So I ended up with a Zygo 7702 dual freq head. I rigged up a power supply and it just would not lock eventually the service light coming on. Reading though the laserfaq they mentioned that one of the test points needed to swing though at least +/-7v as the tube does it's mode sweep and sure enough mine was sweeping about -4 to 8v. The tube has about 30k hours on it so it was just getting weak, still putting out about 400uw but just not enough.
There was a mention about possibly adding an extra opamp to amplify the signal a bit to get over the threshold. There is a LF347 quad opamp on the board that amplifies the signal from the photodiodes so I figured there ought to be a way to modify that to get it back into range. Tracing back the test point I found the quarter that handled the diode and traced out the basic circuit, just a pretty generic inverting amplifier. So I traced out the feedback resistor which is R17 for those playing along at home which is a 1 Meg resistor. I popped that off with SMT resistors and added a 1Meg axial to the pad and then started adding resistance till I got it to sweep in the range it wanted, which turned out to be about 1.6Mohm. Eventually the thing finally locked!
So, it would probably be easier to put in a small multi turn trimmer than do what I did, but it worked. So for reference to anyone else that gets one of these heads and the power is too weak to lock in, try this.