Interesting Troubleshoot on a Dewalt Circuit Board??? - Page 1
Forum section: electronics / beginners

author: JayMan07
author_rank: Regular Contributor
timestamp: January 08, 2017, 08:34:18 pm
content: I have a Dewalt shop vac that runs on both batteries and 120VAC so it has a circuit board in it controlling those functions.Here's my troubleshooting results so far (I'm only concerned with it powering down while running on a battery)...1) The motor intermittently loses it's ground via a FDD 8874 Mosfet losing gate voltage...voltage into the circuit board does not drop out during this time3) I looked at the current waveform of the motor and it's very normal.2) I traced this gate voltage all the way back to a small transistor (X1EV 5D). When the vacuum is operating normally the pins on this transistor measure C=19V E=19V B=2.8V however when the vacuum shuts down the pins measure C=19V E=0V and B=???The reason for the ? on the base voltage is thatevery time that I probe that pin with my multimeter the vacuum instantly cuts onI don't think there's a connectivity issue with the pin since I'm actually probing a test pad located right beside the pin.My meter is a BK 2709B with 10M Ohm impedance in VDC mode. Since I thought that maybe the meter was acting like a resistor I connected a 5Mresistor from ground to the pin. It actually made the motor which was running half way (i.e. partial ground), at the time, completely die...after that I probed the pin with the multimeter again and it fired right up. What the heck is going on here?!! I use this vacuum every day so this has been truly annoying.I may be able to post a pic of the circuit board but I figured it wasn't really necessary...there are a lot of SMDs so I'm not sure you could see much anyway.Would the multimeter be adding some sort of capacitance to the circuit? If so could I bodge a cap into the board?

author: JayMan07
author_rank: Regular Contributor
timestamp: January 09, 2017, 12:58:44 pm
content: Bump...I know I added a lot of details for anyone who needed them but I don't want that to drown out my basic question (which I know someone here can answer). In what small way does a DVOM affect the circuit you are measuring? Does it just act like a 10Mresistor and nothing else? Since the transistor was a PNP maybe the incredibly small amount of current flow out of the base when probed by the meter was enough to turn it on???

author: klunkerbus
author_rank: Unknown
timestamp: January 09, 2017, 01:34:21 pm
content: The probe wire could be acting like an antenna, adding noise into the circuit. The wire (and the person holding the probe) will add capacitance to the circuit, which could cause or kill an oscillation. I'd first put more effort into ruling out intermittents. Board flex caused by probing a board at one point can aggravate an intermittent anywhere else on the board - probe that same spot (and then other spots around the board) with something non-conductive and see if that reveals anything.From your write-up, I found it unclear when the vacuum is turning on but shouldn't, or turning off but shouldn't.

author: klunkerbus
author_rank: Unknown
timestamp: January 09, 2017, 02:51:39 pm
content: Unless DeWalt has had more than one shop vacuum with a battery, I may have that same model. Thinking back, I may have had the same problem long ago. I seem to remember scrambling to pull the battery a few times when I was done using it.What I remember reading at the time is that the power switch was notorious for failing since it is an unsealed switch mounted where it is exposed to dust, etc. Unfortunately, the repair was long ago and I don't remember if I replaced the switch, took it apart and cleaned it, found something loose, or something else. But the problem wasn't in the electronics.

author: JayMan07
author_rank: Regular Contributor
timestamp: January 09, 2017, 03:39:05 pm
content: Quote from: klunkerbus on January 09, 2017, 01:34:21 pmOk thanks for your thoughts...I had considered the possibility of the meter adding capacitance.After cutting the vacuum on and off like 50 times I never could get it to fail again. The base of the transistor is connected to a 56Kresistor which drops the battery input voltage of 19V to 2.8V and feeds that 2.8V into the base of the transistor turning it on. The solder on that resistor didn't look cracked but it did look a little sketchy (like lumpy as in a cold joint). I re-soldered that resistor and the base of the transistor. If that doesn't work I may just add a 1-10 nF cap into the circuit and see what happens. I'll keep this thread updated. The vacuum fails at least once every day so I'll know if it's fixed by the end of the week.PS To clarify the problem...when a failure occurs, the vacuum will run full throttle for 1/2 a second then die (completely or almost completely). Then while it is dead and the switch is still on I take my DVOM and probe the test pad for the transistor base voltage and the vacuum immediately turns on full throttle. Hopefully that helps clarify.

author: JayMan07
author_rank: Regular Contributor
timestamp: January 09, 2017, 03:44:38 pm
content: Quote from: klunkerbus on January 09, 2017, 02:51:39 pmYes that's true! My first thought was that the switch had intermittent high resistance. The first thing I did was put one channel of my scope on the circuit board feed (obviously post switch) and another channel on the gate voltage of the main mosfet that switches on the motor ground. I saw that the feed voltage remained perfectly steady while the mosfet gate dropped to 0V. That's what led me into troubleshooting the board.

author: klunkerbus
author_rank: Unknown
timestamp: January 09, 2017, 04:57:39 pm
content: Ah, so your symptoms are different than mine. My vacuum just wouldn't shut off. I also now see DeWalt has had a couple of vacuums in their product line, and we're likely discussing different models. I have model DC500 for 12-18v batteries. I know there's an AC/DC supply in mine, but can't recall whether there was much else for electronics. It's been a pretty handy vacuum for my occasional needs.

author: JayMan07
author_rank: Regular Contributor
timestamp: January 09, 2017, 06:16:00 pm
content: Yeah its a DCV581H. It takes a regular 18V dewalt battery but also has a power cord. Once you plug in the power cord a relay clicks cutting off the battery and turning on a switch mode power supply. It's amazing how complicated simple power tools are getting. I have a dewalt gyroscopic screwdriver which I'm sure will be a real pleasure to troubleshoot. Makita has a line of power tools that actually connect to your cell phone so you can change their settings and log usage...will it ever end

author: JayMan07
author_rank: Regular Contributor
timestamp: January 13, 2017, 04:07:28 am
content: Just wanted to give a quick update so this thread wouldn't be left open ended. The vacuum has worked perfectly ever since re-soldering the resistor and transistor. Since the problem was intermittent I'm still not totally confident it's fixed. But if it is in fact fixed, then I would conclude that the resistor had a cold solder joint that was intermittently not feeding the base of the transistor. Probing the board with a multimeter brought it back to life either by slightly flexing the solder joint or by providing a small amount of current or noise or whatever was sufficient for the base to turn on the transistor.