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Old 25th Oct 2020, 9:30 pm   #125
Join Date: Jun 2019
Location: Dortmund, Germany
Posts: 111
Default Re: Telequipment D75 scope.

During some repair work on my Bradley 192 calibrator, the brightness of my D75 suddenly dropped severely in several steps over time. I immediately suspected one of the 10K carbon resistors in the HV-section and measurements revealed that R322 had gone high to 80M. While I was at it, I measured the other partially highly inaccessible carbon resistors of the same type on PC148.

R317, R318, R319, R321 -> ~10 MOhms (should be 10 MOhms)
R322 -> 80 MOhms (should be 10 MOhms)
R323 -> 5.2 MOhms (should be 3.9 MOhms)
R324 -> 4.8 MOhms (should be 4.7 MOhms)
R325 -> 5.8 MOhms (should be 4.7 MOhms)
R326 -> 7.8 MOhms (should be 5.6 MOhms)

The HV Metal film ones 304, 305, 309 and 313 all measured just above 6.7 MOhms with an intended value of 6.8 MOhms, so they should be OK.

Since I cannot find identical resistors, I will probably replace all the carbon resistors listed above with Vishay VR37 high voltage resistors. Unfortunately, it seems to be quite a challenge to find a dealer that can supply all the values without charging a fortune for shipping.

Regarding the accessibility: does anyone know the best way to access PC148 for swapping the resistors without desoldering all the wires?

A second issue showed up on my D755. Just as I thought, I had exterminated all issues, it started behaving strange again. The issues are slight brightness changes and twitching of the trace during start-up and operation. Probably connected to that is that once the scope has warmed up for about 15 to 20 minutes the horizontal and vertical deflection of the trace suddenly gets stronger. This leads to kind of a magnification effect on the trace. The strength of magnification can be influenced by turning the brightness up and down. This can be done only after the magnification effect has appeared. Before that, changing the brightness has no effect on the scale of the trace.

At first I suspected an issue with the PSU because it had been quite troublesome with this scope in the past. However the -24 V, +24 V and 105 V rails seem to be stable and do not follow the trace changes when adjusting the trace brightness. the 30 V rail is a little low, but also seems to be stable. So probably no PSU issues there.

For further testing and trying to rule out issues with the horizontal and vertical amplifiers, I set the V4 to zero coupling, moved the trace up and down via the vertical position control and then changed the brightness. The trace seems to move always away from the center of the screen, the more pronounced the farther away it is at the beginning. In addition to that, the focus of the trace seems to change ever so slightly but somehow proportional to the trace magnification effect. So I am suspecting a general issue with either the deflection or the acceleration voltages. The fact that the effect is linked to the trace brightness while the PSU voltages are stable could be pointing to an issue on PC 148 or PC 128. Unfortunately that is exactly the area of the scope with voltages high enough to overload my DMM, so I have to be very careful there and I cannot measure the voltages at the tube. My next step will probably be to check all testing points in the circuit diagram that my DMM can handle for a proportional reaction to brightness changes. Any hints where to look would be appreciated.
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