Canon i850 PSU Trouble

[Mad Capper]

I have ruined an otherwise good, I believe, PSU as my hand eye coordination isn't what it used to be.

The story so far ...

What we have here is a PSU from a Canon i850 Printer. The printer did not turn on and so I went to check the power supply output voltages thinking it could be there rather than the logic board, which I really wish I knew how and could fix, but everything checked out with the proper voltages. I looked at the logic board and have no clue if it could be fixed or how. (As an aside is there any place out there that assists with fixing those sorts of boards? )

Anyway I thought I would double check the PSU and while doing so the probes slipped and I guess I shorted something as now there is no 5V output but I do get 24v and 27v.

Contacts from output connector from top to bottom are as labeled on case +5V GND +24V GND +27V GND CONT.

I don't know what CONT is or if it matters. Thinking back to what happened I can only think that I connected the 5V and GND but cannot be certain except that there is no 5V out anymore.

So if there is any tests or info that could lead me hopefully to a bad component that could be replaced to resurrect the PSU I would appreciate it, you all are so helpful. If I can give any info on the board that can't be seen in the photo please let me know.

Thanks

[ben7]

Re: Canon i850 PSU Trouble

Check F201 and the big diode on the secondary right next to the secondary capacitors (cant see the diodes #)

[Mad Capper]

Re: Canon i850 PSU Trouble

Thanks ben7

Please bear with me. So I tested the fuse F201 and there is continuity, so that means it is still intact, correct? I don't need to remove a fuse to test it do I?

I believe you are referring to the diode to the left of the empty capacitor spots labeled C303 and C302, correct? The D101 label is directly underneath.

I lifted one leg of the diode. With my meter on the diode setting and the black lead of meter to cathode (striped end) and red lead on anode, shows around 240. Leads reversed shows nothing. Am I correct that the means the diode is good?

Any suggestions as to where to from here?

Thanks

[ben7]

Re: Canon i850 PSU Trouble

Thanks ben7

Please bear with me. So I tested the fuse F201 and there is continuity, so that means it is still intact, correct? I don't need to remove a fuse to test it do I?

I believe you are referring to the diode to the left of the empty capacitor spots labeled C303 and C302, correct? The D101 label is directly underneath.

I lifted one leg of the diode. With my meter on the diode setting and the black lead of meter to cathode (striped end) and red lead on anode, shows around 240. Leads reversed shows nothing. Am I correct that the means the diode is good?

Any suggestions as to where to from here?

Thanks

Yes, both the diode and the fuse are good.

Try checking D201, and read the part number off the transistor-like thing circled in red (not D201).

[Mad Capper]

Re: Canon i850 PSU Trouble

D201 performs just like the other one so it is good too. The part in question is labeled on the board IC201 and on it is says TA76 431AS 2 F.

Seems to google as an Adjustable precision shunt regulator. I think this is the datasheet: . Do I need to take this out to test and how would I test it?

Thanks again

[ben7]

Re: Canon i850 PSU Trouble

D201 performs just like the other one so it is good too. The part in question is labeled on the board IC201 and on it is says TA76 431AS 2 F.

Seems to google as an Adjustable precision shunt regulator. I think this is the datasheet: . Do I need to take this out to test and how would I test it?

Thanks again

I am thinking that the 5v is derived from either the 24v or the 27v rail, using a buck converter.

Check Q201, it probably will be some type of mosfet.

The adjustable regulator should be fine, its likely used as a reference.

[Mad Capper]

Re: Canon i850 PSU Trouble

Q201 is a J378 2 E

I can't seem to find what it is exactly, perhaps a zener diode?

[ben7]

Re: Canon i850 PSU Trouble

Q201 is a J378 2 E

I can't seem to find what it is exactly, perhaps a zener diode?

It is a 2SJ378 P-channel MOSFET.

[Mad Capper]

Re: Canon i850 PSU Trouble

Is there anyway I can test this? If this is the component that is responsible for the 5v is there a way I can see whether it is outputting the 5v and depending, if the problem is that component or elsewhere in the circuit

[ben7]

Re: Canon i850 PSU Trouble

Is there anyway I can test this? If this is the component that is responsible for the 5v is there a way I can see whether it is outputting the 5v and depending, if the problem is that component or elsewhere in the circuit

This is the easiest test I can describe...
Basically connecting the mosfet up in a circuit to see if it will light an LED when the button is pressed. The 100K resistor proves that all the current wont flow through the gate (and through the switch) if the mosfet is shorted gate-to-source. If the mosfet is open circuit, it wont do anything; if the mosfet is shorted source-to-drain the LED will light even when the button isn't pressed.

[b700029]

Re: Canon i850 PSU Trouble

And be careful with MOSFETs, they're quite static-sensitive.

[Mad Capper]

Re: Canon i850 PSU Trouble

Well after looking through the parts box I found the requisite materials and built the circuit in the schematic and the mosfet checks out as good. I.e. when connect to 9v LED is off. When the switch is pressed the LED is on.

b700029
thanks for heads up

I hope there is more that I can try, as everything so far has tested as good.

I really appreciate the help.

[budm]

Re: Canon i850 PSU Trouble

Check and see if you do have 27V feeding the MOSFET, otherwise check those components in red circles.

[ben7]

Re: Canon i850 PSU Trouble

Check and see if you do have 27V feeding the MOSFET, otherwise check those components in red circles.

The 27v rectifier does measure ok, so the 27v rail must be working properly, must be something in the bucking circuit that is broken.

[Mad Capper]

Re: Canon i850 PSU Trouble

Here goes.

For the Rectifier
24v on the side to the top of the photo
27v on the side to the bottom

27v shows on the right side of R207, I think that is where I am supposed to check

Cap for 27v
27v on left
0 on right

The cap you have circled below it
24v on left
0 on right

For the inductor
From GND on output connector to either leg is 0
It has continuity between both legs, not sure if it supposed to.

For the MOSFET
G and S show 27v
D 0

For the two diodes, lifted one leg and they check out ok.
D104
27v on cathode
0 on anode

D204
27v on both sides

for Q202, says C1741A on it, not sure how to test this independently,but
27v on all legs

Fuse has continuity

C201
show 0 on both legs

D201
shows 0 on both legs but as stated in post 5 it checks ok too

Hope I got all that right

[budm]

Re: Canon i850 PSU Trouble

The MOSFET is getting the 27V OK but it is not switching on and off like it should, since you had already checked the MOSFET and it is tested OK, so there may be bad transistor that driving the Gate circuits is bad. I cannot trace it that well from the pictures.
It looks like the Gate is connected to the transistor to the right of the MOSFET, cannot see the designator on the picture, IC201?, which you had indicated earlier as 431? Which is the ref diode.


You can try replacing that, it is commonly used for regulating the switching power supply circuits.

[Mad Capper]

Re: Canon i850 PSU Trouble

I tried to get a better photo to be able to trace the runs and put a few labels on. Let me know if that helps or if I should put some more labels on.

On the IC201, TA76 431AS 2 F
I get
0.8v on cathode
0 on anode and Ref

Is there a way to test it?

Is the Fairchild TL431A the same or an acceptable sub for the TA76 431AS 2 F datasheet I referenced in post 5.

IF this is the faulty component, I happen to have two other 431's

a WS TL431AA 336D from a DVD player power supply. I think this is the datasheet. http://www.alldatasheet.com/datashee...S/TL431AA.html

Also have an AZ431 AZ-AE1 735D78 from a Digital Converter Box power board. Datasheet here I think
http://html.alldatasheet.com/html-pd...4/5/AZ431.html

Would either of these work or are they not to the correct specs?

[ben7]

Re: Canon i850 PSU Trouble

The MOSFET is getting the 27V OK but it is not switching on and off like it should, since you had already checked the MOSFET and it is tested OK, so there may be bad transistor that driving the Gate circuits is bad. I cannot trace it that well from the pictures.
It looks like the Gate is connected to the transistor to the right of the MOSFET, cannot see the designator on the picture, IC201?, which you had indicated earlier as 431? Which is the ref diode.


You can try replacing that, it is commonly used for regulating the switching power supply circuits.

No, the gate is driven by Q202 and Q203

[budm]

Re: Canon i850 PSU Trouble

OK, I seee them now in the new picture, I thought that Q202 was the IC201 (had to flip picture in my head). So he needs to chek those driver transistors then.

[Mad Capper]

Re: Canon i850 PSU Trouble

Test transistors, Ok.

the best data sheet i could find for the Q202, C1741A S QF is
http://www.alldatasheet.com/datashee...1741AS_11.html

according to that datasheet, if I got the correct one, it is ECB orientation starting with pin 1 as designated by circle on board. (I think that is what the circle would mean)

I think this is the correct testing methodology for a transistor

Meter set to diode
Red on - Black on
E - B nothing
B - E 747

C - B nothing
B - C 743

E - C nothing
C - E nothing

That means it is good, correct? Am I doing this right?

As for the surface mount one, Q203, I hesitate taking it off to test it as I am afraid my desoldering skills, or lack thereof, would put too much heat to it and destroy it. Also, as it has no marking I do not know how to identify which is B, C or E. Can it be tested while on the board?

As you can tell I don't know anything so I really appreciate everyone's help.