Not a MASTR Exec, but a low band MASTR II mobile for use as a 6m beacon. I'm working on using the ProgRock in place of the crystal in the channel element. Once I get the low band beacon going in good order, I plan to expand to 2m, 1.25cm & 70 cm. I started this project over a year ago, set it aside for several months, and just today returned to it.
I plan to inject the RF from the ProgRock directly into the channel element connections on the exciter board, and key it on/off with a small memory keyer. It works just fine using a simple relay to key, but I want a solid state switch instead of a mechanical relay, so I've been working toward that.
I tried inserting the on/off keying in the bias & tripler circuits downstream of the channel element, but that didn't work well at all; the resulting RF was dirty & splattered across the narrow beacon sub-band. The original mic PTT simply turned the power to the oscillator in the channel element on/off, so that seems to be a good starting point.
Presently I have good stability using the ProgRock stand-alone (no GPS PPS discipline); it's running less than 10 Hertz off my desired frequency. I expected much better performance with GPS 1PPS discipline, but after trying 3 different GPS modules, I've not had better success than without. I'm beginning to question the 1PPS I've been giving the ProgRock relative to the ProgRock 1PPS requirement; perhaps the ProgRock needs a wider pulse width than 1mS, so I'm considering trying a pulse stretcher stage; perhaps 10mS will do the trick.
I did find a significant increase in stability by thermally bonding the 27MHz. crystal on the Si5351A Synth module to the heat sink of the 5V regulator AND the aluminum Bud box enclosure, so if you don't want to deal with the GPS discipline, by all means make the effort to thermally stabilize the 27MHz. crystal & then thermally isolate the ProgRock enclosure from the environment. By the way, it's an inverse relationship...warmer crystal shifts the frequency down. Simply holding my finger on the crystal is enough to move the oscillator.
QRPLabs offers a OCXO module to replace the crystal on the Si5351A Synth module; you might consider using it. There's info in the archives of this email list on the various issues that may entail. I haven't tried that module so I can say anything about it's performance.
I'm happy to share anything I learn along the way; feel free to contact me.
Robin Midgett K4IDC