30 Sep
2007
30 Sep
'07
5:02 p.m.
hey thanks for that. Do you still attend radio
rallies, and if so are you still able to find
interesting stuff? I haven't attended ham shows in
years, except for the MIT Flea 2 weekends ago, so I
don't know what turns up these days. Back in the early
90's you could find all sorts of useless stuff LOL
LOL. Like 19" open frame monochrome monitors and such.
They always asked a lot for their *junk* though. Often
I was put off.
OT - Tony, what are you doing about finding yourself
(or ME LOL) and early English T & LM lathe. If you
don't know what I'm talking about, lathes.co.uk should
have a profile on it. Lindsay Pubs. has a small
booklet that describes the construction (contains a
reprint from "American Machinist" from the early 20th
century). Since you're in the country it was
manufactured, don't take offence, but you'd be crazy
to not at least keep yer eyes open for one. Or
advertise around. Yer know ;)
--- Tony Duell <ard at p850ug1.demon.co.uk> wrote:
____________________________________________________________________________________
Shape Yahoo! in your own image. Join our Network Research Panel today!
http://surveylink.yahoo.com/gmrs/yahoo_panel_invite.asp?a=7
> problems caused by extra components [ask if you
want to know _that_
> story], and aligning those wonderful
voice-coil
floppy drives...
OK, I bought a QX10 at a radio rally (hamfest), and
the seller threw in
an extra CPU box which he said didn't work properly,
but it might be
useful for spares.
Well, of course, ard being ard, I decided to find
out just what was wrong
with this spare machine. The PSU checked out fine,
so I fired it up. It
seemed to boot OK, but after a bit of screen
scrolling the display got
terribly corrupted. I forget the exact details, but
I think the text
appeared in the wrong part of the screen, stuff that
had scrolled off
came back in strange locations, and so on.
It looked like a video RAM problem. Now the QX10 has
a daughterboard
containing all the video circuitry, it contains an
7220, a gate array for
the actual dot shifting/combining (early versions
have TTL logic chips
for this...), a character generator ROM, a bit of
TTL and 16 DRAM chips.
According to the techref, the last can be either
4116s (16K) or 4164s
(64K), there are links on the board to select which
type are in use. I
checked, everything was set correctly. The board had
4164s on it, all the
links were in the '64K' postiion.
So, being young and foolish, I replaced the complete
set of (socketed)
DRAMs, intending to put the originals back until I'd
found the one that
was playing up. No, that made no difference. Nor (as
I said I was young
and foolisH) did temporarilu replacing the 7220 and
gate array with the
ones from the good machine. And the ones from the
defective machine
worked fine in the 'good' machine.
So I had to think about it. I decided to see what
the RAM size links did,
these are shown on the schematic.
One of them connected pin one of all the DRAMs to
-5V, or left it floating.
Another connected pin 8 of all the DRAMs to +12V or
+5V
And another connected pin 9 of all the DRAMs to +5V
or an address output
from the logic.
Suddenly, it hit me. Not shown on the schematic, but
present on the
board, were decoupling capacitors. And pin 9 of the
DRAM array had 8 or
so decoupling caps, 0.1uF each, between it and
ground.
Fine if 4116s were fitted and that pin was a +5V
supply. Not so good if
it was a logic-level signal. That extra capacitance
added a ridiculous
delay to that address line.
So I desoldered the capacitors. With all the
original chips in the board,
the machine sprang to life and worked fine.
Presumably a previous owner had replaced 4116s with
4164s, either to get
the higher video RAM capacity, or because one of the
former had failed
and he only had the latter. He'd correctly re-set
the links, but hadn't
realised about the decoupling capacitors.
-tony