How to calibrate the vertical sensitivity of an old analog scope (Hameg HM203-6)?

by anrieff   Last Updated May 20, 2018 17:25 PM

I have a Hameg HM203-6 20 MHz analog 'scope I bought from a flea shop. There's about 20% mismatch of the readings of the two channels, with CH1's vertical amplification being about 20% higher than CH2 when both are at the "CAL" positions of their respective variable amplification knobs. Using a reliable 5V DC source, I've checked that CH2 shows about 5 volts exactly, whereas CH1 is close to 6 volts. On this model, the "CAL" position is at maximum CCW, and sensitivity cannot be reduced below the calibrated one.

So I assume the "calibrated position" must be recalibrated. The problem is that there's no mention on how to do this in the manual I was able to find. I see those "CH1/CH2 balance" holes on the underside of the case, but they serve a different purpose.

I'm also almost sure that the sensitivity can be calibrated through the VR703 ("Y-GAIN CH1") pot, slightly below the center in this photo.

So my questions are,

  1. Is there anything else to it, besides the pot adjustment?
  2. Is it a terribly bad idea to do the adjustment while the scope is naked and powered? I'm aware that exposed CRTs are lethally dangerous.

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