Fluke 45- no negative DC measurements above 3V

[David Aurora]

Might be a while before I get a chance to dive in and troubleshoot properly so just thought I'd check if someone has come across the same problem before and can save me the effort.

I got shafted on a Fluke 45 on eBay recently and at first thought the damage was just cosmetic as things appeared to work OK at first. Fast forward a couple weeks and I'm measuring something and flip from slow to fast rate and the measurement drops to zero. Flip back to slow and my measurement is back. Flip to medium- 0V again.

I noticed though that at voltages below about 3V it DOES work for negative inputs.

Anyone come across this before?

[Jory E]

I have a unit with the same defect but was not able to find any glaring fault within the unit. I am doubtful I'll ever return to troubleshoot this one though..

[stj]

sevice manual in this thread,
https://www.eevblog.com/forum/repair/fluke-45-repair/

[David Aurora]

Thanks guys, yeah I haven't gotten around to digging any deeper yet. Kind of just accepted I got ripped off and can't really be assed spending a lot of time on it, it'll most likely just stay in the modes that work for the rest of it's life

[J-R]

Service manual link in that thread is dead.  Attaching a copy.

The troubleshooting section is pretty good.  I'd suggest at least going as far as you can to start with.

Also, maybe this fact will help narrow down the fault:

"In the medium and fast measurement rates, the a/d converter uses one of two ranges: ±300 mV and ±3 V full scale.
In the slow rate, the a/d converter uses one of two additional ranges (±100 mV and ±1000 mV full scale), for a total of four ranges."

So if you can isolate the problem to specific functions and circuit paths, it may more quickly direct you to a faulty component.

[robert.rozee]

hi David,
    it may be a long-shot, but i have read of folks having problems with strange behavior when the Fluke 45's 5v rail goes out of tolerance - in particular relays not switching when they should. i believe there are a couple of fixed-value trim resistors that can be added/removed to adjust the 5v rail. it could be worthwhile, at the least, checking that the 5v rail is indeed spot on.


cheers,
rob   :-)