Access to the RF Unit is required.
Remove bottom cover and then remove cover to RF Unit - see below.
To increase overall Scope gain:
Lower the value of R963 (100k as fitted).
I put a 56k 1/4w through-hole resistor across R963 - see above - making the combined resistance approx 36k.
(One resistor leg was soldered to pin 2 of IC-961 and the other leg to the junction of R966 and R964)
To be neater you could remove the small 0603 SMT resistor and replace it with a 39k 0603 part.
To adjust Scope base-line:
Trimpot R966 will adjust the base-line of the Spectrum Scope.