c3850fs

[adecanmin]

Gentlemen.
I have a theory of operation question. I looked in the service manual could not find the answer. How are the 24 volts initialized? Is the signal generated from the mfpb to turn on the 24 volts in the power supply or does the prcb perform the action?

[allan]

Are you repairing a power supply?

[adecanmin]

Its a shop as well as a parts machine. It was working somewhat. I was reseating one of the ribbon cables to the scanner because I was getting a getting a scanner service code, it came loose and touched the mfpb board. Of course I had the machine powered on. It went dead. I have 5 volts but no 24. Not a big deal if I can't get to power up but I would like to know which board I took out. I'm thinking mfpb.

[allan]

The power switch is connected to DCPS then it looks like its gives 5V power to the MFPB thru pins 4 5 and 6 on CN5.
CN3 of the DCPS pin is Input Power Monitor and on pin 4 Sleep Remove that goes to CN2 on the PRCB.
CN4 on the PRCB goes to CN16 on the MFPB taking with it Sleep Remove but not Input Power Monitor.
3.3V gets converted on the MFPB and supplied to the PRCB and itself.
Check for 3.3V if its not there the MFPB could be dead.
The Sub Power Switch goes from the control panel to the MFPB.



My guess would be the PRCB controls the 24V but if 3.3V does not get supplied to the PRCB it would no work anyway. Think one of the chips would be shorting that out.

AC Power Off is monitored by the MFPB.

Sorry for the edit on the vodka and bored.
Check for 3.3V on the MFPB.

[adecanmin]

Thanks! Have you had to replace any scanners on these machines? Unfortunately it is an assembly and an expensive one. I had one that was giving the scanner lock code intermittently and the other one had a bad ribbon cable causing a code . I was able to take the ribbon cable off the one that was giving the scanner code so I was able to avoid having to replace two instead of just one. It is cheaper to buy a used machine with a good scanner then to replace it to tune of $860.

[allan]

Would that ribbon cable happen to be CN103 and not CN102 on the MFPB?
You could be lucky and just lightly roasted an ICP.

[adecanmin]

cn102

[allan]

There are no 3.3V pins on CN102 but CN103 does have.
Ok check for 3.3V. Put your tester to peak hold and have the probes connected at power up.
And check ICP's and other fuses.

[allan]

I have solder repaired the ribbon cables before to avoid replacing them.
find the crack in the ribbon or where its worn thru and scrape the insulation off with a blade and flow solder from the one end to the other.

[adecanmin]

Ok, I've read about the ICPs. I don't know where they are located on the circuit board. I've only found two fuses both on the power supply. Thank you for your help.