Hi All -
I recently snagged a 335A and have begun going through it. It has not yet been powered up. The only glaring thing found so far is an overcooked, burned out resistor (R3) on the A5A6 current limiter PCB. The schematics I've found thus far (and the board shown in the other recent 335A thread) are somewhat different than the one I have - it appears that mine is a newer iteration (marked 335A-3060 Rev H), as it is missing some of the resistors shown in the earlier schematics.
The older schematics show a series of 2W resistors feeding the range indicator lights - R3, R8 and R25; 150, 120 and 120 ohms respectively. My board has only R3, and it is 24? ohms (red-yel-something unreadable as the multiplier band is cooked off). This does not appear to be a field mod, as the reference designators for the missing resistors are not present on the board (though the layout appears unchanged from the earlier version; the holes and traces are present but are jumpered out). Based on a quick look at the paste-together schematics I've found thus far, it appears that all this does is to feed the range indicators, which on the older schematics appear to be incandescent lamps. On my 335A, they are LEDs, so as they are fed from what looks like a 45-50V line, I'd expect the resistor to be in the 2.4k range (~20mA @45V). Does anyone have info on the newer(?) iterations of the board and know what the value of that resistor
should be? At 20mA, it seems it should dissipate roughly 1W, so a 2W resistor might be a bit marginal there (I was taught to go 3X anticipated power dissipation), but plan to dig into the circuit further to see if anything else is amiss there - it definitely got VERY hot.
Board:
Cooked resistor:
Additional photos of the instrument are at
https://pmanning.smugmug.com/Electronics/Fluke-335A-DC-Voltage-Standard-Differential-Voltmeter-NullThanks for any info!
-Pat