LiteOn lit on fire in HP Scanjet 5

[clearchris]

Re: LiteOn lit on fire in HP Scanjet 5

Pin 4 needs a min. of 2 volts to turn on the regulator, if the opto has 5 volts across the led, the output pins 3&4 should be low connecting pin 1 to pin 4 via the 2.2k resistor.
you could short pins 3&4 of the opto and see if the -12 is at the output of the reg.

That's much easier than replacing the part to test. It worked! Now to replace the part.

Thanks!!

[clearchris]

Re: LiteOn lit on fire in HP Scanjet 5

Ok, I'm clearly missing something here. So I'm going back to basics.

1. If I bridge emitter and collector of the optocoupler, I get -12v at the outputs.
2. I have replaced the optocoupler (LTV-817) twice with PC123Y22FZ0F. I even checked the provenance of these pieces, I bought them from digikey two years ago, so they shouldn't be fakes.
3. I have replaced the regulator since I zapped it. Woops.
4. No change in behavior.
5. I rechecked the voltage at the anode and cathode of the optocoupler against ground. I get 5v at the anode and 10v at the cathode.

What should I expect to see at the cathode? I expected to see 5v or maybe a little less because the cathode gets attached to ground and maybe some voltage drop across the led, but strangely, I see 10v. What am I missing here?

Really have to be careful in troubleshooting, I measured 5v earlier across the anode and cathode because 5 - 0 = 5 (what I assumed I measured) and also 10 - 5 = 5 (what I actually measured). Sneaky broken power supply.

[R_J]

Re: LiteOn lit on fire in HP Scanjet 5

I'm not sure what that 702p transistor is, (2n7001 mosfet?) but when the regulator turns on it will supply -12 volts to the (Source) the fault might be somewhere before the 000 and then 181 res. or maybe that line 513 res. that goes to the (gate)

It may need power good signal to turn it on? The PG orange wire looks like it does find its way to the gate via a couple jumper wires (jp46?)
Have you tried connecting this power supply to the scanjet to see if it does work? It may be working just fine.

[clearchris]

Re: LiteOn lit on fire in HP Scanjet 5

You were right again. It did need power good to turn it on. I traced it back to the chip that does provide power good and found that the supply was only getting 3.6v when the chip requires 5-35v. That was caused by a resistor that was supposed to drop 40v to something in the range, but was dropping the voltage too much. The 330 ohm resistor must have been damaged, I replaced it and now it's working.

Thanks!