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Old 6th Aug 2019, 7:50 pm   #49
Pinörkel
Pentode
 
Join Date: Jun 2019
Location: Dortmund, Germany
Posts: 161
Default 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.
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