Odd rectifier behavior in lawn mower

[norry]

This is probably a dumb question for the pros, but I can't wrap my head around why it's acting this way.

Trying to fix an old Black & Decker electric lawnmower, the one Mom got many years ago. Never had to replace anything other than brushes, and perhaps a switch. Now it doesn't do anything at all when you hit the switch. Brushes are good.

Motor was sounding pretty rough when I turned it manually, so I oiled it, and now it turns a lot more smoothly, but still doesn't do anything.

Tried testing for DC voltage at the motor (it has a rectifier and a DC motor) - none. Ahah, I thought, it must be the rectifier or the cord. But not so fast: when I take out a brush and test, it's now 17 volts at the brushes. Put the brush back in the motor, 0 volts. Motor windings all test around 1.x ohms.

What am I missing here?

Thanks in advance!

 

[Ram]

Set meter to AC setting and measure what is on the power cord, then out of of rectifier.

 

[justsam]

When you remove the brush and see 17 volts, are you seeing AC or DC?

Measure both AC and DC at the brushes, with the brush removed so you have your 17 volts.

Is there a step down transformer before going into the rectifier?

What is AC voltage out of the transformer but before the rectifier?

I assume this is not a rechargeable device but rather requires AC plug in power all the time?

What is rectifier configuration. Is a 4 terminal device, i.e. bridge rectifier?

 

[norry]

Thanks for the replies!

Measuring DC voltage has produced anywhere from 17 to 25 vdc forward (red to red, black to black) and 0 vdc backward. It seems consistent within each time I connect the cables and fire up the switch, but inconsistent between times. Measuring AC voltage is 8 vac forward and 16 vac backward. All of this is at the brushes with one brush removed from the windings.

AC input voltage would be a useful thing to know, as the switch seems to serve as some sort of rheostat, but I ran out of time for the evening and wanted to post what I did get.

Setting the multimeter to ohm mode and putting it on the two DC leads of the rectifier results in a low ohm reading, regardless of which 'direction' the multimeter is going (i.e., red to red or red to black). Ditto with the multimeter on diode check mode. From my reading, it sounds like that's not a good sign about the rectifier?

There is no step down transformer, and you're correct that there's no battery either.

There are two AC terminals entering the rectifier and two terminals leaving the rectifier. This is my first experience with rectifiers in a long, long time - is that the same as the 4 terminal configuration you're describing?

Thanks again for the help!

 

[justsam]

Sounds like there is no transformer so you should be seeing more like 150VDC at the brushes, (careful this is enough to bite a little!)

The rectifier has failed based on your readings. A new bridge rectifier to handle the required voltage and current is cheap, and available at Radio Shack or others.

Here is a site that may help!
http://nickviera.com/electrical/lawn_mower/bandd/index.php

 

[norry]

Sounds like there is no transformer so you should be seeing more like 150VDC at the brushes, (careful this is enough to bite a little!)

The rectifier has failed based on your readings. A new bridge rectifier to handle the required voltage and current is cheap, and available at Radio Shack or others.

Here is a site that may help!
http://nickviera.com/electrical/lawn_mower/bandd/index.php

That makes sense, that 110vac shouldn't magically turn into 17-25vdc. I'll check to make sure that the rectifier is 'seeing' 110vac input (i.e. that the switch hasn't failed) but as long as that's the case, I'll go ahead and order another rectifier. Thanks for the help!

 

[kbs2244]

I would chek for burned contacts in the switch before digging in too deep.

 

[norry]

Update: the comments about voltage at the rectifier were correct that there was insufficient voltage there. Which got me wondering what the input voltage was, and intriguingly, it also seemed too low.

Tracing the circuit towards the plug, I found that the voltage at the main junction between the cord (about a one-foot-long pigtail) and the mower was 40-something VAC. Hmm, that's interesting. (A quick check of the wall voltage revealed that it was, indeed, the expected 110v). Went and bought a replacement pigtail, cut the end off the old one to salvage the stress relief, and managed to cut directly through the bad part! A wire had flexed too many times and failed, with evidence of heat damage to the surrounding insulation.

I ended up cutting off a little more of the damaged section, splicing and soldering the original pigtail back onto the mower, and now getting full voltage all the way to the brushes.

Thanks for helping me diagnose Mom's mower!

 

[gatchel]

Update: the comments about voltage at the rectifier were correct that there was insufficient voltage there. Which got me wondering what the input voltage was, and intriguingly, it also seemed too low.

Tracing the circuit towards the plug, I found that the voltage at the main junction between the cord (about a one-foot-long pigtail) and the mower was 40-something VAC. Hmm, that's interesting. (A quick check of the wall voltage revealed that it was, indeed, the expected 110v). Went and bought a replacement pigtail, cut the end off the old one to salvage the stress relief, and managed to cut directly through the bad part! A wire had flexed too many times and failed, with evidence of heat damage to the surrounding insulation.

I ended up cutting off a little more of the damaged section, splicing and soldering the original pigtail back onto the mower, and now getting full voltage all the way to the brushes.

Thanks for helping me diagnose Mom's mower!

I once picked up a non-working Streetfighter 2 Arcade Game for $125 because of the exact same issue.

Good find!