On Thu, Feb 24, 2022 at 11:42 AM, Alex_03 wrote:
The input signal for the captures was not the calibrator, it was an external function generator. I chose triangle because it makes it easiest to see the distortion on the screen, sine shows it too and obviously square does not, hence the external function generator.
I completely forgot that I had my scope in AC coupling for those captures. new ones are attached along with pics of both sine and triangle on the screen, which I just realized I never attached to my original post, oops.
Attached in order of pins 3/6, 4/5, and 12/13 of U1(hopefully). I also captured the final vertical output(AC coupled so you can actually see something).
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The measurement that is most puzzling is pin 7 of U2 (=pin 13 of U1) is at 27.77V, pin 5 of U2 (=pin 12 of U1) is at 27.5V but pins 2&3 of U2 are nicely balanced at 42V. If internals of U2 is exactly like the figure, 270mV difference across the input transistors should steer the current completely to pin 2 which should cause different voltages at pin 2 and pin 3 of U2.?
With zero input could you re-measure pins 12&13 of U1 and pins 2&3 of U2? Possible scenarios:
1) 165-ohm terminations at the other side of the plates drifted and not balanced anymore.
2) Look at 53.3VF supply for ripple and DC voltage while you are feeding in the signal.?
3) There may be a large voltage drop from pins 12/13 of U1 to pins 5&7 of U2, and in reality voltage at pins 5&7 of U2 could be closer at the IC terminals. If you measure DC voltages touching the IC pins vs touching components (e.g. R24/R25) do you see a difference? You may try carefully removing U1 and reseating it. Removing, cleaning terminals of U2 and reseating is also a good idea but it is trickier so you have to decide whether you are comfortable with it after reading the service manual.
I recommend focusing on why pins 5&7 of U2 are 270mV apart but pins 2&3 of U2 match to the same voltage.?
As you also wrote this is not a push-pull stage so there is no NPN/PNP cross over. Also U2 has cascoding inside but its function is beyond a simple cascode, it is built as a transimpedance amplifier.?
Ozan
