Need help in fixing lexmark printer

[felto]

Hi all,
Can anyone help me in fixing my lexmark printer?
I have a multiprinter type (scanner, fax, print) and
the problem is in the main circuit board of the printer.
When I connect the power cord I can see a spark from the
connection but I get no power to the unit. The board does not
have any fuses where I can check. The only thing I can find is
a diode thats right next to the power connection of the board. It
gives me an open diode with my voltmeter. I preceeded to change
that diode with a new one, but the same thing happens to the diode. Something weird that I dont understand, when I unsolder the diode from the board it give me a .6V reading which is good, but when I solder it to the board it reads an overload, which is a bad diode. Anyone can explain this mystery to me and any help in diagnosing my printer. The board does not have any transformers to. It is part of the plug itself. For your information I have checked all of the other diodes that are soldered to the board and they all come out ok. Its only the one nest to the connector. thanks for your help.

 

[ante1]

Hi felto,

I like to help you but I need the schematic for the circuit board. The problem is probably in the power supply section but without some documentation I am lost here.

 

[felto]

I checked the power supply already with a voltmeter and
it checks out ok. I get 30 Volt output which is what it calls
for. Something on the board itself burnt out. I think it may be
the voltage regulator on the board. Lets hope that is it. It a
7812CT version made by Motorola. I just bought a new one from
Radio Shack and am going to solder the new one in tonight. I think it is bad because it gives me an open circuit alert on my voltmeter, which is capable of checking diodes also. Anyway do you know exactly how to check the regulator with a voltmeter. It has three pins to it. One for the input, middle for ground and the third pin is the output. Thanks for your help

 

[ante1]

Well a 7812 is very easy to test, just put the neg lead to pin two and the pos to pin three. If you get 12.0V its ok if not its broken. To check the input voltage (which have to be present) just move the positive lead to pin one, here should be at least 15V and maximum 35V for the regulator to work.

 

[MP1]

Felto,
You asked this question under the opto sensor topic. Please also look there for my reply.

MP