I have also a problem with my YIG. I measure the following on the connector:
FM- = -3.8mV
FM+ = -1.2mV the resistance between FM- and FM+ is 0.48 Ohm !
When I disconnect the connector FM+ measures +7.5 V (with an 139khz signal on top of the DC), this is with a span of >10Mhz. If the span is <=10Mhz the signal jumps hence the DC is unstable (coming from the analogue board)
Main Coil has 13 Ohm resistance and 13V as well as 14.95 V respectively
I cant get a signal on FM- with or without connection.
Do you see any indication that the YIG is definitely damaged (too low resistance on FM?)?
It seems I might have two issues, one on the YIG with FM and another with the <>10Mhz span fed from the analogue board ?
Would be great to get your insides. BTW my HP is the 8590L, I attach the relevant schematics.