I have repaired a number of PG506's and these are the common problems:
Intermittent operation of the flat white relays at the back of the instrument
On older instruments, there are two small metal-cased relays near the front. These switch in a low-pass filter for some standard-amplitude settings to reduce noise from the switching power supply.
Cam switch contacts. To get to these, it is necessary to remove the DVM daughter board
The standard amplitude cal trimpots R340 and R205. I replace the single-turn pots with multi-turn trimmers
To clarify the 50-ohm vs 1 megohm load question, the Standard Amplitude output is intended to drive a 1 megohm load at all ranges. At 10 V and below, it can drive 50 ohms, in which case the output should be exactly half the 1 megohm load value. The High-Amplitude output can drive either a 1 meg or 50-ohm load. The output to a 50-ohm load will be about a tenth of the 1-meg value.
The fast-rise outputs must drive a 50-ohm load.