6 Apr
2009
6 Apr
'09
5:07 p.m.
To keep you posted :
I made a error by stating the -12V was wrong, it was the +12V that's why I
replaced the uA723. (mea culpa etc....)
The TMS4060's don't have a -12V connection they get +5,+12,0,-5V.
The board is working again, the VCC pin of lowerbit 9 had a resistance of
12.5 ohm to the VCC-line.
The A10 pin of the same chip didn't connect to anything.
A little bypassing did the job, so thanks for your advice.
-Rik
-----Oorspronkelijk bericht-----
Van: cctalk-bounces at classiccmp.org
[mailto:cctalk-bounces at classiccmp.org] Namens Tony Duell
Verzonden: zondag 5 april 2009 18:57
Aan: cctalk at classiccmp.org
Onderwerp: Re: HP 9825 16K 09825-66523 memoryboard failure
I thought hte -12V rail came from a 3-terminal regulator
(7912 or similar). There's a 723 in the _+12V_ supply IIRC though.
I'm going
> > destroyed all the rams TMS4080.
>
> This puzzles me a bit. The -12V rail is regulated down to
-5V using
> a resistor/zener circuit,. and it's the
-5V that's used
by the RAMs.
I assume
you've checked this voltage.
Or did the -12 disappear, removning the bias from the DRAMs and
cooking them?
The -12V was defective (not on this machine but on another witch I
fixed
(uA723 replaced) and put a 8k board in it.) That wasn't the problem, the errors are a bit
erratic
always the same
bit 9 but not always the same address.
Today I replaced the address buffer ('LS368) but that wasn't the
solution too.
I think you're going to have to do some measurements before
reolacing odd components at random. As I mentioned. some of
these old HP boards don't like being resoldered too often,
the through-hole plating fails and leads to more problems.
Please don't ask how I discovered this!
> I've had vias fail on old HP boards when
soldering/desoldering
> components. My next task would be to check that all pins
on all RAMs
go to the
rignt places and that none are left floating.
I'll think I'm going to do that I got a current tracer, so to use that first.
Using an ohmmeter on the board (with it powered down, of
course) will verify that all the address lines are connected
between the RAMs, etc.
-tony