7 Dec
2014
7 Dec
'14
5:38 a.m.
PS: When buying replacement chips, since many are NOS, which manufacturers
should be avoided, and which are good? I recall there was some discussion on
here a while back about the topic of which vendors seemed to have the most
problems with chips failing, but I can't find it (and a bit of Google
searching couldn't turn up anything on the topic - all my searches turned up
too much other stuff).
And should I always install the replacements in sockets, or is it OK to just
go ahead and solder them straight in? (The socket obviously doesn't cost
much, and I'm less likely to damage the chip installing it like that, and of
course if I get it in and it's U/S, it's easy to swap out from a socket, but
I'm wondering if the use of a socket has any downside, electrically.)
Again, thanks for any advice (and I won't be surprised if the last starts
the usual debate... :-)
Noel
7 Dec
7 Dec
7:32 a.m.
Something like a 74LS373, that's still in production and commonly
available; you can get it from Jameco, Digi-Key, Mouser, etc. I don't think
there's really any difference in quality between manufacturers these
days... if you go to Jameco the manufacturer is often "TBD" anyway, LOL.
I would replace all with "LS" parts, don't bother trying to find an exact
replacement for that 74S part.
Some people really like to put a socket in when they replace an IC. I
suppose people figure if it failed once it could fail again, and of course
you want to limit the amount soldering/desoldering so you don't damage the
board... I think it's really personal preference; I think the MTBF on
whatever IC you've replaced is going to be pretty high... if you do go
ahead and put a socket in there, be sure to check vertical clearance and
also I think it's worth spending the extra pennies for a nice turned-pin
socket... much nicer than the stamped-pin sockets and they'll hold up
better over the long run.
Best,
Sean
On Sun, Dec 7, 2014 at 8:38 AM, Noel Chiappa <jnc at mercury.lcs.mit.edu>
wrote:
PS: When buying replacement chips, since many are NOS,
which manufacturers
should be avoided, and which are good? I recall there was some discussion
on
here a while back about the topic of which vendors seemed to have the most
problems with chips failing, but I can't find it (and a bit of Google
searching couldn't turn up anything on the topic - all my searches turned
up
too much other stuff).
And should I always install the replacements in sockets, or is it OK to
just
go ahead and solder them straight in? (The socket obviously doesn't cost
much, and I'm less likely to damage the chip installing it like that, and
of
course if I get it in and it's U/S, it's easy to swap out from a socket,
but
I'm wondering if the use of a socket has any downside, electrically.)
Again, thanks for any advice (and I won't be surprised if the last starts
the usual debate... :-)
Noel
9:16 a.m.
On 12/07/2014 09:32 AM, Sean Caron wrote:
Something like a 74LS373, that's still in
production and commonly
available; you can get it from Jameco, Digi-Key, Mouser, etc. I don't think
there's really any difference in quality between manufacturers these
days... if you go to Jameco the manufacturer is often "TBD" anyway, LOL.
I would replace all with "LS" parts, don't bother trying to find an exact
replacement for that 74S part.
If they use a 74S part there, there is probably a timing
reason, S is at least
5 ns faster than LS, sometimes even 10 ns.
Jon
11:44 a.m.
And should I always install the replacements in
sockets, or is it OK to just
go ahead and solder them straight in? (The socket obviously doesn't cost
much, and I'm less likely to damage the chip installing it like that, and of
course if I get it in and it's U/S, it's easy to swap out from a socket, but
I'm wondering if the use of a socket has any downside, electrically.)
Sockets have basically 5 problems :
1) Extra stray capacitance between the IC pins. This is the normal reason for not using a
socket in high-
speed circuitry.
2) Extra inductance of the connection to each pin. This can affect certain ICs which need
external decoupling
(e..g for a clock multiplier PLL) as close to the pin as possible
3) Extra thermal resistance. This is a reason for not putting some power devices in
sockets
4) Extra height above the board. In your case Q-bus is tightly spaced anyway, so check
there is enough
space for the socket you are using.
5) Reduced reliability. My experience is that formed-pin (cheap) sockets are a pain.
Turned pin (machined
pin, whatever) are fine. I have never had a bad contact on the latter. Yes, if you are
doing military or medical
work it will matter but for classic computer systems I don't think that a turned pin
socket will degrade
reliability at all.
Personally, if there are no problems due to the above I solder common TTL parts and the
like in directly.
I socket anything expensive, anything hard to find, or anything complicated. And of course
a programmed
device (ROM, PAL, etc) gets socketed if at all possible.
In yuo case I'd socket the Q-bus buffer chip, but not the TTL latches.
-tony
9:41 p.m.
When I was at Living Computer Museum, we were using a small pin that had a
socket (hole) in it. Solder N of those into the board after removing an
IC, and you'd never have to desolder another one - an important benefit
considering that some of these boards are losing integrity in the bonding
of copper to epoxy (or never had such integrity, e.g., Data General).
They're low profile (maybe a fraction of a millimeter higher than a
directly soldered-in chip), which can be crucial in some of the crowded
backplanes we see. ISTR that Mouser sells them. -- Ian
On Sun, Dec 7, 2014 at 11:44 AM, tony duell <ard at p850ug1.demon.co.uk> wrote:
--
Ian S. King, MSIS, MSCS
Ph.D. Candidate
The Information School
University of Washington
An optimist sees a glass half full. A pessimist sees it half empty. An
engineer sees it twice as large as it needs to be.
And should I
always install the replacements in sockets, or is it OK to
just
go ahead and solder them straight in? (The socket
obviously doesn't cost
much, and I'm less likely to damage the chip installing it like that,
and of
course if I get it in and it's U/S, it's
easy to swap out from a socket,
but
I'm wondering if the use of a socket has any
downside, electrically.)
Sockets have basically 5 problems :
1) Extra stray capacitance between the IC pins. This is the normal reason
for not using a socket in high-
speed circuitry.
2) Extra inductance of the connection to each pin. This can affect certain
ICs which need external decoupling
(e..g for a clock multiplier PLL) as close to the pin as possible
3) Extra thermal resistance. This is a reason for not putting some power
devices in sockets
4) Extra height above the board. In your case Q-bus is tightly spaced
anyway, so check there is enough
space for the socket you are using.
5) Reduced reliability. My experience is that formed-pin (cheap) sockets
are a pain. Turned pin (machined
pin, whatever) are fine. I have never had a bad contact on the latter.
Yes, if you are doing military or medical
work it will matter but for classic computer systems I don't think that a
turned pin socket will degrade
reliability at all.
Personally, if there are no problems due to the above I solder common TTL
parts and the like in directly.
I socket anything expensive, anything hard to find, or anything
complicated. And of course a programmed
device (ROM, PAL, etc) gets socketed if at all possible.
In yuo case I'd socket the Q-bus buffer chip, but not the TTL latches.
-tony
9:54 p.m.
When I was at Living Computer Museum, we were using a
small pin that had a
socket (hole) in it. Solder N of those into the board after removing an
IC, and you'd never have to desolder another one - an important benefit
considering that some of these boards are losing integrity in the bonding
of copper to epoxy (or never had such integrity, e.g., Data General).
They're low profile (maybe a fraction of a millimeter higher than a
directly soldered-in chip), which can be crucial in some of the crowded
backplanes we see. ISTR that Mouser sells them. -- Ian
I've used them (Farnell -- so I guess Newark -- sell the Harwin brand) but they
obviously need a larger hole in the PCB than an IC pin on its own will go into.
And in a lot of cases the hole in the PCB is not large enough for these sockets
(obviously you can't drill out holes in a plated-through-hole PCB)
I must admit I was pleasantly surprised when working on an HP11305 disk
controller. That machine is tight inside. I removed the microcode ROMs to
dump them and realised that if I put 2 of them in sockets they would foul
the mains transformer (the other 3 were OK). Only after I had desoldered them
did I realise that the holes for those 2 were large enough for these little pin sockets
(I can't remember about the other 3, but I used normal sockets there anyway). So
one order later and all my ROMS were socketed. It appears that the 'old HP'
thought that people might want to socket their ROMs...
-tony
3666
days inactive
3667
days old
5 comments
5 participants
participants (5)
-
ard＠p850ug1.demon.co.uk -
elson＠pico-systems.com -
isking＠uw.edu -
jnc＠mercury.lcs.mit.edu -
scaron＠umich.edu