pin diode attenuator

I have an older HP/KeySite HP8656B RF signal generator. It has one problem in that the diode attenuator circuit is not functioning properly. This attenuator circuit has 3 PIN diodes in series. Most the the circuits I see use a quad diode circuit, and I have seen quite a few discussions on this forum about PIN diode attenuators. So maybe someone here can be of help. One of the 3 series diodes in the attenuator circuit looks bad when comparing the loss across all 3 diodes. The HP manual doesn't give a 1N number for this diode, only the HP part number that's not very helpful in finding a generic. So I am looking to replace the diode with a generic diode. Are PIN diodes so very different in their specs, that I will have to worry about going out and finding a diode replacement? Hope someone can advise me about trying out a generic PIN.