Haunted shredder

[ogulpal]

Hi all,

I have this shredder that has 3 modes of operation: off, reverse, and auto. The auto mode works by sensing paper and turning the motor on when paper is present. It has a photoresistor and an IR LED, the paper blocks the path from the IR LED to the photoresistor and that's how it knows there is paper to shred.

The issue is that the auto mode doesn't work properly. It either keeps the motor on constantly the moment it is switched on without any paper in the slot; or it starts off when there is no paper, shreds whatever paper is introduced as it should, and then never stops anymore.

Things I've tried (with a lot of help from #TEAtime on Discord):

- Replaced the photoresistor with wire so I can close or open the circuit: no effect, motor stays on.
- Tested the voltage across the IR LED and the IR photoresistor. LED is at 1.06V, photoresistor is at 2.39V when it sees the LED and 5.99V when the LED is blocked with a piece of paper (to simulate paper insertion in the slot)
- Desoldered and tested capacitors C{3,5,6,7} (electrolytics) and C2 (film) with a DMM on 2M resistance mode and checked the resistance goes up over time towards infinity: all seemed ok.
- Desoldered and tested resistors R{1,3,4,6,7,12,13}: all matching their color coded values
- Replaced Q1 and TR1 to no effect

I have drawn the schematic in Kicad based on pictures I took and tracing the paths, but I'm very new to electronics and so I don't really understand how the thing works. I couldn't see what the diodes are, so I used the generic 1N4001 but that doesn't necessarily match reality. Except DZ1, I desoldered to check the reference on it.

I'd appreciate any help figuring out what is failing on this board so I can repair it, but also understanding what the discrete "units" are on this board and what they do so I can learn. For instance, I see many capacitors and resistors in parallel: how this that called and what does it do?

And based on understanding this circuit's theory of operation, what component might be failing so I can test it and why would it cause the shredder to behave the way it does?

Thanks a lot!

[BrokenYugo]

Any time you have a "capacitive dropper" type power supply (C2 limits current via capacitive reactance, DZ1 clamps voltage to desired level)  you have to remember two things.

1. Every point in this circuit is no safer to touch than the inside of a light socket, every point is to be considered live with 240VAC, be careful.

2. The dropper capacitor C2 is common fail point, they take damage and lose capacity over time. If you don't have a means of measuring capacitance you might just replace it (edit: capacity value must match here, equal or higher voltage rating) and get lucky.

[tooki]

Double-check your schematic, I think there’s some mistakes. The motor white/R13/D8 section doesn’t seem to match the PCB.

[amyk]

Besides the dropper capacitor, check the triac too.

Did you check capacitances? Just checking that they charge up is no guarantee that they're the right capacitance (nor ESR). Even cheap DMM with capacitance measuring function will be sufficient for former, but ESR meter will be necessary for latter.

[BertB]

Some other non-circuit board ideas:

Is there a button to turn the shredder on (overriding auto-mode)? Perhaps if there is such a button it has gone intermittent. It has happened that PC power buttons had PCs turn on at some random time. Perhaps temporarily disconnect such a button if it exists.

I trust there is nothing silly going on like dust dust or a thread dangling in front of the IR sensor. The angles of the sensors are in tact?

If you can, are you able run some small wires to the IR sensor to see when is happening with it? That way you can see if it being triggered or not.