UK Vintage Radio Repair and Restoration Discussion Forum - View Single Post

Pinörkel · 6th Sep 2019, 9:38 pm

Hmmm, too bad. I really hoped for some images of the units.

@mhennessy: Thank you for participating in this thread. I found the report on your website regarding the D63 repair really interesting and useful. However, I can report that the vertical position sensitivity reduction, obtained by inserting those 680 ohm resistors is barely noticeable. I compared this in my D75. The V4 in my defective D755 had this factory modification already installed.
Regarding the adjustment of reverse engineered calibration units, I assumed that the correct settings could be derived from nominal component values and the nominal voltages in the circuit diagrams of the main unit. In any case, the settings in the calibration unit should not depend on the real world values of components in the scope, since they must work for all scopes of that type. However, it is difficult to speculate on something, I have so little information on.

@MotorBikeLes: Those dummy units you mentioned sound indeed like an interesting starting point for custom modules. Unfortunately it is impossible to search for these units on ebay, or even find images of them, since they were probably mostly dumped when they were replaced with vertical plugins. On the web, there are some circuit diagrams for component testers available which are intended for use with older oscilloscopes, e.g. the Hameg HZ65. Maybe the circuits can be adapted to work with the set of available voltages in the D75.

Luckily, I discovered with my newly acquired function generator that the calibration of my D75 is not too far off. Therefore, one of my next steps will be to thoroughly check the components in the power supply to minimize any remaining voltage drifts. I addition to that, I will have a look at the trigger circuit. At the moment, the scope triggers very stable, but the trigger point gets a little bit jumpy during adjusting the trigger level, even for a clean sine wave. It is well possible that this is normal for this scope but I know that setting the trigger level on Hameg scopes of the same age is way more stable. Maybe I will also document the current state of the scope photographically, since it is now in a working, nearly 100% original state (except for the newly produced Time/Div knob).

6th Sep 2019, 9:38 pm	#69
Pinörkel Pentode Join Date: Jun 2019 Location: Dortmund, Germany Posts: 161	Re: Telequipment D75 scope. Hmmm, too bad. I really hoped for some images of the units. @mhennessy: Thank you for participating in this thread. I found the report on your website regarding the D63 repair really interesting and useful. However, I can report that the vertical position sensitivity reduction, obtained by inserting those 680 ohm resistors is barely noticeable. I compared this in my D75. The V4 in my defective D755 had this factory modification already installed. Regarding the adjustment of reverse engineered calibration units, I assumed that the correct settings could be derived from nominal component values and the nominal voltages in the circuit diagrams of the main unit. In any case, the settings in the calibration unit should not depend on the real world values of components in the scope, since they must work for all scopes of that type. However, it is difficult to speculate on something, I have so little information on. @MotorBikeLes: Those dummy units you mentioned sound indeed like an interesting starting point for custom modules. Unfortunately it is impossible to search for these units on ebay, or even find images of them, since they were probably mostly dumped when they were replaced with vertical plugins. On the web, there are some circuit diagrams for component testers available which are intended for use with older oscilloscopes, e.g. the Hameg HZ65. Maybe the circuits can be adapted to work with the set of available voltages in the D75. Luckily, I discovered with my newly acquired function generator that the calibration of my D75 is not too far off. Therefore, one of my next steps will be to thoroughly check the components in the power supply to minimize any remaining voltage drifts. I addition to that, I will have a look at the trigger circuit. At the moment, the scope triggers very stable, but the trigger point gets a little bit jumpy during adjusting the trigger level, even for a clean sine wave. It is well possible that this is normal for this scope but I know that setting the trigger level on Hameg scopes of the same age is way more stable. Maybe I will also document the current state of the scope photographically, since it is now in a working, nearly 100% original state (except for the newly produced Time/Div knob).