>From: "Philip Pemberton" <philpem_at_dsl.pipex.com>
>
>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
Hi
5.04 is an OK value. I wouldn't stick another Z80/780 in there until I
knew that there wasn't something else connecting a bad voltage to
the processor. You should first power up without the processor plugged
in and measure the voltages on all of the pins. Once you've confirmed
that there is no +12V or something where it shouldn't be, you can
then look for drive contention issues. With the processor removed,
find all of the CPU pins that control Writing and reading( I don't
have a Z80 pinout handy ). Use pullups to put these into their
off states ( no bus activity ).
Now check the voltages on the data and address lines. Most TTL
cause soft pullups to about 3.5V and any resistor pullup would
bring the lines to +5V. These pullups are usually on the order of
5K or larger so you should be able to pull the line down, noticably
with a 1K pull down. This will find most any of the issues with
some other part that is hard driving against the bus lines.
Once you are sure that there is nothing hard driving against
the CPU, you could then plug it back in and look for other
issues. Just plugging parts in at random would likely be a waste
of time and may even do additional damage.
Dwight
>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. Also, the schematics are almost
>unreadable. I got them from home-micros.freeserve.co.uk. Anyone got a better
>copy?
>
>Thanks.
>--
>Phil.
>philpem_at_dsl.pipex.com
>http://www.philpem.dsl.pipex.com/
>
>
Received on Thu Dec 05 2002 - 16:42:01 GMT