel some of that drift.

It looks to me though that the designers were struggling with temperature drifts which is why they added the thermistors in x5 mode. Also as I recall those dual fets do run warm. Looking at the board layout, there is a different physical spacing between the thermistors and the dual fets for the two channels. Try bending the CH2 thermistor (TH602) away from the body of the fet(TR607) so the the physical spacing of it from the fet is more like the channel 1 situation.
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Old 26th Jul 2019, 5:58 pm   #43
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Default Re: Telequipment D75 scope.

Quote:
Originally Posted by Alistair D View Post
I have 3xV4s here. These are my results for the D604 readings.

V4/1 V4/2 V4/3
0 mins 10.473 10.429 10.430
5 mins 10.657 10.581 10.589
10 mins 10.678 10.600 10.612

On all 3 units the voltage rose quite rapidly during the first 30 seconds or so.

Regarding the other measurements you want, can you give me the exact circuit references just to be sure I am measuring the exact point you require. I assume that these readings are to be taken after 10 minutes.
Hello Al,
thank you for the measurements. As I can see from your measurement values, you measured the voltage across the diode instead of the voltage to ground. I just did the same on my unit and got the following values:
10.32V 0 min
10.57V 1 min
10.63V 3 min
10.646V 5 min
10.648V 10 min
This looks a lot like the diodes in your V4s, so I think, its normal.

Regarding the additional measurements, I will prepare a marked circuit diagram and a PC152 photo for you later, if I run out of other debugging options. I do not want to steal more of your valuable time than necessary.

Quote:
Originally Posted by Argus25 View Post
With regards to the 10V zener, it is expected to have a positive tempco, meaning the voltage across the diode will increase with heating. That could be expected to slightly alter the balance but most of the drift on that power rail should be largely ignored , but it could be worth trying a new one and as suggested adding a series diode to help cancel some of that drift.

It looks to me though that the designers were struggling with temperature drifts which is why they added the thermistors in x5 mode. Also as I recall those dual fets do run warm. Looking at the board layout, there is a different physical spacing between the thermistors and the dual fets for the two channels. Try bending the CH2 thermistor (TH602) away from the body of the fet(TR607) so the the physical spacing of it from the fet is more like the channel 1 situation.
Hello Argus25,
thank you for your expertise regarding the 10V zener. I did not expect the drift at this point to be kind of unimportant. Your analysis of the role of the thermistors sounds very interesting. I just did what you suggested with TH602 and the result was a much more consistent convergence of the CH2 drift. It now settles after about 5 minutes. However, it still has a kind of inconsistent convergence point which I attribute to the highly wiggle sensitive CH2 vertical position potentiometer. CH1 is perfectly usable after only 1 minute of warm-up and does not have any wiggle problems at the corresponding potentiometer.

Since the initial deviations from previously optimized vertical positions on cold startup of CH2 are 5 times more off than for CH1, I still have to find the component causing this. I have also the impression, that CH2 reacts more sensitive to the potentiometer than CH1. Maybe I should also try a second cleanup of the double vertical position potentiometer of CH2, although I already did that very thoroughly on both double potentiometers.

Denis
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Old 28th Jul 2019, 8:39 am   #44
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Default Re: Telequipment D75 scope.

The voltages I measured were across D604 as you said. When I am next working on the scope I will do one set ground referenced.

No need to mark up a diagram, a list of component references will be fine.

Al
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Old 29th Jul 2019, 10:51 am   #45
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Default Re: Telequipment D75 scope.

Just a heads up here and I hope this doesn't violate the ebay policy on here but there's a load of telequipment plugins appeared on ebay here: https://www.ebay.co.uk/itm/362703426707
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Old 29th Jul 2019, 5:21 pm   #46
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Default Re: Telequipment D75 scope.

Quote:
Originally Posted by Alistair D View Post
The voltages I measured were across D604 as you said. When I am next working on the scope I will do one set ground referenced.

No need to mark up a diagram, a list of component references will be fine.
I did some additional measurements to further pin down the location of the fault, but did not succeed. From the fact that channel 1 works comparatively well, I conclude that the voltage drift at D604 can only be a small part of the problem. A second thorough cleaning of the corresponding vertical position double potentiometer did not solve the problem. At least It killed some of the pots wiggle sensitivity. Maybe I will get an optical interface adapter for my DMM to log the voltage drifts on my PC.

Quote:
Originally Posted by MrBungle View Post
Just a heads up here and I hope this doesn't violate the ebay policy on here but there's a load of telequipment plugins appeared on ebay here: https://www.ebay.co.uk/itm/362703426707
Thank you very much for the hint. I'll keep an eye on that. Some of these seem to have a fair bit of cosmetic damage and all serial number plates have been removed. So maybe they could be good for spare parts. They seem to only ship to UK though.

Apart from that, I could not stop myself from acquiring a D755 Opt 66 with a V4 and an S2C amplifier which will arrive this week (X-Y mode. Yay!). Unfortunately the units case was painted deep blue by the former owners and modified with a custom power cord (drilled a hole in the case for that). That makes it unsuitable for a restauration. The modules seem to be sort of OK, but the mainframe has some serious corrosion issues due to moisture. As I could see on some supplied images, nearly all the trim pots on PC147 and the PCBs around the tube have to be replaced. Hopefully the unit does not get damaged during transport. IMHO the most interesting aspect of this unit is, that the modules use much newer electric and mechanical parts than the ones on my D75. The push switches and black plastic button rods resemble those, I once saw in a Tek 465b. Most electronic components like resistors are much newer models than in my D75. However I will first concentrate to fix my D75. Maybe the modules from the D755 can help with that.
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Old 2nd Aug 2019, 9:53 pm   #47
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Default Re: Telequipment D75 scope.

I tried to do some comparative measurements on my newly acquired partially defective D755 to further pin down the drifting problem in the vertical amplifier of my D75. To my great surprise, the D755 had a different dual FET type installed at positions TR601 and TR603, which reads as TFD1551 instead of Tek 151-1036-00. Even more surprised, I found the voltages behind the FETs being at 0.42V for channel 1 and 0.27V for channel 2, which are supposed to be 0.8V. Nonetheless, both channels on this V4 amplifier do not show any drift like the one in my D75. The vertical positions on the V4 of the D755 are spot on right from the start, as well as the DC-balance, and do not need any recalibration after powering the unit on. In addition to that, the D604 zener has a similar, but somewhat faster voltage drift like the one in my D75 that does not cause the traces to drift vertically. Maybe I searched for the error at the wrong place. In addition to that, the voltages at the test points shown in the circuit diagram of the vertical amplifier are roughly as inaccurate in the D755 as in the D75. So they also do not seem to be the cause of the drifting issue. A confusing but challenging issue.

That aside, I tried to analyze the D75 power supply by self probing the +24V and -24V power rails with the scope. Both rails exhibit a small amount of ripple that can be seen at 5 V/div. In addition to that, the +24V rail has kind of a superimposed sawtooth wave at double the line frequency. Since I could not observe this sawtooth wave on the D755, this is probably something I should fix in the power supply. The first image shows the ripple on the +24V (top) and the -24V (bottom) rails. The second image shows the sawtooth ripple of the +24V rail with the overlay of an inductively coupled sine wave from the scopes power cord. Looks like a capacitor induced rectifier issue to me, maybe C405.
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Old 3rd Aug 2019, 12:48 pm   #48
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Default Re: Telequipment D75 scope.

Just noticed a small but important typo in my last post: The ripple that can not be seen at 5 V/div but only at 5 mV/div. Would be catastrophic otherwise.
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Old 6th Aug 2019, 6:50 pm   #49
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Default Re: Telequipment D75 scope.

Today, I had some time to work on the D75 again. Regarding the sawtooth on the -24V rail, I removed and checked the two 2200μF Erie electrolytic capacitors in the power supply, but the capacitance of C405 and C406 looks OK, being at 2600μF and 2800μF. Since the ripple does not have a noticeable effect on the scope's operation, I left it that way for now.

Regarding the trace drifting issue, I tried to identify temperature sensitive components in the suspected area of the vertical amplifier. So far, I noticed no hot and overheating components, which means that the drifting issues probably happen in a temperature range below 60 degree Celsius. This made freezer spray seem a little overpowered. Thus, I build a small device from a straw and tape that allowed me to selectively cool down components by blowing into the straw without affecting other nearby components. The results were pretty interesting. I discovered that the carbon film resistors R615 and R616 in the channel 2 circuitry(see attached image) and the respective R612 and R613 in the channel 1 circuitry are extremely temperature sensitive. By blowing onto R616, you can effectively move the trace in 5x mode down by a full division in a matter of seconds. Blowing onto R615 moves the trace in the opposite direction. Even touching the legs of the resistors with a room temperature DMM probe moves the trace in 1x and 5x mode. After that, the trace slowly returns to its former position. IMHO my issues with channel 2 could be easily explained if R615 and R616 have a different temperature coefficient. I am somewhat puzzled that Telequipment used highly temperature sensitive components at a seemingly critical location like this. Assuming that those resistors behave like common 5% tolerance carbon film resistors, their temperature coefficient should be around 350 to 500 ppm/K (according to some resistor datasheets). Now, to tackle the issue, I could try to replace these resistors on both channels with 0.1% tolerance metal film resistors with 25 ppm/K temperature coefficients. In addition to that, I could try to temperature compensate the D604 zener diode (would have to read some lectures on that first). However, having to do that would be a pity since the components would no longer look original. Besides, I am still a little bit unsure if my above analysis sounds reasonable.

On a side note, the respective resistors on the V4 amplifier in my defective D755 behave in the same way when cooled. However, in the D755 there is no observable vertical trace drift. In addition the V4 amplifier of the D75 shows the temperature drift even when put in the D755. This should rule out the power supply being the source of the drift.
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Old 8th Aug 2019, 11:04 am   #50
Alistair D
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Default Re: Telequipment D75 scope.

I was thinking about how isolate which section of the amplifier is drifting. The idea I have come up with is to use two meters. Connect one to the 2 gates of TR601, connect the other meter likewise to TR603. Monitor the voltages as the scope warms up. Both meters should drift by the same amount. Repeat this process with TR605/607 etc until you find where the two meters disagree. Hope it helps.

Al
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Old 9th Aug 2019, 10:32 am   #51
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Default Re: Telequipment D75 scope.

I wonder if you are expecting too much.

My experience is that 'scope amplifiers drift all over the place when cold but the drift slows down as they get towards thermal equilibrium - but they never actually stop drifting! For example, my Tektronix 7A22 amplifier (very high gain) can easily drift the trace right off the screen from cold! Even after an hour I will be constantly switching between ground and measurement to null out the drift.

dc
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Old 9th Aug 2019, 10:53 am   #52
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Default Re: Telequipment D75 scope.

That's not a bad point. Even my modern digital one drifts a bit.
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Old 9th Aug 2019, 3:27 pm   #53
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Default Re: Telequipment D75 scope.

Quote:
Originally Posted by Alistair D View Post
I was thinking about how isolate which section of the amplifier is drifting. The idea I have come up with is to use two meters. Connect one to the 2 gates of TR601, connect the other meter likewise to TR603. Monitor the voltages as the scope warms up. Both meters should drift by the same amount. Repeat this process with TR605/607 etc until you find where the two meters disagree. Hope it helps.
Al
Thank you for sharing your thoughts on this. Differential measurements were a good idea. Until now, I just measured the voltages to ground. Unfortunately, I do not have two suitable meters. So I got me an optical PC interface for my DMM, which enables me to log measurements with a 1 second sampling interval. The measurements were conducted with half an hour cool down time for the scope in between the individual measurements to have a kind of reproducible starting point. The results are interesting:
  • The issue with channel2 consists of largely off values on startup combined with a nasty drifting behavior which does not stop even after an extended amount of time.
  • Since the drifting issue of channel 2 can not be observed with channel 1 and with both channels of the second V4 amplifier from my D755, I consider it to be a repairable issue. In addition to that, it completely prevents the unit from being calibrated with respect to any kinds of DC offsets.
  • Swapping TR601 with TR603 did not swap the drifting issue between channel 1 and 2. Thus, both FETs should be OK and the issue comes from another component.
  • Swapping TR605 with TR607 did not swap the drifting issue between channel 1 and 2. Thus, both FETs should be OK and the issue comes from another component.
  • Measuring between the drains of TR601 and TR603 shows no voltage drift, which means that the drifting zener voltage is passed with a constant offset to both channels over R617 and R618. Since channel 1 works acceptably, the issue is most likely not located here.
  • Voltage drifts between the gates of TR601B and TR603B were too small to be measurable with my DMM.
  • Measuring between the gates of TR601A and TR601B, as opposed to the gates of TR603A and TR603B clearly showed my drifting issue(see attached image). The graph for TR605 looks like its quickly stabilizing. The graph for TR607 in channel two, on the other hand, only converges very slowly.
  • IMHO this narrowed down the issue to the gate of TR607 and the connected source of TR603. Since both FETs are not linked to the issue(ruled out by swapping), the issue should be linked to the connected R615 and R616. So I measured the voltage drop over time across these resistors and their counterparts R612 and R613 in channel 1. The results can be seen in the second attached image. While the resistors in channel 1 seem to maintain a constant offset, this is not the case for the resistors in channel 2. I think, a constant offset should be expected from resistors with equal value and load. When plotting the difference of the voltage drops over the two resistors in channel 1 and 2(see third image), the ones in channel 2 show a curve that resembles the one at the gate of TR607. Considering the trace position is extremely sensitive to changes of both resistors(checked by individual cooling through blowing) the non equal temperature drift of the resistance of R615 and R616 is maybe the source of my issue. The voltage in between the two resistors is exactly the same like at the corresponding measurement position in channel 1.

The shape of the temperature curve of R615 and R616 both do not match the shapes for R612 and R613. Fixing the issue could require to find a pair of resistors with matching temperature curves. I am not sure if it is a good idea to swap all four resistors with some 0.1% tolerance metal film ones like described in my previous post. After all, the temperature drift of these resistors may part of a design to compensate the temperature drift of other components. Maybe it counteracts the voltage drift at the zener diode.

Denis
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Old 10th Aug 2019, 7:50 am   #54
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Default Re: Telequipment D75 scope.

Well done tracking down this problem. Like intermittent faults drift can be very difficult to trace.

Personally I would fit resistors of the same specification as the originals. Since more modern resistors may have different characteristics to the ones fitted it could be worth replacing all 4. Try doing the faulty channel only and see what results you get.

Al
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Old 11th Aug 2019, 5:55 pm   #55
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Default Re: Telequipment D75 scope.

I just tried to switch R615 and R616 with some 1% tolerance metal film ones I had lying around. The result was only sort of a success. When previously properly zeroing out and then switching the scope on after a 30 minute cool-down, the 1x trace of channel 2 was now nearly at zero(off by 0.5 tick), whereas it was off before by about -0.5 major divisions (5 ticks = 1 division). The 5x trace is now off by 0.5 major divisions, whereas it was off before by about -1.5 divisions. So a clear improvement here. What bugged me was that the general shape of the voltage drift function over time did not not change. IMHO this means that the resistors with the non-matching temperature coefficients only amplified the actual issue but are not responsible for the qualitatively differing shape of the drifting voltage function. To verify this, I harvested a pair of matching carbon film resistors from an old TV board and tried those. The result was very similar to the one I got with the metal film ones, just a slightly higher temperature drift. The next thing I suspected was the TR603 FET which acts as a source follower to provide sufficient current for TR607. So I swapped in the original TR603 instead of the one I got from MrBungle. After re-zeroing the channel and a cool-down, the result was close to the previous ones which means TR603 should not be the source of the issue.

The attached image shows a comparison of the measurements taken so far. A comparison measurement of the TR607 gates in the V4 of my D755 showed that they qualitatively behave exactly like channel 1 in my D75. So, I am pretty sure that channel 1 should be used as the reference target here.

Summarizing, this does not leave many options:
  • The issue shows at the gate of TR607, so it should be located before this FET.
  • According to my observations, it is unlikely that R618 at the drain of TR603 or R706 are the issue. Blowing on them did not result in visible trace drifts and their temperature drift converges much quicker than the phenomenon I am looking for.
  • The issue could be located before the gate of TR603. If so, a voltage drift should be measurable between the gates of TR603A and TR603B. At least with my DMM I can not detect such a change, only fluctuations with a magnitude of 0.01 mV to 0.02 mV with the total difference being at 2.00 mV. Maybe my measurement equipment is not up to the task. So, I looked further and measured the temperature drifts of the voltages across R604 and the center pin of the R602 DC balance pot to ground. Both exhibit exactly the type of drift I am looking for. However, so do R603 and R601 in channel 1 and channel 1 is working near perfectly.
  • A quick measurement indicates that the temperature drift function of the new carbon film resistors resembles the one of the original ones. So it is somewhat unlikely that the newer resistors I swapped in have qualitatively non-matching temperature behavior.

At the moment, I have run out of ideas but I am not going to give up this easily. Any analysis ideas and thoughts are welcome.

Denis
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Old 11th Aug 2019, 10:09 pm   #56
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Default Re: Telequipment D75 scope.

..hmm in theory a metal film resistor would have a positive tempco, but I believe it can be either way sometimes. A carbon resistor would have a negative tempco normally. Here is a link to some information:

https://www.pc-control.co.uk/resistor-types.htm

One thing to try, unlikely the cause but not impossible, set up a replacement resistor pair for the two suspect resistors and a new 1uF bypass capacitor, just on the off chance the capacitor is passing some current (unlikely by the type of capacitor it is).

As you say the drift might be originating in the gate circuit but its too difficult to see on the meter. You could connect the gate circuits of the two channels together and see if the drift transposes into the other channel. For example it might drop in half but then be seen equally in both channels. As least you do have one channel that is working as a reference. (maybe link them with a 470R resistor rather than a wire just to limit the current if the link accidentally gets connected to the wrong place)
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Old 11th Aug 2019, 11:11 pm   #57
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Default Re: Telequipment D75 scope.

....also if the drift is only happening in the x5 mode you could try swapping the two thermistors.
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Old 12th Aug 2019, 8:03 am   #58
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Default Re: Telequipment D75 scope.

It may be worth going through the DC part of the calibration procedure before making any further measurements.

Al
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Old 12th Aug 2019, 11:06 am   #59
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Default Re: Telequipment D75 scope.

Quote:
Originally Posted by Argus25 View Post
..hmm in theory a metal film resistor would have a positive tempco, but I believe it can be either way sometimes. A carbon resistor would have a negative tempco normally. Here is a link to some information:

https://www.pc-control.co.uk/resistor-types.htm

One thing to try, unlikely the cause but not impossible, set up a replacement resistor pair for the two suspect resistors and a new 1uF bypass capacitor, just on the off chance the capacitor is passing some current (unlikely by the type of capacitor it is).

As you say the drift might be originating in the gate circuit but its too difficult to see on the meter. You could connect the gate circuits of the two channels together and see if the drift transposes into the other channel. For example it might drop in half but then be seen equally in both channels. As least you do have one channel that is working as a reference. (maybe link them with a 470R resistor rather than a wire just to limit the current if the link accidentally gets connected to the wrong place)
Thank you for your reply. I read about the inverted temperature coefficient of metal film resistors as opposed to carbon resistors before and hoped this might actually compensate some bad temperature effects of the carbon resistors in the circuit. Unluckily, it did not.

I will try to have a look at those capacitors. However, I do not have a device to measure capacitor leakage or ESR.

Interconnecting the circuits of channel 1 and 2 in the suggested area and monitoring the trace is kind of impossible. As soon as anything gets connected to the circuitry in that area, even if it is only a short unconnected cable, it acts as an antenna that couples all of the available EMR into the circuit. This blurs the trace so much that it can barely be seen on the screen at all.

Quote:
Originally Posted by Argus25 View Post
....also if the drift is only happening in the x5 mode you could try swapping the two thermistors.
The issue also affects 1x mode. The strength of the effect on 1x and 5x mode is approximately as strong as the trace drift in both modes when adjusting the DC balance potentiometer R602. That is also the reason I started looking in that particular area of the circuit.

Quote:
Originally Posted by Alistair D View Post
It may be worth going through the DC part of the calibration procedure before making any further measurements.
I did that multiple times (DC balance, X5 balance, variable gain balance and invert balance) just to discover that the issue prevents a stable balance calibration to be set. Since this is not an issue in the V4 of my D755, I deduce that the issue must be fixed before a calibration can be made. Nonetheless, I will continuously check if my modifications affect the possibility to calibrate the DC offsets.

Denis
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Old 16th Aug 2019, 4:29 pm   #60
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Default Re: Telequipment D75 scope.

I did some further debugging and followed the idea of Argus25, cross-wiring stuff. Swapping the 47K DC calibration pots R601 and R602 suggested that they are not part of the error, since channel 1 worked perfectly with the other pot. In the same way, I could also rule out the Volts/Div switch connected to the gate of TR603A via the 100R resistor. Looking at the voltages across the resistors in the gate circuits of TR601 and TR603 over time, I did not find any obvious differences either. Swapping the resistors in the gate circuit of TR603B with new ones also did not cure the shape of the voltage function between the two gates of TR603. A small leakage of C602 should not be critical, since there are resistors to ground anyways. Still need to check C623, but I do not have an appropriate replacement at the moment. Its counterpart in the circuitry of channel 1 is too complicated to reach without desoldering tons of wires.

Now I was kind of desperate, so I tried to fight the issue by using the issue to my advantage. I had noticed that the difference of the voltage development over time between using the original resistors for R615 and R616 and using new carbon ones was approximately the difference of the voltage function I measured with the pair of new carbon resistors to the one I was looking for. So I simply tried putting in the original resistors, but swapping their places to produce the opposite effect. To my surprise, this stupid idea kind of worked. As can be seen in the attached image the voltage difference between the gates of TR603A and TR603B now converges much quicker and stays at the final value. Now, the DC offsets of the V4 amplifier can be calibrated and remain sort of stable. However, this is for sure not the way it is intended to work. I cannot believe that the Telequipment engineers manually selected resistors from the same series with specific temperature functions for each scope. In addition to that, channel 2 still does not converge as quickly as channel 1. All In all, I still do not consider the issue to be fixed but only mitigated by using stupid tricks.

While doing all the component swapping, I also detected a small voltage drift of the +24V rail. Since it does not have a noticeable impact on the operation of channel 1, which I consider to be good, I will leave it that way for now and fix it later.

Since Argus25 also suspected the thermistors I had a look on those. They are probably designed to compensate temperature drifts of R698 and R699, which they manage to do quite satisfactory. Measuring the voltage across the resistor/thermistor pairs did not reveal any significant drift.

Denis
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Last edited by Pinörkel; 16th Aug 2019 at 4:45 pm.
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