[Pinörkel]

I tried to do some comparative measurements on my newly acquired partially defective D755 to further pin down the drifting problem in the vertical amplifier of my D75. To my great surprise, the D755 had a different dual FET type installed at positions TR601 and TR603, which reads as TFD1551 instead of Tek 151-1036-00. Even more surprised, I found the voltages behind the FETs being at 0.42V for channel 1 and 0.27V for channel 2, which are supposed to be 0.8V. Nonetheless, both channels on this V4 amplifier do not show any drift like the one in my D75. The vertical positions on the V4 of the D755 are spot on right from the start, as well as the DC-balance, and do not need any recalibration after powering the unit on. In addition to that, the D604 zener has a similar, but somewhat faster voltage drift like the one in my D75 that does not cause the traces to drift vertically. Maybe I searched for the error at the wrong place. In addition to that, the voltages at the test points shown in the circuit diagram of the vertical amplifier are roughly as inaccurate in the D755 as in the D75. So they also do not seem to be the cause of the drifting issue. A confusing but challenging issue.

That aside, I tried to analyze the D75 power supply by self probing the +24V and -24V power rails with the scope. Both rails exhibit a small amount of ripple that can be seen at 5 V/div. In addition to that, the +24V rail has kind of a superimposed sawtooth wave at double the line frequency. Since I could not observe this sawtooth wave on the D755, this is probably something I should fix in the power supply. The first image shows the ripple on the +24V (top) and the -24V (bottom) rails. The second image shows the sawtooth ripple of the +24V rail with the overlay of an inductively coupled sine wave from the scopes power cord. Looks like a capacitor induced rectifier issue to me, maybe C405.