Help for repair; A Resistor value of Amprobe AM-240 - Page 1
Forum section: electronics / repair

author: N-Gonka
author_rank: Newbie
timestamp: April 21, 2013, 10:05:20 am
content: Hi Guys,A Resistor in my Amprobe AM-240 released his magic smoke so I can't see his color code. Could anybody who owns this meter could look up the value of resistor R41 on a rev 3.0 board? It's located directly above the com Input (see attachment).The resistor has a five band colorcode and the outer colors are green and black.Thanks very much.

author: Lightages
author_rank: Unknown
timestamp: April 21, 2013, 04:44:57 pm
content: I am curious. How did this happen?

author: N-Gonka
author_rank: Newbie
timestamp: April 21, 2013, 07:10:26 pm
content: That was a bit strange. I was just measuring different currents and let the meter connected to the circuit. After some time it didn't want to measure currents anymore. The other ranges seems to be fine.

author: Lightages
author_rank: Unknown
timestamp: April 21, 2013, 07:14:15 pm
content: So I assume you were on the milliamp range and had it near the upper limit of the range?The resistor should not have died first, the fuse should have. Is the fuse still good? Is it the right one for the current range used?

author: c4757p
author_rank: Super Contributor
timestamp: April 21, 2013, 07:15:44 pm
content: Current sense resistor? Looks like the cheap-asses undersized the resistor, assuming you'd only be measuring current for a short period of time, and you cooked it. If you can find another AM-240, just use this one to probe its input jacks while set to mA and that's your resistor value. You'll have to recalibrate, though.

author: N-Gonka
author_rank: Newbie
timestamp: April 21, 2013, 07:20:23 pm
content: The fuses are ok. That was the first thing I checked.@c4757p:Thats the a problem, I just have this and a cheap one. I bought the Am-240 just a few days ago :-( It was the only one I could afford.

author: firewalker
author_rank: Super Contributor
timestamp: April 21, 2013, 07:26:07 pm
content: Use the warranty if you haven;t done anything wrong.Alexander.

author: c4757p
author_rank: Super Contributor
timestamp: April 21, 2013, 07:27:41 pm
content: It should be easy to replace. Looks like there are trimmers on the board, so it's not a software adjustment. You just need to find someone with a known good meter to help you briefly. First, to measure that resistor so you can replace it, and second, to calibrate. (Just run a stable current through both meters in series, then adjust the trimmer so it reads correctly.) I suggest using a physically larger resistor so this doesn't happen again; it should also be a low temperature coefficient resistor, though actual tolerance doesn't matter a rat's ass.Quote from: firewalker on April 21, 2013, 07:26:07 pmOr this.

author: N-Gonka
author_rank: Newbie
timestamp: April 21, 2013, 07:35:49 pm
content: But how can I get the resistor value if I can't see the colorcode? This little bastard is burned and brown all over and has little cracks in it.Quote from: firewalker on April 21, 2013, 07:26:07 pmThats the last possibility but its faster, cheaper and easier to do it myself if I have the right resistorvalue.And by the way, thanks for your help

author: armandas
author_rank: Frequent Contributor
timestamp: April 22, 2013, 07:19:36 am
content: R41 is a 0.99R, 0.5% tolerance resistor. The colour code is black-white-white-silver-green.

author: N-Gonka
author_rank: Newbie
timestamp: April 22, 2013, 08:08:24 am
content: Thank you very much armandas. I suspected such a weird value.

author: SeanB
author_rank: Super Contributor
timestamp: April 22, 2013, 05:52:10 pm
content: Simplest way to get it is to parallel a 1R 0.1% 1W resistor with a 100R 0.25% 1% resistor. Either that or buy 50 1R resistors at 0.5% tolerance and select on test those that are closest to 0.99R and use one.

author: commongrounder
author_rank: Frequent Contributor
timestamp: April 22, 2013, 07:37:11 pm
content: Don't forget to get "flameproof" resistor(s). Otherwise, if this happens again, you will get a very large carbon deposit on your meter circuit board, and lots of magic smoke.

author: BillyD
author_rank: Regular Contributor
timestamp: September 13, 2013, 06:52:50 pm
content: Quote from: armandas on April 22, 2013, 07:19:36 amThanks armandas for the info. I blew the 500mA range on my AM240 a while back when I went to take a voltage reading having earlier taken a current reading and forgotten to switch back to volts. Eventually got round to ordering spare fuses for it and just discovered that R41 is also burnt out. Googling it brings up your response. This will help me get it back going again, cheers.