HP 8557A center frequency display issue.

[Raindog]

Greetings all. I have been working on a HP 8557A plug-in module and going through the frequency control adjustments laid out in section 5-17 of the service manual, now that that part is completed I have hit an impasse. I am now at the point where I have to set the center frequency display, however the only service manual I was able to download is for an earlier revision of this module with the mechanical frequency display. Since I do not have the correct service manual the center frequency display does not properly track as you tune up in frequency. The center frequency display gains about 40 kHz for every MHz as you tune up from 0 MHz. The display reads high, at 10 MHz the display reads 10.4 MHz, at 175 MHz it reads about 7 MHz high and at 350 MHz it reads 14 MHz high, the tuning marker in full span mode also reflects this.

 There are three adjustments on the digital display board labeled R6, R8 and R20, I’m assuming these are some sort of linearity adjustments but I don’t know since I have been unable to acquire the proper service manual. Has anyone out here serviced one of these plugins? Or know knows what these trimmers do? I have retired this unit from service but would like to keep it operational anyway since it’s  the only “portable” spectrum analyzer I have.

*Edit*

I did some poking around and discovered that R6 and R8 compensate for frequency. R8 adjusts for the lower end and R6 for the upper end. Not knowing the reference frequency's to use I defaulted with 350 MHz for the upper end and 175 MHz for the lower end. Now the display has a tracking linearity of +/- 500 kHz, I still don't know what R20 does so I won't bother with it till I can find the service manual specific to my plug in module. These adjustment have also corrected the issue I was having in full span as well.