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Old 4th Jun 2019, 2:23 am   #1
Pinörkel
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Default Telequipment D75 scope.

@Anode_to_Joy: Congratulations on your successful repair!

https://www.vintage-radio.net/forum/...d.php?t=155339

I stumbled upon a D75 in form of a dumpster-find a few weeks ago (picture attached). Unfortunately the unit was in very poor condition: dirty, missing and fractured knobs, missing labeling on the amplifier unit, non-functional or loose push buttons, and missing screws. Since I have been planning to buy an oscilloscope for years, I could not resist trying to bring the unit back to life. I have no specific experience with repairing oscilloscopes but have done a lot of successful repair work on defective CRT and LCD displays and some tube radios. So repairing the D75 sounds feasible for me, if I get some kind hints from more experienced tinkerers. I already tried to find solutions for my current issues online, but the descriptions for most D75 and D83 related issues are PSU or EHT related.

So far, I performed an overall cleaning of the unit, acquired the service manual and made a basic checkup of the unit for potential short circuits, especially regarding the PSU section. After fixing a few things, I powered the unit on and did a calibration of the output voltages, which are now spot on. Unfortunately, I was not able to check on the cathode voltage due to insufficient measuring equipment. The trace looks OK to me and reacts like I would expect it, when doing CRT related adjustments. After fixing the push buttons, I tried to conduct a basic function check of the unit. Most functions seem to work to some degree including the delayed timebase, with the most obvious problems being inconsistent Y-offsets, wonky triggering and the missing knobs.

In principle, the Y-offsets for channel 1 and 2 can be adjusted properly including an acceptable match with the 5x magnification. Aside from the overly sensitive 5x trimmer, the potentiometers are a little bit jumpy and need some inside cleaning. The traces do not show any continuous drift. However, if the position knobs have been moved significantly or some channel switching has been performed, the match with the 5x magnification is lost and needs to be trimmed again to avoid jumping of the trace when switching the magnification on and off. Also, the zero position of the trace in GND mode is off by a few ticks sometimes for no obvious reason. So far, I have no idea what is causing this. In the same way, the horizontal position adjustment seems to change sometimes by 2 or 3mm, when working with the unit. An in depth cleaning of the CH2 position and trim potentiometers resulted in a more stable operation of the knob, but did not resolve the offset problem. Are there some known usual culprits for the D75 that could be related to this problem?

The triggering seems to mostly work. If a periodic 1kHz sine or rectangle signal is applied, the scope triggers and shows no drift. Unfortunately I get unexpected behavior when moving the knob for trigger level A. As far as I remember, putting the scope in AC or DC trigger mode should result in a smooth change of the trigger start position of a sine wave, as long as I am in a valid triggering interval. Also, the lamp of the trigger should illuminate when a valid trigger point has been found and be off otherwise. On my D75, rotating the trigger level knob does not consistently change the trigger level. The triggering stays mostly at the same position and erratically jumps somewhat up and down, but only while the knob is being moved. This movement has a clear correlation with the rotation direction of the knob but is very jumpy. As expected, leaving the valid triggering interval in any direction causes the triggering to be lost. Regardless of the triggering being successful or not, the trigger status lamp is always on. My first try to solve the problem was to try cleaning the trigger level potentiometer. However, I failed to fully open its case which is hold together by four screws but has a kind of sealed inner part.

For the knobs, the time base section is not only missing the plastic knobs, but also the metal rings, which secure the buttons on the shafts. I think, they are called shaft collars in English (picture attached). However, I was not able to find any information on where to get the utilized variety with a serrated exterior side and the specific dimensions required to fit the shafts. Especially the lower one for the Time/Div-knob seems to be impossible to find, since it needs two grub screws in an 120° angle. In addition to that, machine parts that use imperial units are very hard to find in Germany. Is a special name for this type of component that I just do not know but which is required to find spares on the net? Unlike the missing shaft collars, replacements for the plastic parts of the knobs are no problem since I can simply reverse engineer the knob geometry in digital 3D and create new knobs with a 3D printer. This even works for the transparent delay time knob. Nonetheless, original spare knobs would be very nice and also solve the problem of the missing shaft collars.
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Old 5th Jun 2019, 6:35 am   #2
Diabolical Artificer
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Default Re: Telequipment D75 scope.

I think I may have a square volts/div knob and am pretty sure someone was offering D75 spares a while back but can't find the thread. I'll try and dig it out.

Andy.
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Old 5th Jun 2019, 7:58 pm   #3
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Default Re: Telequipment D75 scope.

Hello Andy, thank you for taking the time to read my post and for the kind reply. Your offer of an original D75 Volts/Div square knob sounds very interesting and could solve at least 1.5 of my current part acquisition issues. I will also try to find the user, you mentioned, which offered D75 spares in earlier posts. My previous attempts to get some spare parts like knobs and screws online always resulted in accumulated costs that would be more than sufficient to buy a defective scope on ebay for parts, which would unfortunately waste a lot of parts that could be useful elsewhere. If said user maybe has a non-repairable D75, I could try to buy several parts from him. Especially since I discovered some severe cracks on other knobs in the meantime.

That aside my reverse engineering attempts for the plastic parts of the Time/Div, Volts/Div and horizontal position knobs went really well, especially the transparent Time/Div knob. However the silver-gray color of my printing material does not really fit the original color of the other knobs. I will try to take some pictures and post them here. If anybody wants to try printing their own knobs I can also post the 3D model files on request.

Greetings
Denis
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Old 5th Jun 2019, 11:03 pm   #4
Alistair D
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Default Re: Telequipment D75 scope.

Well done on the work you have done so far on saving another Telequipment scope from the scrap heap.

It was me who was offering D75/D83 parts for sale. The only knob I have left is the smaller A timebase knob. The cost is Ł2.00+p&p. I have no idea what the postage cost will be but I can find out for you. If you buy it I will include the remains of a B timebase knob as seen here. https://www.vintage-radio.net/forum/...d.php?t=135544 Perhaps you can fabricate something from it.

The collar in your second picture is actually an integral part of a Telequipment knob. The plastic parts have broken off it.

Al
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Old 6th Jun 2019, 12:07 am   #5
Pinörkel
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Default Re: Telequipment D75 scope.

Quote:
Originally Posted by Alistair D View Post
Well done on the work you have done so far on saving another Telequipment scope from the scrap heap.
Hello Al, thanks for your reply. No way I can let something like this just sit in a dumpster, waiting to die.

Quote:
It was me who was offering D75/D83 parts for sale. The only knob I have left is the smaller A timebase knob. The cost is Ł2.00+p&p. I have no idea what the postage cost will be but I can find out for you. If you buy it I will include the remains of a B timebase knob as seen here. https://www.vintage-radio.net/forum/...d.php?t=135544 Perhaps you can fabricate something from it.
An original D75 A-timebase knob sounds perfect, I'm in! Until now, I used that shaft collar core in the second picture, which is the remains of the CH2 Volts/Div knob to occasionally operate the A Time/Div which happens to have the same shaft diameter. Having that knob would be extremely helpful. Also a shaft collar core of the B-timebase knob (if it is the one you showed here) would enable me to build a near perfect replica of that knob using an SLA-Printer and transparent resin. Maybe you can determine postage costs and drop me a message. I could pay via PayPal in advance.

Denis
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Old 8th Jun 2019, 1:10 pm   #6
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Default Re: Telequipment D75 scope.

Found the knobs I have, see pics. Some may or may not be Telequipment. Drop us a PM if any are of any use to you.

Andy.
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Old 8th Jun 2019, 3:11 pm   #7
Alistair D
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Default Re: Telequipment D75 scope.

Top row knobs 1 & 2 look like V/div knobs. TQ used 2 identical looking but different diameter knobs on some of their modules. I would be worth double checking the diameter before sending them off.

The V/div knobs on a V4 module are 22mm dia.

The other size knob is 28mm

Al
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Old 8th Jun 2019, 6:12 pm   #8
Pinörkel
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Default Re: Telequipment D75 scope.

Quote:
Originally Posted by Diabolical Artificer View Post
Found the knobs I have, see pics. Some may or may not be Telequipment. Drop us a PM if any are of any use to you.

Andy.
Hello Andy,
thank you for digging the knobs out. Unfortunately, I can tell from the pictures that none of the knobs is from a D75. The square ones with four indices are a lot bigger than the ones required for the Volts/Div on the D75 and also have slightly different proportions.

I attached a render of my attempt to reconstruct the Volts/Div knob in 3D and a photo showing three printed knobs that I plan to use as substitutes until I get my hands on original knobs. The geometry is very accurate. However, the 3D printing resolution is of course no match for injection molding and I need to find a better matching PLA color.

Denis
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Old 8th Jun 2019, 9:28 pm   #9
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Default Re: Telequipment D75 scope.

I reckon six of Andy's knobs are "modern" TQ, most of which are for 63,75 and 83 series. Most of the rest are "old type" TQ, with a couple of strugs thrown in for good measure.
Les.
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Old 10th Jun 2019, 6:49 am   #10
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Default Re: Telequipment D75 scope.

Oh well, that's a shame, your copy looks well though.

Thanks Les.

Andy
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Old 10th Jun 2019, 5:53 pm   #11
Pinörkel
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Default Re: Telequipment D75 scope.

Today I had some time to clean the front panels and do a test fitting of my temporary replacement knobs for CH2 Volts/Div, Horizontal Position, Time/Div A and Time/Div B, as can be seen

here

(sorry for the link, but the forum does not accept higher resolution images for upload). The first two knobs already work but the Time/Div knobs are still non functional due to the missing shaft collars. The Time/Div B knob is still a prototype, which I did only for experimenting with different ways of material post-processing to get the desired transparent look. I'm still unsure whether I will try to reproduce the original knob as closely as possible or go with the evenly toothed exterior shown here, which has better grip.

The glossy stuff on the V4 amplifier is a piece of OHP transparent film on which I printed a reconstruction of the front panel labels. This is a test for checking the accuracy of a label reconstruction I created because many parts of the original labels are damaged or missing. I am planning to print the final labels on a waterslide decal paper that can be used to transfer highly detailed labels onto a surface by placing a thin foil with the help of water.

Unfortunately, a closer inspection of the knobs on the V4 revealed that most of them have cracked. At the moment, I am still unsure if this can be fixed. Trying to glue the cracks back together would require the internal shaft collars to be removed in beforehand to get rid of the mechanical stress. However the knobs will surely break again upon re-insertion of the shaft collars.
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Old 12th Jun 2019, 10:19 pm   #12
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Default Re: Telequipment D75 scope.

Thanks to Alistair D, I got some Time/Div knob components today. The special shaft collar from the resin remains of the Time/Div B knob seems to perfectly fit my prototype knob. When I tried to put the Time/Div A knob on the respective shaft I noticed that the internal shaft collar is not firmly connected to the plastic exterior. This causes the knob to wiggle. Upon decomposing the knob into its three parts (shaft collar, cap and shell) I discovered that the issue had its origin in what looks like kind of a construction error. The shaft collar, which oddly also has a 120° angle between the set screws instead of 90°, does not have a linearly grooved exterior side but a diamond pattern instead (see image). This does not allow the shaft collar to be pressed into the plastic shell without damaging it, finally resulting in a wiggly connection. At the moment I am trying to find a solution for this, without having to glue the shaft collar into the plastic shell.

With the new knob components I noticed that the shafts of the Time/Div knobs require a very high amount of torque to be rotated and the position snapping produces very hard clicks. In contrast, the Volts/Div knobs can be rotated easily. Does anybody know if this is supposed to to be like this on a D75? I suspect that the internal mechanism in which a spring-loaded brass roller rotates around a gear wheel is either set too hard or the gear wheel geometry is worn out. Unfortunately, I couldn't find a way to reach this mechanism, as the two outer circuit boards in the S2A unit do not seem to be removable without desoldering tons of wires. Does anybody know a way to get to the Time/Div switching mechanisms without desoldering half of the unit?
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Old 12th Jun 2019, 11:16 pm   #13
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Default Re: Telequipment D75 scope.

Sorry about that Denis. I did not know that the knob was loose on it's collar. I am more than happy to give a you a full refund on the cost. Perhaps a little epoxy adhesive will sort the problem.

I have owned 3 scopes that used the S2A module. On every one of them the timebase knob is much stiffer to operate than the V/div control. I think that the extra stiffness is due to the timebase switch having more wafers.

Al
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Old 12th Jun 2019, 11:31 pm   #14
Pinörkel
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Default Re: Telequipment D75 scope.

Quote:
Originally Posted by Alistair D View Post
Sorry about that Denis. I did not know that the knob was loose on it's collar. I am more than happy to give a you a full refund on the cost. Perhaps a little epoxy adhesive will sort the problem.
No need for a refund, everything is fine. The parts are as perfect as can be expected from used 1975 items. And the really important parts, the special purpose shaft collars are intact. Nothing there that can not be fixed.

Quote:
I have owned 3 scopes that used the S2A module. On every one of them the timebase knob is much stiffer to operate than the V/div control. I think that the extra stiffness is due to the timebase switch having more wafers.
Thanks for this information. I do not think the extra wafers are causing this, but I will try to clean and lubricate them once I find a way to safely access this part of the switch mechanism.
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Old 13th Jun 2019, 11:45 am   #15
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Default Re: Telequipment D75 scope.

Hi, you can access the switch roller mechanisms if you remove the top and bottom centre aluminium panels which run from front to back, the only draw back is you have to take front panel of to access screws that hold panels in place, (same as back panel) and you will need to set time ten control up afterwards.

Regards Mike.
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Old 13th Jun 2019, 6:45 pm   #16
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Default Re: Telequipment D75 scope.

Quote:
Originally Posted by micheal View Post
Hi, you can access the switch roller mechanisms if you remove the top and bottom centre aluminium panels which run from front to back, the only draw back is you have to take front panel of to access screws that hold panels in place, (same as back panel) and you will need to set time ten control up afterwards.
Ahh, that was also my first guess but I failed to find a secure way to remove the Delay/Time Mult knob for getting the front panel off. Removing the knobs front part was simple but the back part with the printed on coarse scale would not come off. I removed the larger copper ring(see image) which did not result in any progress in removing the knob. Maybe the inner most silver ring with the two 180° notches needs to be rotated? Are there any known traps in setting up the time ten control again? I would just set the knob to 5, remove the front part and not rotate the shaft afterwards. Then I would screw on the front part in the position that is determined by the set screw marks on the shaft.
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Old 14th Jun 2019, 8:56 am   #17
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Default Re: Telequipment D75 scope.

I have looked at a functionally similar 'Duodial' turns counter in my spares box and on that one, the stationary part of the counter mechanism is just pegged to the panel by a stand-off washer with a broad peg on the counter side and a smaller round peg on the panel side. The rotating knob is fixed to the mechanism by a circlip.

If you have removed the knob, then it seems to me that there is no need to remove the counter body to remove the panel, as the connection is merely via the ten turn pot shaft, which you have disconnected.

I'll post some photos of the Duodial turns counter underside shortly.

Ron
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Old 14th Jun 2019, 9:51 am   #18
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Default Re: Telequipment D75 scope.

The inner collar does unscrew. I normally use a small pair of circlip pliers with right angled tips to loosen them.

No need to worry about damaging anything as I have an almost complete S2A that you can have any parts from.

Al
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Old 14th Jun 2019, 11:27 am   #19
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Default Re: Telequipment D75 scope.

You have to undo slotted recessed nut i use a pair of pointed nose pliers and two hands one on handles of pliers and the other on the jaws.
As for recalibrating try not to turn the thin shaft, and when putting back together again align knob screw with indentation on shaft , there are calibration pots which allow you to fine fine tune the delay time multiplier control in alignment and set up instructions in s2a manual .

Mike
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Old 15th Jun 2019, 11:45 pm   #20
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Default Re: Telequipment D75 scope.

Thanks to your hints, I managed to remove the Delay/Time Mult knob. Now, there is something stuck near the labeled plastic cylinder of the Time/Div knob that is preventing me from taking the front panel off. Seems like I have to figure this out another time.

Today, I thoroughly cleaned the vertical position double potis including the trimmers. Most of their hick ups are gone now, however, the trimmers are still very fiddly.

Upon pressing the shaft collar into the modified Time/Div knob I got from Alistair D, I noticed a small crack on it that started to widen. This was easily fixed with some epoxy and two cable ties pressing the knob together.

For calibration of the scope I acquired two Cinch 81 731 AA 16 edge connectors (see images). These are just like the build in Cinch 81 774 AN 16 module connectors, except they have gold plated contacts on both sides and do not have the reverse polarity protection gap. I am planning to use these and some custom cut PCBs to build extension cables for both modules for live measurements and calibration.

This week I was allowed to play with an equally old Hameg 412-5 scope. I was amazed how responsive and stable it was compared to my D75. No trace drifting over time, no 5x trace offset, a consistently and stable working trigger control, virtually no warmup time and absolutely no trace hick ups on using any switches. At the moment I can't judge how a D75 should behave in perfect condition. Do you have any experiences on that? This would make it much easier for me to identify aspects that should be fixed by, for example, locating and replacing bad and heat sensitive components.
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