Seeking shredder repair help

[tinker_1000]

I've got a cheapo Chinese paper/cd/credit card shredder that I bought over a year ago. It worked perfectly until just a few weeks ago. Now I have to operate it in manual mode to shred, then switch it off. It runs constantly in Auto mode.

The only info I have on it is that it's a Model CD71, but without any name brand on it. Upon taking it apart, I see that it uses what I believe are photo-transistors because the circuit board connections are labelled as transistors and the transistors have only an E(mitter) and a C(ollector), no B(ase).

My question is: How do I troubleshoot these components? If anyone has any ideas, I am open to any suggestions. It's not a big issue, but I'd like to just know what happened with this shredder.

 

[unclejed613]

you should have an infrared LED and a phototransistor, either across from each other, or next to each other. if they are across from each other, the motor starts when the transistor sees no IR from the LED. if they are next to each other, the motor starts when the IR from the LED gets reflected off of a piece of paper. in the first case, the transistor is conducting while IR from the LED is hitting it. when the beam is broken, the transistor goes open circuit, and a logic circuit inverts this signal and applies power to the motor. if they are next to each other, the transistor is open circuit until the beam reflects off of a piece of paper, the transistor turns on, the signal goes through a buffer circuit that applies power to the motor. if the LED and sensor are across from each other, it's possible the transistor is shorted or leaky. this will show as a low resistance when you check between the C and E of the transistor with an ohmmeter. if they are side by side, the LED could be bad, either shorted or open, or the transistor could be open (and without a base lead on it, it's very hard to test for that with a meter, except by removing it from the circuit, connecting a meter across it and holding it up to an INCANDESCENT light bulb (CFL lights don't emit enough IR to turn the transistor on). you want the positive lead of the meter on the collector lead of the transistor, and the meter in diode check mode to test the transistor. IR LEDs usually have a low enough forward drop to read on the meter. in one direction it reads open, in the other direction, you will see the forward drop voltage on the meter.

 

[Nigel Goodwin]

Most aren't that clever, they just have a microswitch operated by the paper been inserted. Bit's of paper get jammed in the mechanism and either jam the switch, or break it from it's mounting jamming it in either the ON or OFF positions.

Shredders are incredibly unreliable, they seem to self destruct all the time

 

[Torben]

This is good information. The shredder in the office has started just running continuously when in Auto mode. It tried to eat our accountant on Wednesday. I'll see about taking a look at the paper detection mechanism.


Torben

 

[tinker_1000]

unclejed613

Thanks for that helpful explanation. First of all, the led/phototransistors in my shredder are across from each other. In reading thru your answer, you mention that the transistor is conducting until the light from the led is interrupted. That makes perfect sense and would seem to indicate that one of the leds is no longer working. However, when I switch the shredder to Auto, and it begins running right away, the moment I place a sheet of paper in either the cd slot or the normal paper slot, the motor shuts off, indicating that there is communication between the led/phototransistor pairs.

What would explain that kind of operation?

 

[unclejed613]

that is odd behavior for a circuit that should be very simple. try looking for a set of jumper pins. or a wire or surface mount jumper on the board. sometimes these circuits are used in multiple products, and one model might be using sidby side sensors, and some opposite. rather than design 2 separate circuit boards, they make one and use jumpers to set up the circuit for side by side or opposite sensors. so a jumper or a resistor in one position causes the logic to invert, the other (or jumper open) causes it to not invert. sometimes in an envireonment where there's a lot of vibration, solder connections work themselves free. a shredder certainly has a lot of vibration while operating, so you may have an open solder joint somewhere, or even a crack in a circuit board. the sensor certainly seems to be working (albeit backwards)., so neither the transistor or LED are bad. something is causing the circuit to operate backwards. the simplest curcuit that might be used here is a photosensor driving a MOSFET, but there are no inversion steps here. next would be a sensor, some type of active signal conditioning (a trasistor to amplify the output current of the sensor, or a logic gate), and an output device to drive the motor. the intermediate step, whether it's a transistor or logic gate is probably what is at fault here. somewhere between the sensor and dmotor drive, you are missing an inversion step but a signal is somehow still getting through.

 

[tinker_1000]

You certainly seem to know what you're talking about. There is a quad 2-input nand gate on the circuit board, but I've changed it out with another and it still does the same thing. There is also an 8050S transistor on the board but I'm not sure how to check it since there is no pinout labelling on the circuit board. It seems to be a rare beast since I have trouble finding anything on it when I google for it. Lastly, there is a relay which seems to be working because I can hear it click just before the motor comes on.

 

[unclejed613]

is the transistor driving the relay? if so, you know the transistor works.
on most transistors produced in the far east, the center lead is the collector. for TO-92 transistors (small D-shaped package) the pinout looking at the flat side is ECB. so if the trace from the collector (center pin) goes to one side of the relay coil, that's the relay driver transistor.

i would check pullups on the inputs of the nand gate. if one of the gate inputs is supposed to be tied high, and it's low, that would account for the inverted state of the output. there may be several inputs used, so you need a pinout of the chip, and trace where the inputs and outputs of the chip go to. draw it out as you go, and you can build your own schematic.

 

[noop]

I've got a cheapo Chinese paper/cd/credit card shredder that I bought over a year ago. It worked perfectly until just a few weeks ago. Now I have to operate it in manual mode to shred, then switch it off. It runs constantly in Auto mode.

We had the exact same problem here. To fix it, we first unplugged the machine and hand picked as much paper debris as we could see. Then we plugged it in and pressed the paper forward button for a minute, (scrapped off more junk), press the back button (scrapped off more junk)

Then it worked fine. I am sure a vaccume wouldn't hurt either.

Hope this helps!
-noop

Also, we did purchase a new one and the guy at Office Depot told us that this can happen more frequenty without oiled "grinders" (those metal shredder things). I guess you are suppose to oil those every once in awhile to help prevent paper buildup.

 

[tinker_1000]

unclejed613

I'll take it apart and do what you suggested. It may take a few days, but I'll get back to you with my results. Thanks.

 

[Hero999]

People wearing ties that get trapped in shredders have been strangled - they seem pretty dangerous to me.

 

[Torben]

People wearing ties that get trapped in shredders have been strangled - they seem pretty dangerous to me.

Once they've been strangled, people in ties are usually perfectly safe. Before that, all bets are off.


Torben

 

[Nigel Goodwin]

Once they've been strangled, people in ties are usually perfectly safe. Before that, all bets are off.

If they wear a tie they deserve strangling!

 

[tcmtech]

I worked for about two years as a copier and office equipment repair tech. I was at a local business fixing a copier one day when I was asked to take a look at the big paper shredder they had there. It just needed cleaning and a little oiling. The office manager wanted to make sure it was working good before I left so he grabbed a bunch of inter office junk mail and paper.
About 20 sheets of paper and a dozen envolopes went through with no problem. Then I saw him quikely reach into the machine and rip one envolope back out that was about half shredded at that point.

It was the one with his monthly paycheck in it!

 

[tinker_1000]

People wearing ties that get trapped in shredders have been strangled - they seem pretty dangerous to me.

I never wear ties so I should be safe. lol

 

[tinker_1000]

unclejed613,

Well, I was ready to trace out your connections as you instructed in your last reply, but for some odd reason, I decided to check the voltages on the phototransistors first. The one by the paper slot was reading zero volts or close to it, indicating it was turned on by its led. Then I measured the voltage on the phototransistor by the cd slot and it was reading around 4 volts.

I removed the led from its slot and just shone a light from a mini flashlight at the phototransistor and it worked. The voltage dropped to almost zero, just like the other one. I took an infrared led from an old remote control I no longer use, wired it in place, taking care to hook it up correctly and, Voila!. I have a properly working shredder again.

I want to thank ALL you people for replying. You are friendly and the replies are fast. Excellent!

 

[unclejed613]

glad to be of some help......

might have been ambient light bouncing off the paper that turned the motor off....