On Dec 5, 18:08, Philip Pemberton wrote:
> Hi all,
> I've just opened up my Jupiter Ace and I think the problem I'm having
> with it is far more severe than a blown CPU. It looks like while I was
> testing it this morning the PSU was accidentally powered up, the
+12(unreg)
> connection made contact with the barrel of the power jack (-ve) and the
> PSU's -ve line made contact with one of the first five expansion bus pins
on
> the topside of the board.
> The CPU was getting very hot - I've since swapped the original NEC
D780C
> (1982 datecode) with a Sharp LH0080 Z80-A-CPU IC. The replacement is also
> getting hot (takes about a minute for either of them to hit 50deg C).
Output
> from the 7805 is 5.04V steady according to my Fluke 25 DMM. Video is
being
> output and my TV can lock onto it, but the output is total garbage, no
> difference if I remove the CPU and ROMs or have them installed. The
garbage
> is always the same, too, in case it matters. The replacement CPU is
> known-good - it came out of a working Toshiba HX-10 MSX.
> How should I proceed with this repair? I've got a Fluke 25 DMM and a
Tek
> 466 storage scope at my disposal.
It would be worth looking to see if signals look consistent (ie outputs of
gates behave in the way you'd expect from what you see happening on the
inputs).
If you're thinking of replacing every chip at once, or one-by-one, I'd
advise against it. You won't be able to tell what's really wrong, and
random swapping might just destroy an otherwise perfectly good IC.
The 74LS166 is a serial-out shift register, and I expect that's what
generates the video stream. Since you say you always get a good consistent
picture, with characters (even if they're junk) or some cosistent pattern,
the shifter and video timing is working. That's probably where most of
your 393's are, too (they're dual 4-bit counters).
Am I right in thinking the character set is soft-loaded from the ROM? ie,
not in a character generator ROM? Then you wouldn't expect to get
recognisable characters unless the CPU can run, and the ROM is OK.
Try swapping the memory chips around. If that gives different (but
self-consistent) video, at least some of it is working. If it makes no
difference, either it's *all* fried or the buffers have gone west.
Other obvious things to check are the CPU clock and /M1 lines. The Z80
clock needs to be pulled high, your scope should be able to show if the
clock is a nice square wave that goes up to almost 5V (minimum acceptable
is about 4.5V, IIRC). The /M1 line goes low once for each instruction
fetch; it should be pulsing. The data and address lines should be pulsing.
If the CPU is free-running becasue it can't read instructons, it might be
executing NOPs or RSTs or just some random instruction, depending on
whether the data bus is stuck all-high, all-low, or at some random value.
One possible cause of CPU and SRAM getting hot is if the data bus which
they're trying to drive is stuck with an active signal. See if you can
isolate the bus, and if the CPU or video behaves differently.
I'm just giving general advice here, as I've not used an Ace in decades,
and have no idea what the circuit looks like :-)
--
Pete Peter Turnbull
Network Manager
University of York
Received on Thu Dec 05 2002 - 16:57:01 GMT