Yup. The 800 is a 1200 with smaller transistors. Think it was ORIGINALLY designed around the MRF454... I STILL have a pair of boards with the 454s in them from a 16 pill amp
The 1200 is the M2000, cut in half and using the basic board assy from the 4v, rather than the boards from the M2000. The earlier M800 and M1600 (which never really made it to production, I believe 6 to 10 where made of the M16, and they where ALL sold in San Diego to locals, at least that I know of. The FIRST M16 became the first M2000, and it's sitting 5 feet away, hence the reason I've got the M16 boards)
The small relay off to the side, so to speak, is for bias. Messenger uses a seperate relay to control the bias, as its MUCH MO BETTAH to ground the base of the transistors when you're NOT keyed up.... Reduces the chance of oscillation when the load is removed from the collectors (the relay unkeys), and reduces the change of 'white noise' being produced (another fancy name of a different type of oscillation, this one being broadband in nature) that you hear on receive.
I <<BELIEVE>> you're OK with the 2879 conversion. The problem with the M4V being converted to 2879s is with the output combiners. If you have ANY VSWR, you'll smoke them. I have a 4V that literally caught on fire after being converted. I've spoken with Sam about it indirectly (CB Doctor BDW argued with me on CB Tricks about this NOT being the case.... about 4 days later he confirmed that Sam concurred with my statements.... Stand up individual for admitting he was wrong like that, all other personal bullshit aside). The 2290 and 2879 boards differ only in the input and output xformer capacitors I think..... Anyway, for 2879s, 5 turns on the output with 1200 pf across it will get you 1200 watts or better
The resistors in the hi / mid / low probably need to be changed to corrolate to the slightly different input impedance (if you want the ratios to remain the same.... I use high power, anything else is nothing but an SWR issue)
Lemme know how it goes.... Sounds like a good project, and sounds like bad xister in the drive section, or maybe a bad cap....
--Toll_Free