Just worked out a preliminary conversion for the Mark 1 Sentinel (earliest model) and it comes out as a PL12 construct.
Sentinel Mk1; PL 12 Construct; Init +1; Defense 14/13 flat-footed (+5 defence ; 10 pp, -2 Size); Spd 30 fly 30; Atk +7 melee (+6 attack; 18 pp) Dmg: (+12 S, punch), +5 ranged (+6 S/L, energy blast); Hardness 15;
SV Dmg +17, Fort --, Ref +1, Will --, Str 18, Dex 12, Con --, Int --, Wis 16, Cha 8. (23 pp)
Skills: Spot +5 (5 pp)
Feats: Detect (Mutants), Standard Construct Immunities, Accurate Attack, Assessment, Darkvision, Toughness, Radio Broadcast, Radio Hearing (14 pp)
Powers: Growth +8 (Flaws

ermanent; 40 pp), Super-Strength +8 (from Growth), Immovability +8 (from Growth), Protection +12 (from growth and +4 extra; 4 pp) Flight +6 (Power Stunts: Super Flight; 18pp), Energy Blast - Fire +6 (Power Stunts: Dual Damage; Extra: Additional Energy - Electricity, Cold, Sonic, Kinetic, Magnetic 44 pp), Disintergration +6 (12 pp), Datalink +1 (2 pp)
Weakness: Programmed (Follows strict set of directive, simply will not act outside of this) (-10 pp)
total = 180 pp (PL 12)
This is the earliest model sentinel and lacks a lot of the weaponry, add ons and also the adaptability all the successive models had.