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Probes shouldn't make any difference to the ability
of the scope to trigger.

However, depending on whether the probe is a 1X, or
a 10X probe, different attenuator sections in the scope
will be used. The attenuator relays get thin films of
corrosion on their gold leads, and sometimes won't switch
reliably.

If the scope wasn't working properly, QService should not
have calibrated it until it was... It really isn't possible
to complete calibration with problems like you are describing.

I fix a lot of my customer's scopes that were supposedly
working perfectly, and were ready for calibration, but
had problems found during calibration. This is normal.

Bear in mind that the opamps you were probing are intended
to be sample and hold cells. The opamp is a unity gain
follower, with essentially infinite input impedance. It
is measuring the charge put in the hold capacitor by the mux.
The hold capacitor is only about 0.1uf, and will discharge
almost instantly when loaded by the input impedance of a DMM
or a scope probe.

-Chuck Harris

Panos wrote:

Yeah maybe it's better to do so. Nevertheless I thought about making an effort, since after the calibration the oscilloscope is working properly, apart from this detail.
To be fair opposite to QService, the calibration cost did not include any minor repairs to the machine.

In the meantime as I looking the reason for this problem, I made some progress. As I check the waveforms around the U500-U700-U800 according to the service manual, I see some instabilities on the signal during the interference.

Specific from the Inverted A Trigger output of U500 pin 18 to Trigger In of U700 pin 2.
After the same from the ramp output of U700 pin 35 to the A Sweep Input of the U735 pin 9.
Finally the same from the U735 pin3 to U800 A Sweep Input pin18.

That gave me the idea to look before the U500 the the trigger level lines A and B. There in A5 digital board, is a double operational amplifier marked as TL072, which drive the A and B trigger lines to U500.
Measuring the amplifier input and output with a voltmeter, I was surprised by the very low voltage across the full range of the trigger level control knob. The measurement was lower than +/- 500 mVolt across the range and in the trigger status about 200 mVolt in line A, contrary to 2.5 volt and 600 mVolt of line B.

I replaced the TL072 operational amplifier and now things was improved! As for the level of the voltage during trigger moment came at the same level for both A and B lines. About 600 mVolt.
Although it seemed that the problem was completely solved, something strange appeared using a different type probe.
As a remind, the interference was appeared when the volt/div selector was in the range of 20 mVolt and below (200 in my case using a x10 probe).

But enough with the written. I have to make a video.