Craftsman 82369 Multimeter mod

       After doing some renewable energy system installs, I decided I needed a clamp meter with DC amps capability. Not having to insert a DC ammeter inline is a big advantage especially on low voltage, high current loads.

       A recent trip in the US and lots of shopping around, I stumbled on this Craftsman 82369 Multimeter with 40A and 400A AC/DC amp range for about $60 plus tax. (and a $5 off coupon!)

       I took it back home, popped in a couple Eneloop rechargeables in it and used it and it worked no problems. Just last night, I decided to go and measure something in the home system and then the amp ranges only display "OL". What? It was working the day before and suddenly, it doesn't? All the functions work correctly (ohms, volts, temp, continuity, etc) but the 40A DC, 400A DC and 40A AC only shows "OL". 400A AC seems to work but I did not have a means to verify as I don't have a readily accessible high current AC line in the house.

       I didn't want to return it for warranty as the store is several thousand kilometers away on the other side of the globe, though it would be the worst case scenario if I could not find a way to fix it. The functions returned once but then failed again so I thought it would be an intermittent issue I could probably fix. Time to take it apart!!!

The multimeter and included pouch bag and accessories (inside, not shown).

The meter looks like a rebadged Extech MA440 AC/DC Clamp meter.

Two screws and you can take the case apart.

The apparent difference in build quality compared to more expensive brands is obvious. I just need this for the DC clampmeter function and it does it fine (back when it was working) so it is ok for my purposes.

unsoldering a few wires and removing five screws gets the PCB out.

This is the PCB front side.

Back side is um. Yeah, could have been better.

The wire was soldered to the PTC with a bit more than needed so it pooled under the PCB.

Long story short, I found out that the OP07 op amp is not operating in linear mode as it should be and realized that the negative supply pin is about 1.2V. Traced it back to a cheapo Chippower branded 7660 negative voltage supply that stops working at 2.6V even though the low batt indicator is not yet showing. Maxim's version is guaranteed to work down to 1.5V!

Scrounging my parts bins, I found an LT1044 which is a pin compatible part with much better specs than the original and also spec'd to operate down to 1.5V.

Testing showed that the multimeter amps functions still worked to 1.7V battery voltage but accuracy is now questionable at that level.

Note to self. Pointlessly turning pots on the PCB thinking it could have been a wiper losing contact is the least possible cause.

This is the calibration setup. An ammeter, constant current power supply and ten turn coil allowed me to calibrate the higher ranges with a 3A powersupply.

       Hopefully this works much better than the original and would serve me as well as the old-ish 82008 I have on the workbench.

Here's another mod I decided while I was at it.

The LCD backlight is not well distributed and the rather dim "Nokia green".

Here is the clear panel that distributes the light behind the LCD.

The LED is a chip on board type with the LED dies bonded directly to the PCB and a drop of epoxy on top to protect it.

I took an xacto knife to it and removed it exposing the pads.

I originally tried these white LEDs but the supply is taken straight from the two cells which is not enough to run gallium nitride based LEDs.

I used these 0803 size amber LEDs instead which have a similar voltage drop to the green LEDs.

Worked much better and brighter too!

Page created and copyright R.Quan ©19 Mar 2015.