? Voltage Regulator Output Pin Connection

Hi,

I'm repairing my flatbed scanner but I can't figure out what the output pin of the 7905 connects to.

Here's some snaps:

Front:

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Back:
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Reverse:
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I can see where the input voltage and ground pins lead, but the output pin doesn 't seem to lead anywhere. Is it a layered PCB or am I missing something?

It makes sense that the output voltage isn't connected because the scanner does nothing when plugged in; the power LED doesn't even light.

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Alec S.
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Alec S.
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That would be a 7 *8* 05 then ... Just a comment as the pinning is different between a 78 and a 79. The board has a t least two layers as evidenced by the nearby vias. Is there actually any voltage appearing on the output pin ? If not, is there voltage on the input pin. If there is, is the device getting hot ? There should be an output decoupling cap nearby - probably around 1 to 10 uF - with possibly a 0.1uF across it. You should be able to follow the print from those. It should be easy enough to 'pick up the trail' from the 78's output pin with your multimeter set to ohms, anyway.

Arfa

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Arfa Daily

doesn

As Arfa said, it is a 7805, and the silk screen clearly shows 'I'nput, 'G'round, and 'O'utput. in the first photo. Use a digial ohmmeter and connect one lead to the output terminal, and probe the board to see where it goes.

What are the other IC part numbers? Have you looked for datasheets?

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Reply to
Michael A. Terrell

Thanks for the info guys. There?s good news and bad news.

The good news is that I now know what the problem is, and if I?m lucky (which unfortunately history has shown otherwise), it may be easily fixable.

The bad news is that you guys were both right and wrong about it being a 7805, not a 7905. You?re right because that is indeed what is supposed to be there. You?re wrong because that?s not what?s currently there. When I posted this a month ago, I was advised to remove the three suspect components (the voltage regulator, a power resistor, and a cap near the bottom of the board) to find what was causing the short. I did so and found the short (it was B52 between the power resistor and regulator). I removed B52, which resolved the short, but when I was putting the three components back, somehow I put in a 7905 instead of the

7805 (I?m still looking for the 7805 right now).

I guess it makes sense that it?s not working now. :) I don?t suppose that I can expect it to work when I put the correct one back can I? Would having a 7905 in there (while on) have killed the board?

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Alec S.
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Alec S.

Ah yes, I rember the original thread now, and the problem coming down to that component, and why its descriptor should have been a "B". I think we eventually decided that it was probably a cap, and the "B" was for " B - ypass".

It is highly unlikely that the board will have suffered any damage, although the 7905 may have. Usually, if you hit the input of one of these monolithic regulator ICs with the wrong polarity for its type, it just sighs gently, and ignores you, so get the right one back in there, and fingers crossed ... :-)

Arfa

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Arfa Daily

Unable to determine the extent of the damage to the rest of the board by the incorrect substitution of a 7805 (positive) regulator with a 7905 (negative) regulator. You absolutely can't expect the board to work properly with such an obviously wrong part. Your next step is to replace the regulator with the proper type, then test the board for the correct regulated voltage from the 7805 regulator. If you get anything other than 5.0 (+/- 5%) volts, you still have trouble.

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Dave M
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DaveM

On Sun, 28 Sep 2008 11:55:58 -0400, "Alec S." put finger to keyboard and composed:

The pinout of a 7805 is I G O. The pinout of a 7905 is G I O.

IIRC, the unregulated input voltage was +12VDC. This would mean that the 7805's IGO pins would have had voltages of +12V/0V/+5V.

The 7905 would then have seen -12V on its input, which is correct, and this would then mean that its output pin would have been sitting at 5V below its ground pin, ie +7V. AISI, this may have done irreparable damage to your +5V logic. The 7905 is probably quite happy, though. :-(

BTW, when looking for a short, you don't always need to remove the entire part, you only need to isolate it by desoldering whichever pin is connected to the short.

- Franc Zabkar

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Franc Zabkar

can

in

Which is how the board is silk screened in his photo..

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Michael A. Terrell

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