Swapped out the FSU today, normal service resumed.
Curiosity got the better of me and I went to the trouble of measuring out the resistances across all possible pin combinations to compare the old one with the new one. I don't quite understand what's going on, but it may be a useful reference point for someone who wants to estimate how much useful life is left on their FSU.
This is a legend to indicate the 5 pins on the FSU, to be used with the table:
And here are the measured resistances across all pin pairs:
<table width="241" style="width: 181pt; border-collapse: collapse;" border="0" cellspacing="0" cellpadding="0"> <colgroup><col width="64" style="width: 48pt;" span="2"> <col width="113" style="width: 85pt; mso-width-source: userset; mso-width-alt: 4132;"> <tbody><tr height="20" style="height: 15pt;"> <td width="64" height="20" style="border: 0px black; border-image: none; width: 48pt; height: 15pt; background-color: transparent;">
Pins</td> <td width="64" style="border: 0px black; border-image: none; width: 48pt; background-color: transparent;">
Old</td> <td width="113" style="border: 0px black; border-image: none; width: 85pt; background-color: transparent;">
New</td> </tr> <tr height="20" style="height: 15pt;"> <td height="20" style="border: 0px black; border-image: none; height: 15pt; background-color: transparent;">
A-1</td> <td style="border: 0px black; border-image: none; background-color: transparent;">
107.1 kΩ</td> <td style="border: 0px black; border-image: none; background-color: transparent;">
0.634 MΩ</td> </tr> <tr height="20" style="height: 15pt;"> <td height="20" style="border: 0px black; border-image: none; height: 15pt; background-color: transparent;">
A-2</td> <td style="border: 0px black; border-image: none; background-color: transparent;">
106.1 kΩ</td> <td style="border: 0px black; border-image: none; background-color: transparent;">
23.22 kΩ</td> </tr> <tr height="20" style="height: 15pt;"> <td height="20" style="border: 0px black; border-image: none; height: 15pt; background-color: transparent;">
B-1</td> <td style="border: 0px black; border-image: none; background-color: transparent;">
0.2 Ω</td> <td style="border: 0px black; border-image: none; background-color: transparent;">
0.2 Ω</td> </tr> <tr height="20" style="height: 15pt;"> <td height="20" style="border: 0px black; border-image: none; height: 15pt; background-color: transparent;">
B-2</td> <td style="border: 0px black; border-image: none; background-color: transparent;">
1.008 kΩ</td> <td style="border: 0px black; border-image: none; background-color: transparent;">
0.614 MΩ</td> </tr> <tr height="20" style="height: 15pt;"> <td height="20" style="border: 0px black; border-image: none; height: 15pt; background-color: transparent;">
C-1</td> <td style="border: 0px black; border-image: none; background-color: transparent;">
7.94 kΩ</td> <td style="border: 0px black; border-image: none; background-color: transparent;">
1.212 MΩ</td> </tr> <tr height="20" style="height: 15pt;"> <td height="20" style="border: 0px black; border-image: none; height: 15pt; background-color: transparent;">
C-2</td> <td style="border: 0px black; border-image: none; background-color: transparent;">
6.94 kΩ</td> <td style="border: 0px black; border-image: none; background-color: transparent;">
0.601 MΩ</td> </tr> <tr height="20" style="height: 15pt;"> <td height="20" style="border: 0px black; border-image: none; height: 15pt; background-color: transparent;">
A-B</td> <td style="border: 0px black; border-image: none; background-color: transparent;">
106.9 kΩ</td> <td style="border: 0px black; border-image: none; background-color: transparent;">
0.634 MΩ</td> </tr> <tr height="20" style="height: 15pt;"> <td height="20" style="border: 0px black; border-image: none; height: 15pt; background-color: transparent;">
A-C</td> <td style="border: 0px black; border-image: none; background-color: transparent;">
112.8 kΩ</td> <td style="border: 0px black; border-image: none; background-color: transparent;">
0.607 MΩ</td> </tr> <tr height="20" style="height: 15pt;"> <td height="20" style="border: 0px black; border-image: none; height: 15pt; background-color: transparent;">
B-C</td> <td style="border: 0px black; border-image: none; background-color: transparent;">
7.96 kΩ</td> <td style="border: 0px black; border-image: none; background-color: transparent;">
1.212 MΩ</td> </tr> <tr height="20" style="height: 15pt;"> <td height="20" style="border: 0px black; border-image: none; height: 15pt; background-color: transparent;">
1-2</td> <td style="border: 0px black; border-image: none; background-color: transparent;">
1.008 kΩ</td> <td style="border: 0px black; border-image: none; background-color: transparent;">
0.611 MΩ</td> </tr> </tbody></table>