"Challenge" Electric Stapler

[waveydavey]

Hello,

I have an electric stapler that has stopped working. It's a Challenge Xtreme brand (UK cheapie)

When I pull the trigger it is completely dead. I've traced the 230V which is present on the board so it is not a fuse/wiring issue.

I know these are often simple devices (basically a solenoid), but this one has some sort of anti-latching circuit, and I'd like to keep it standard instead of modifying/bypassing it completely.

I think there is a component that switches the solenoid on/off as part of the anti-latching circuit, and may have failed. When desoldered and connected to a transistor tester, it is reporting it as an NPN transistor, but reading a Vbe/hFE of zero



My issue is that there is no model number or identifier whatsoever on the component, so I can't just buy a replacement to see if it fixes it.

The tool is rated at 720 watts on the label, and the voltage across the solenoid will be 230V.

Does this really look like an NPN transistor? Is it likely that it is switching 700+ watts? Maybe silly questions, but I'm a novice.

Thanks for any help!

[CapLeaker]

Clean that transistor up with 99% IPA and look at it under the microscope. If that doesn't work try Acetone. The transistor isn't ground off, but there seems to be a coating of something or other on it. That must come off and then hopefully there is a very faint print on it.

[waveydavey]

Thanks for the response.

I've cleaned it with IPA and put it under the microscope, but it looks completely blank, like it was never laser etched.

I'll try with some acetone tomorrow, too, and report back.

[CapLeaker]

Maybe look at it when wet?

[CapLeaker]

Can we see the whole board, so we can see the circuit? I think it is something else, not a transistor

[waveydavey]

The black wire at the bottom is live (230V), the blue neutral.

The component I've removed is marked BT, which I guessed to be "Bipolar Transistor".

There is a 47ohm resistor at the top of the board which is out of frame.

Thanks!

[stj]

it's probably an SCR or triac - some of those have a "BT" prefixed part number.
component testers hate triacs - they dont have a high enough voltage to test them.

remove and test the film capacitors - i think it uses a capacitive dropper circuit - those kill film caps.

[CapLeaker]

It looks to me there is supposed to be an SCR in there, not a transistor. Just like stj already mentioned. Measure it with a regular DMM in diode mode, not with a component tester.

[harp]

My bet is that is specialized chip ic with integrated triac inside and pwm controller of constant current or so, what is setup via external resistor of 47 ohms... I see simillar chip on some led lights, and messing arround with settup resistor can control power and heat disipation, so, longer life...

[redwire]

I think it's just a triac, PCB label is "BT" likely a BT136 or BT137.
Test it the old fashioned way - use a multimeter on Diode-Test and then on Ohms.

Many triacs have a built-in gate resistor so a low ohms reading between Gate-MT1 is normal for high-commutation triacs to get their better dV/dt rating. It's not mentioned in datasheets.
Around 150-250Ω or so, some people read 400Ω but these are no-name triacs from the gutters of guangdong. If you read that resistance, the part might be OK.
Check C5 is not shorted and 12VDC power is across the zener. I think this is just a one-shot timer for the solenoid, made up of a couple transistors and maybe an IC? There's no snubber for the triac so it does see stress.

[harp]

Something like this...

[R_J]

Is this a J-123? if it is, have you checked the anti firing safety switch?

[waveydavey]

remove and test the film capacitors - i think it uses a capacitive dropper circuit - those kill film caps.

I've pulled both. One is marked 474J and measures 458nF and the other is marked 103J which measures 10.15nF, which I believe is correct based on the 3 digit capacitor marking?

I think it's just a triac, PCB label is "BT" likely a BT136 or BT137.
Test it the old fashioned way - use a multimeter on Diode-Test and then on Ohms.

Many triacs have a built-in gate resistor so a low ohms reading between Gate-MT1 is normal for high-commutation triacs to get their better dV/dt rating. It's not mentioned in datasheets.

Around 150-250Ω or so, some people read 400Ω but these are no-name triacs from the gutters of guangdong. If you read that resistance, the part might be OK.
Check C5 is not shorted and 12VDC power is across the zener. I think this is just a one-shot timer for the solenoid, made up of a couple transistors and maybe an IC? There's no snubber for the triac so it does see stress.

I think you are right, it is a triac! I've never tested these before, so it took a while for me to test it properly..

I measured no voltage drop in diode mode on the multimeter between pins 1 and 2 (both polarity), but around 200Ω resistance (as you said) between pin 1 and pin 3 (gate). I used my current limited power supply to load pin1 and pin3 (gate) with around 35mA at low voltage and diode mode on the multimeter then shows a 0.7v voltage drop between pin1/pin2.

Both C4/C5 are fine, the three transistors are all NPN BJT and measure fine too

I will check the Zener.

Is this a J-123? if it is, have you checked the anti firing safety switch?

The label shows J-105, and the PCB is marked DW J-105.

The anti firing safety switch on this model is just a mechanical interlock. You can't physically pull the trigger unless you depress the metal guard. With the case open, and the mechanism removed, the anti-firing is bypassed completely.

[waveydavey]

Check C5 is not shorted and 12VDC power is across the zener.

With the Stapler powered on and at rest, the multimeter shows 5.1VDC with probes at each end of the Zener. I've pulled the Zener and it is correctly recognized as a 12V zener on the multifunction tester..

[CapLeaker]

I don’t see an electrical reason then why this thing doesn’t fire. Maybe an issue with the trigger or some safety crap?
I mean it’s got a few transistors, some diodes, a triac, some caps and that’s about it.

[stj]

did you meter the solenoid coil?

[redwire]

What are the markings on the TO-92 semi's on the board.

[waveydavey]

Great news, it lives!

I decided to call it quits - reassembled it with the intention of throwing it back In the garage.

The last thing I soldered to the board was the solenoid connections. I gave it one last test, and it works flawlessly.

My only conclusion is that the solenoid solder joints (or one of the other components I've removed) were cracked.
The board was originally coated in anti-vibration goo, and when this thing activates, it is pretty violent. I can see why it could crack joints over time.

Things I have learned:

(1) Always start with the basics (The first things checked were voltage presence on the board, the switch, and the solenoid resistance, but I should have checked solder joints under the microscope - the anti-vibration goo was a clue).

(2) That a triac will not often be detected on a cheap multifunction tester, that they often have an internal resistor between MT1 and Gate, and how to test them.

(3) That Badcaps rocks! Thanks for your patience and all of the help, guys!

[stj]

dont forget to remove the safety interlock - that bit is boring

[CapLeaker]

dont forget to remove the safety interlock - that bit is boring

Muhaha! Congrats!