Re: Tek 465: CH1 No signal, CH2 signal reads a fraction of input, V Pos distorts signal


Greenpitu@...
 

Hello!

I attempted some calibration adjustments today and as per service manual to adjust the V amplifier bias (R455) and output centering (R429).

*** Vertical Amplifier Bias ***
As in previous days, Ch1 AC div/pos 0.1V connected to onboard calibrator with 300mV square signal.

Before making any adjustments, and as per previous posts, the signal displayed is just 1/3 of the input calibrator signal i.e expands one vertical division only.

Adjusting the variable resistor R455 allowed me to increase the deflection x3. So in essence, I now was able to see the full 300mV signal, expanding across 3 divisions as expected.

If I turn the V Pos in either direction the signal no longer expands by 50%, it now compresses by approx 20%.

So I guess something still not quiet right here? Also, is this the correct way of calibrating the V amplifier? I ask because the service manual says "calibrate for maximum deflection", it doesn't say "calibrate until you see what you expect to see" which is essentially what I am doing?

** Adjust Output Centering **
No input signal, Ch1 on .2 volts/div as per manual

Connected TP322 and TP324 with DMM, and turned V Pos dial until I read 0mV on meter. At this stage, the trace was at +1.1 divisions over the center graticule line.

I then tried to adjust R429 as per manual so that trace line is exactly at the center of the graticule. But I was going nowhere! No matter how many turns I went on each direction, the trace simply didn't move, it stuck at +1.1 divisions.

Does it mean its fully open and needs replacement? Is this related somehow to the signal compression problem I mentioned above?

Many thanks and best regards
Luis

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