Hung Chang (Protek) 3502c oscilloscope

[Mikeva]

Hi guys,
I recently bought my first oscilloscope used and although all the functions work ok,I'm having problem with the vertical gain on the first channel.It can't measure the voltage right.
i connect the probe into the 0.5vcal point and it measures only 0.3Vp-p instead of 0.5Vp-p.The same happens if i try to measure dc voltage from my power supply.For 10 vdc it reads about 7.5-8v.The channel B  works ok instead.
I found the schematic and tried to calibrate it but the trimmer which is responsible for this is almost fully turned clockwise giving the measurements i described.
I checked all the voltages(except the 1.9kv ) and all seems within the specs.
I didn't found any obvious fault on the components measurements.
There is an 15 pin hybrid ic marked AX-1015 (2 per channel) that maybe is the cause for the fault but i cannot find any information about it.Does anyone knows what ic is that?(pics)
Or any advice on how to proceed?
Any help will be greatly appreciated.
Thanks
 

[siggi]

Input the same signal on both channels and trace it through, figure out where they diverge. Hopefully this happens in the discrete so before the hybrid.


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[Roeland_R]

Does the cal potentiometer knob work correct?

[Mikeva]

yes it works alright .It has the same gain as the B channel.