MODERATOR: Read carefully below! New links given are valid. Tech Note 484-B on the Synergy website, "Single Board, Drop-In Replacements for Motorola VP, UT+ and GT+ Oncore GPS Receivers..." shows replacement GPS receivers for various HP and Datum/Symmetricom GPSDO units. The Synergy GPS receiver replacement board for the HP Z-3801A is Model SSR-VP1/R, P/N 16U54523G-B1. As described here by others, it is a drop-in replacement for the original Motorola GPS receiver but with enhanced performance. Note that Synergy just changed the model number of this module and the Tech Note Number. The web address for the tech note is: https://synergy-gps.com/wp-content/uploads/2022/05/SSR-VP-UT-and-GT-Drop-In-Replacements011624.pdf The webpage address for this tech note was apparently recently changed by Synergy. This is a new address link. This link works perfectly in my Firefox browser. If you are unable to make this link work, go to: https://synergy-gps.com/ Then click: GPS (Tab at top of page) Then click: SSR Series GPS Drop-In Replacement Boards... (Picture on right) Then click: Data Sheet (Wide blue bar) Several members here have reported success with these modules received recently. So far as I can tell, the orders for these reported-on receivers were placed long ago. I have just placed an order but am waiting for confirmation of pricing and delivery. My understanding is there is a long lead time for the u-blox receiver and that will delay the replacement module, which is assembled in San Diego. The approximate price is just under $200. I know nothing about the LED digital clock some have built into their HP Z-3801A GPSDO. But, if all you want is a really nice LED GPS clock, check this: https://www.tindie.com/products/nsayer/gps-clock/ This link works perfectly for me in my Firefox browser. If you are unable to make this link work, go to: https://www.tindie.com Then enter "GPS Clock Geppetto" in search box Click: GPS Clock (in drop-down list) This clock receives multiple GNSS and displays time with tenth-seconds. The display mimics the old Heathkit GC-1000 Most Accurate Clock. I have several of these and they work very well. -- Best wishes, Larry McDavid W6FUB Anaheim, California (SE of Los Angeles, near Disneyland)