@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.