OKI 590 Dot Matrix Printer w bad power supply.

[Jluxster]

Got this OKI ML-590 dot Matrix printer DOA with a note on it saying "no power".

Sure enough. Does not power up. No lights, no startup sounds. I've ruled out the plug switch and fuses, no interlocks on the model that prevent power on. The PS is definitely getting power from the wall.

Removed the cover and had a good listen. No noise of any kind from the power supply, and no bulging caps (though perhaps one of you Jedi masters can spot one the photos are good resolution).

On closer examination I see some charring below a set of three light blue components on the top of one of the boards. There are also some charred looking resistors on this same part of the board.

Any thoughts on what's wrong with this board and wether swapping a few components would put me back in business?

Does the charring make this a lost cause?

I think that's my issue, but I'm not sure.

[lti]

Re: OKI 590 Dot Matrix Printer w bad power supply.

Those three blue components are also resistors.

The black stuff on the smaller resistors and the back of the board looks like electrolyte leaking out of the nearby caps. You will probably have to replace all of the brown Nichicon caps and clean the board. There could be other problems as well.

[stj]

Re: OKI 590 Dot Matrix Printer w bad power supply.

yea, caps have all leaked out onto nearby parts

[Jluxster]

Re: OKI 590 Dot Matrix Printer w bad power supply.

Hmmm.
Any need to replace the resistors is that is the case?

[Per Hansson]

Re: OKI 590 Dot Matrix Printer w bad power supply.

Yes, the corrosion looks so bad that the legs might be corroded off.
In any case if they are not now given a few years they eventually will...
Electrolyte is some really corrosive stuff...
P.S: The datecode of the caps shows they are 25 years old, so they don't owe anyone any favors anymore

[stj]

Re: OKI 590 Dot Matrix Printer w bad power supply.

meter them to know your answer.

[Jluxster]

Re: OKI 590 Dot Matrix Printer w bad power supply.

Ok. So what I'm hearing is that the caps on that part of the board are the issue, and that I just need to check the resistors with my meter. That makes sense.

In past PS repair, some times one blown cap meant many others blown downstream. Based on the part of the board this is, is that likely the case here?

I'm trying to talk myself into bothering with the repair.

[PeteS in CA]

Re: OKI 590 Dot Matrix Printer w bad power supply.

The date code on the "C339" (LM339 clone) is late 1995. The Nichicon PL series is the pre-RoHS version of the current PM series and was introduced in the late 1980s, IIRC. Another clue to its age is the Marcon brand input capacitors. Marcon was bought by Nippon Chemi-Con quite a while ago. The output capacitors should probably be replaced due to their age, and the PM series would be the correct equivalent.

Anyway, it uses a flyback topology, in which the primary of the "transformer" is actually a winding of a coupled inductor. The switch device is a MOSFET, 2SK1523 (Shindengen?). I see some very darkened PCB between the "transformer" and the MOSFET, so either some snubber components got very hot or there is a worse problem in the primary circuit. Check the input fuse, and if it is open, check to see if the MOSFET is shorted.

If the P/S has a PWM controller (instead of being self-oscillating), check to see if the start-up capacitor (often ~47uF, 35V or 50V or ...). is OK. This gives the PWM enough energy to start up, but once started the PWM is powered by a winding on the "transformer".

[Jluxster]

Re: OKI 590 Dot Matrix Printer w bad power supply.

Anyone want to throw me a bone and identify the startup capacitor on my board?

[Per Hansson]

Re: OKI 590 Dot Matrix Printer w bad power supply.

It is not possible to see it in your pictures.
Possibly it is IC101 that is the controller.

[sam_sam_sam]

Re: OKI 590 Dot Matrix Printer w bad power supply.

One other clue it would probably be a capacitor is less than 200uf
But I would recommend replacing all the capacitors if I were doing this repair

You would have to look up the ic chip part numbers to find a chip that is either a switching power supply regulator or a PWM controller of some kind