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? Voltage Regulator Output Pin Connection

A

Alec S.

Jan 1, 1970
0
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: http://img517.imageshack.us/img517/9646/23249045py3.jpg
Back: http://img524.imageshack.us/img524/84/13459958xc0.jpg
Reverse: http://img524.imageshack.us/img524/7761/30152421ct5.jpg

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.
 
A

Arfa Daily

Jan 1, 1970
0
Alec S. said:
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: http://img517.imageshack.us/img517/9646/23249045py3.jpg
Back: http://img524.imageshack.us/img524/84/13459958xc0.jpg
Reverse: http://img524.imageshack.us/img524/7761/30152421ct5.jpg

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.

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
 
A

Alec S.

Jan 1, 1970
0
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?


--
Alec S.
news/alec->synetech/cjb/net
 
A

Arfa Daily

Jan 1, 1970
0
Alec S. said:
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?
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
 
F

Franc Zabkar

Jan 1, 1970
0
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.
... somehow I put in a 7905 instead of the 7805 ...

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?

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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