22 May
1999
22 May
'99
8:57 a.m.
Finally, it occured to me that I had flipped the
chassis over to put
some new rubber feet on the bottom and wondered if I could have shifted
something, like a loose screw or nut. So I removed the motherboard and
looked underneath. LO and BEHOLD! A previous owner had replaced an
LS series chip and its socket and had not CUT OFF the excess leads on
the bottom of the board!!! When I shifted the machine, it pushed the
leads against the bottom of the chassis effectively grounding them!
Once those were clipped and a piece of anti-static foam inserted beneath
the motherboard, the machine now talks to the terminal FAR more reliably
than before.
Um... I thought that anti-static foam was conductive? Wouldn't that
be bad? Maybe I'm wrong.
Bill Sudbrink
22 May
22 May
9:06 a.m.
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9:49 a.m.
> > Once those were clipped and a piece of
anti-static foam inserted beneath
> > the motherboard, the machine now talks to the terminal FAR more reliably
> > than before.
Anti-static foam _is_ slightly conductive, but the
resistance would be
too high to cause problems with TTL (or LS TTL) signals. Had this been
CMOS logic (much higher input impedance, so it's possible to drive the
inputs from high impedance sources), I would have been more worried.
Although I do wonder why you didn't simply cut off the excess leads
yourself...
DOH! =-D Actually, I thought about that. I was going to put some
plastic underneath but it tends to slide around unless you tape it
down. I could put tape underneath but that gets icky after a couple
years. I remembered an Apple II mb a friend had mounted in a PC
case, which didn't fit the mounting screws, so he used some anti-static
foam underneath to support it and it worked fine. I've seen it used
and used it this way all over the place so I guess it never occured to
me that it could cause a problem.
It certainly hasn't. If anything the machine is more stable now.
There's just something about a board full of chips with their leads
an 1/8th inch away from an aluminum ground plane that bothers me.
At any rate, I DID clip the excess leads but since the angles make
it about impossible to see UNDER the motherboard when it's mounted in
the chassis, I decided not to take any chances. Plus the back half
of a N* motherboard is NOT well supported. The center tends to sag
I bit I notice and I also wanted it supported so I wouldn't get
intermittants due to cracks in the aging PCB traces.
Note: BTW, I found a reference in the N* newsletters I have to the A2
16k ram board being susceptible to noise on the bus. The A3 board,
which was available also as an upgrade to the A2, has alot better noise
immunity.
Anthony Clifton
10:02 a.m.
If you _really_ wanted to check the distance from the leads to the plate, just
put a piece of clay under it and assemble it, then unassemble and look at the
clay.
Anthony Clifton - Wirehead wrote:
> > Once
those were clipped and a piece of anti-static foam inserted beneath
> > the motherboard, the machine now talks to the terminal FAR more reliably
> > than before.
Anti-static foam _is_ slightly conductive, but the
resistance would be
too high to cause problems with TTL (or LS TTL) signals. Had this been
CMOS logic (much higher input impedance, so it's possible to drive the
inputs from high impedance sources), I would have been more worried.
Although I do wonder why you didn't simply cut off the excess leads
yourself...
DOH! =-D Actually, I thought about that. I was going to put some
plastic underneath but it tends to slide around unless you tape it
down. I could put tape underneath but that gets icky after a couple
years. I remembered an Apple II mb a friend had mounted in a PC
case, which didn't fit the mounting screws, so he used some anti-static
foam underneath to support it and it worked fine. I've seen it used
and used it this way all over the place so I guess it never occured to
me that it could cause a problem.
It certainly hasn't. If anything the machine is more stable now.
There's just something about a board full of chips with their leads
an 1/8th inch away from an aluminum ground plane that bothers me.
At any rate, I DID clip the excess leads but since the angles make
it about impossible to see UNDER the motherboard when it's mounted in
the chassis, I decided not to take any chances. Plus the back half
of a N* motherboard is NOT well supported. The center tends to sag
I bit I notice and I also wanted it supported so I wouldn't get
intermittants due to cracks in the aging PCB traces.
Note: BTW, I found a reference in the N* newsletters I have to the A2
16k ram board being susceptible to noise on the bus. The A3 board,
which was available also as an upgrade to the A2, has alot better noise
immunity.
Anthony Clifton 
12:46 p.m.
Check the quality of the decoupling capacitors on that board. A lot
of noise can be caused by low grade decoupling caps. If in doubt replace
them with speedy MKM-type 100n caps.
Putting the board in a slot near one of the terminators may also be
helpfull.
Does this machine has active termination on the bus?
Sipke de Wal
Anthony Clifton - Wirehead wrote:
Note: BTW, I found a reference in the N* newsletters I have to the A2
16k ram board being susceptible to noise on the bus. The A3 board,
which was available also as an upgrade to the A2, has alot better noise
immunity.
Anthony Clifton
9:15 a.m.
Finally, it occured to me that I had flipped the
chassis over to put
some new rubber feet on the bottom and wondered if I could have shifted
something, like a loose screw or nut. So I removed the motherboard and
looked underneath. LO and BEHOLD! A previous owner had replaced an
LS series chip and its socket and had not CUT OFF the excess leads on
the bottom of the board!!! When I shifted the machine, it pushed the
leads against the bottom of the chassis effectively grounding them!
Once those were clipped and a piece of anti-static foam inserted beneath
the motherboard, the machine now talks to the terminal FAR more reliably
than before.
Um... I thought that anti-static foam was conductive? Wouldn't that
be bad? Maybe I'm wrong.
Bill Sudbrink
10:10 a.m.
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12:52 p.m.
Allways check the foam you intend to use. Not every brand has the same
specs. An ohmmeter reading of more than 1 meg accross an inch will be
Ok.
For fixing board sagging problems I love those rubber pencil ereasers.
You can cut them up in any shape you like and fix them with a dab of
epoxy
so they won't come loose.
Sipke de Wal
Tony Duell wrote:
> Um... I thought that anti-static foam was
conductive? Wouldn't that
> be bad? Maybe I'm wrong.
[...]
yeah, I thought anti-static foam was made from
carbon specifically so it would
conduct static charges away.
I've just taken a piece of such foam and stuck my ohmmeter probes in it,
about 0.1" apart. The resistance was about 5-10M. I don't think there's
much chance of that shorting out a TTL signal.
-tony
9347
days inactive
9347
days old
7 comments
6 participants
participants (6)
-
ard＠p850ug1.demon.co.uk -
bill＠chipware.com -
bluoval＠mindspring.com -
jim＠calico.litterbox.com -
sipke＠wxs.nl -
wirehead＠retrocomputing.com