? Voltage Regulator Output Pin Connection

A

Alec S.

Guest
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.


--
Alec S.
news/alec->synetech/cjb/net
 
"Alec S." <nospam@127.0.0.1> wrote in message news:gbkdbm$v1l$1@aioe.org...
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.


--
Alec S.
news/alec->synetech/cjb/net
Click to expand...
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
 
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



"Arfa Daily" <arfa.daily@ntlworld.com> wrote in message
news:7pmDk.3726$Ax7.3035@newsfe14.ams2...
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.
Click to expand...
"Michael A. Terrell" <mike.terrell@earthlink.net> wrote in message
news:v_mdncmQhtRd8kPVnZ2dnUVZ_oHinZ2d@earthlink.com...
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.
Click to expand...
I wrote:
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.
Click to expand...
 
"Alec S." <nospam@127.0.0.1> wrote in message news:gboa8l$q3j$1@aioe.org...
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



"Arfa Daily" <arfa.daily@ntlworld.com> wrote in message
news:7pmDk.3726$Ax7.3035@newsfe14.ams2...
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.

"Michael A. Terrell" <mike.terrell@earthlink.net> wrote in message
news:v_mdncmQhtRd8kPVnZ2dnUVZ_oHinZ2d@earthlink.com...
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.

I wrote:
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.
Click to expand...
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.

--
Dave M
MasonDG44 at comcast dot net (Just substitute the appropriate characters in the
address)

Life is like a roll of toilet paper; the closer it gets to the end, the faster
it goes.
 
"Alec S." <nospam@127.0.0.1> wrote in message news:gboa8l$q3j$1@aioe.org...
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


Ah yes, I rember the original thread now, and the problem coming down to
Click to expand...
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
 
"Alec S." wrote:
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.
Click to expand...

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?


--
http://improve-usenet.org/index.html

aioe.org, Goggle Groups, and Web TV users must request to be white
listed, or I will not see your messages.

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There are two kinds of people on this earth:
The crazy, and the insane.
The first sign of insanity is denying that you're crazy.
 
On Sun, 28 Sep 2008 11:55:58 -0400, "Alec S." <nospam@127.0.0.1> put
finger to keyboard and composed:

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?
Click to expand...
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
--
Please remove one 'i' from my address when replying by email.
 
Franc Zabkar wrote:
On Sun, 28 Sep 2008 11:55:58 -0400, "Alec S." <nospam@127.0.0.1> put
finger to keyboard and composed:

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.
Click to expand...

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


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
--
Please remove one 'i' from my address when replying by email.
Click to expand...

--
http://improve-usenet.org/index.html

aioe.org, Goggle Groups, and Web TV users must request to be white
listed, or I will not see your messages.

If you have broadband, your ISP may have a NNTP news server included in
your account: http://www.usenettools.net/ISP.htm


There are two kinds of people on this earth:
The crazy, and the insane.
The first sign of insanity is denying that you're crazy.
 
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