26th Jul 2019, 2:13 pm | #41 |
Nonode
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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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26th Jul 2019, 4:47 pm | #42 |
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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. |
26th Jul 2019, 6:58 pm | #43 | ||
Pentode
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Re: Telequipment D75 scope.
Quote:
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:
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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28th Jul 2019, 9:39 am | #44 |
Nonode
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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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29th Jul 2019, 11:51 am | #45 |
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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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29th Jul 2019, 6:21 pm | #46 | ||
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Re: Telequipment D75 scope.
Quote:
Quote:
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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2nd Aug 2019, 10:53 pm | #47 |
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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. |
3rd Aug 2019, 1:48 pm | #48 |
Pentode
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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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6th Aug 2019, 7:50 pm | #49 |
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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. |
8th Aug 2019, 12:04 pm | #50 |
Nonode
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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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9th Aug 2019, 11:32 am | #51 |
Nonode
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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 |
9th Aug 2019, 11:53 am | #52 |
Dekatron
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Re: Telequipment D75 scope.
That's not a bad point. Even my modern digital one drifts a bit.
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9th Aug 2019, 4:27 pm | #53 | |
Pentode
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Re: Telequipment D75 scope.
Quote:
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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10th Aug 2019, 8:50 am | #54 |
Nonode
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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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11th Aug 2019, 6:55 pm | #55 |
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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:
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 |
11th Aug 2019, 11:09 pm | #56 |
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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) |
12th Aug 2019, 12:11 am | #57 |
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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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12th Aug 2019, 9:03 am | #58 |
Nonode
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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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12th Aug 2019, 12:06 pm | #59 | |||
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Re: Telequipment D75 scope.
Quote:
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:
Quote:
Denis |
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16th Aug 2019, 5:29 pm | #60 |
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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 Last edited by Pinörkel; 16th Aug 2019 at 5:45 pm. |