On Wed, Nov 11, 2020 at 10:53 PM, Tom Lee wrote:
You guessed right as regards the 485, Tom.
A quick scan of the 485 Service Manual brings up the following:
Where test equipment for checking and adjusting is specified, a "Dual Input Cable" is listed, with a 067-0525-00 as an "Example of Applicable Test Equipment".
In the "Performance Check" chapter, CMR is checked, both at 50 Ohm and at 1 MOhm.
First, a 50 kHz sine wave is applied to both channels (using the 067-0525-00). Vertical sensitivity is set to 20 mV/div on both channels and amplitude is set to 8 divisions vertical deflection. With CH2 set to INVERTED and vertical mode display set to ADD, CH2 VARIABLE GAIN is set to minimize the displayed resulting amplitude.
Next, 50 MHz is applied and the resulting amplitude is checked against a 0.8 div. maximum spec.
My rough (very rough/wrong?) calculation suggests that apart from the 485's behavior, a 300 ps phase difference between both inputs could cause this. That'd be the result of a length difference of 2.5" between both cables.
The test is repeated with input impedance set to 1 MOhm with 50 Ohm terminations on the inputs.
The 067-0525-00 is also used for checking Added Mode Gain by applying equal amplitude signals to CH1 and CH1. Any good quality signal splitting cable would do the job, as it would for the 7A22