acceptable voltage drop across a MOSFET? looking for clues

[jaunty]

I am trying to troubleshoot (and repair) a device that I'm finding bad power on - namely it's a hard wired 'remote control' for a piece of studio gear - but i won't trouble you with that. It seems to me that a lot of the symptoms are being created by the fact that the logic chips (some 4000 and some TTL SN7 stuff) aren't getting the required 5V they need ... only 3.2 something volts - even the five volt test point reads 3.2 volts - so i started poking around with the DMM and as near as i can figure the culprit APPEARS to be an IRDF 9120 MOSFET (this one: https://www.mouser.com/datasheet/2/427/sihfd912-104368.pdf)  - it's the ONLY place in the circuit where the feed voltage drops from 5 (5.5 actually) to 3.2 volts ... though I KNOW a mosfet probably consumes some power for itself i'm wondering if i should be seeing a voltage drop like this here (obviously depends on implementation etc) - or is there a decent way to test this with normal lab gear (DMM, power supply etc)? Or would i do better to be suspecting ancillary components (caps etc) if this is what i'm seeing? Any hints? I hope i was clear enough to explain this decently ...

[Jeroen3]

The mosfet is probably part of the power supply circuit, and this supply is either defective or limiting.

[jaunty]

The mosfet is probably part of the power supply circuit, and this supply is either defective or limiting.

ha! well yes it's certainly part of the power supply - i'm just trying to verify what part is defective so I can get everything working again

[Jeroen3]

Most likely not the mostfet, since it isn't a short or an open. But the thing operating the mosfet.

[jaunty]

yeah i was playing around with the mosfet a bit and it APPEARS as though maybe it's alright so i guess i'll be looking at the MC34060 driver next

[jaunty]

i dunno man - i replaced that (the 34060 chip) and got no result - seems to me that little mosfet shouldn't be reaching 250F (120C) ??