Dave wrote in message
netamerica...
A friend of mine, who is paralyzed below the neck and confined to a
wheelchair or bed, has a Doorbell-Phone intercom system with which he
communicates to whoever is ringing his doorbell before activating the
automatic door to let them in. Problem is, it apparently got zapped
during
a series of storms some time back, and no longer works as an intercom. I
found a blown cap and replaced that as well as the transistor that was
driving said electrolytic cap, but it still fails to function in intercom
mode and overdrives one power resistor to the point of darkening the PC
board in that spot. Entire board seems to be built around one 18-pin DIP
chip, which I suspect is a PLL. Problem is the surface of the chip has
been
planed, removing any trace of whatever identification numbers etc were
once
there. Calls to Doorbell-Phone Inc. tell me that this chip is
propriatary,
along with any and all knowledge of how the circuit works. Replacement
would run upwards of $200.00. Repair services are not available. So I,
in
my innocent ignorance, am trying to help this guy by offering my own
half-vast espertise (weak grin) as an electronics troubleshooter.
Two 18-pin DIP PLLs come to mind: the NTE7108 and the Motorolla MC145155P.
Can anyone offer any suggestions as to how to tell if this thing even *is*
a
non-working PLL? The two chips I mention above have distinctive pin-outs,
with one pin being ground in one case, and another pin being clock in the
other case. I am thinking I can check for these conditions on the
appropriate pins, but am wondering what else I can do to determing what it
is I am dealing with and want to replace. Anyone have any ideas on how
one
should proceed? I am open to all suggestions/advice. Just trying to help
this friend get his intercom back in working order... If pics would hep,
I
can try to get same but it is likely to be a while, with the holidays and
all.
Any help is greatly appreciated.
Dave
Of course , if it was proprietary then there is little point in removing the
lettering.
Start by identifying rail pins, oscillator or clock pin .
Why a PLL , is it mains bourne carrier ?
--
Diverse Devices, Southampton, England
electronic hints and repair briefs , schematics/manuals list on
http://home.graffiti.net/diverse:graffiti.net/