Royal paper shredder control board

[fleetwise]

My old Royal CIA12x shredder quit and the wife wants it fixed so here I sit scratching my head.

It has a transformerless power supply that is nothing more than a inline cap feeding a rectifier. The DMM shows 121 volts ac coming into the cap and 2.5 going out. The rectifier tests good out of circuit. In circuit the output is almost 0 volts.

What should the typical voltage drop be through the 3.3uf 250 volt 10% cap with a 1m ohm resistor in parallel shown in the first picture? Will the Blue esr meter check a cap like this? (looking for a good excuse for the wife to buy me one)

[lti]

Re: Royal paper shredder control board

What is on the other side of that big burn mark in the last picture?

[kc8adu]

Re: Royal paper shredder control board

shorted 20v zener or 7812.
quite crude.

[fleetwise]

Re: Royal paper shredder control board

yep it is a 7812. It was replaced 9 years ago after it fried the board. Also a trace on the board is now a jumper wire. Like they designed the board so that regulator doubles as a fuse.

Crude isn't what I called it but that is a very good g-rated description.

But for the life of me I can't remember what that cap dropped the voltage to.

[Longbow]

Re: Royal paper shredder control board

But for the life of me I can't remember what that cap dropped the voltage to.

I don't understand your question. Without looking more closely, I would say that the yellow capacitor is a noise filter for the motor. It doesn't have anything to do with dropping voltage.

[fleetwise]

Re: Royal paper shredder control board

I don't understand your question. Without looking more closely, I would say that the yellow capacitor is a noise filter for the motor. It doesn't have anything to do with dropping voltage.

Take a quick look at the back of the board. It really is used to drop the voltage. It ought to be near 12 volts if my math is not totally screwed, which I think it is.

[Th3_uN1Qu3]

Re: Royal paper shredder control board

You cannot calculate the voltage drop of the capacitor in series with the mains without knowing the power drain of the device. Put an amp meter in series before the 7812 and see how much it eats. If it's more than 1A, you have something wrong down the line. If it's close to zero, maybe that cap failed open.

[kc8adu]

Re: Royal paper shredder control board

looks like the zener clamps to 20v.the 7812 does the regulating from there.crude DISPOSABLE junk.

[fleetwise]

Re: Royal paper shredder control board

You cannot calculate the voltage drop of the capacitor in series with the mains without knowing the power drain of the device. Put an amp meter in series before the 7812 and see how much it eats. If it's more than 1A, you have something wrong down the line. If it's close to zero, maybe that cap failed open.

Well it is drawing 149 milliamps at 2.5 volts before the rectifier

[Th3_uN1Qu3]

Re: Royal paper shredder control board

I asked before the 7812 not before the rectifier... but either way 2.5 volts will obviously not power a 7812. Replace the series capacitor and see how it goes. The cap also acts as a current limiter - in fact, a capacitor in AC is a resistor - that's why the need for a zener and regulator after it. So depending on the design the cap may be good and something wrong downstream of it instead.

PS. You have checked for the obvious, like shorted zener and shorted 7812, right?

[fleetwise]

Re: Royal paper shredder control board

I asked before the 7812 not before the rectifier... but either way 2.5 volts will obviously not power a 7812. Replace the series capacitor and see how it goes. The cap also acts as a current limiter - in fact, a capacitor in AC is a resistor - that's why the need for a zener and regulator after it. So depending on the design the cap may be good and something wrong downstream of it instead.

PS. You have checked for the obvious, like shorted zener and shorted 7812, right?

Thanks for the help.
Sorry, It was easier to flip the cap and hang one end off the board than bend a pin up and solder the rectifier to board twice or use jumpers, so I just had to cheat there.
Yep, the 7812 was the second thing I checked after seeing that low voltage and it was what failed long ago.
Replacing the cap is what I'm hoping will fix it, but like you say it very well may still be something downstream.

[PCBONEZ]

Re: Royal paper shredder control board

Probably doesn't apply but I thought I'd chime it in anyway.
I've only worked on one paper shredder and the problem was the magnetic safety switch had failed open.
It senses that the top is sitting on the bin and disables the motor so it can't inadvertently start while the top is off to empty the bin or whatever.
Switch was mounted remote from the PCB near where the metal wire rim of the bin fits into the groove in the plastic top.
.
If yours has that switch and it's mounted remotely you will have to short the two pins that go to the switch in order to disable the 'lid off - motor kill' safety.

[fleetwise]

Re: Royal paper shredder control board

Probably doesn't apply but I thought I'd chime it in anyway.
I've only worked on one paper shredder and the problem was the magnetic safety switch had failed open.

That dang switch is why I'm working on this old shredder. We bought a small cheesy replacement and the basket is so flimsy the switch opens when its about half full or moved slightly. I would bypass the dang thing except I usually end up with a few fingers in the shredder when I lift it off the basket. The old one has a 12 gallon slide out bin, so I don't have to handle the cutters every time I empty it and it doesn't make me stop and fiddle with the bin for 5 minutes trying to close that switch just to shred a few papers.
Long story short - the old one has a bin safety switch that is wired between the power cord and the power switch that stays closed when it should.

[Agent24]

Re: Royal paper shredder control board

If there's enough room in the unit why not just leave out the capacitor\zener etc associated junk and build your own proper power supply...

Or even just add a socket and use an external power brick.