Re: Epson QX-10 Emulator

> 
> > 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

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 <ardp850ug1.demon.co.uk> wrote:

> > 
> > > 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
> 



     
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