Conference 7.286::atarist

733.0. "Problem with ROM driver chips/ 6 ROMs vs 2 ROMs" by BERN01::RUGGIERO () Fri Jan 12 1990 03:45

Hi,
I like to tell you about a hardware problem I had with my Mega2ST and how I
solved it. The problem was also reported in various magazines.

Some of the MegaSTs have two driver chips of type 74LS373 manufactured by SGS.
These chips DO OPERATE AT THEIR LIMIT. They drive the ROMs and the ROMport.

If you hook up something to the ROMport (eg Spectre) you might drive the chips
to the edge. This happened to me: I had (sometimes) strange crashes with the
Spectre or Spectre reporting problem with MacROMs. Yesterday I received the
TOS 1.4 (Rainbow TOS) in ROMs and upgraded my system - I was not even able to
boot or the system crashed right after the desktop showed up. Well, the original
ROMs were 2 large bipolar while the new ROMS were 6 256k CMOS chips. The LS373
was not able to drive the (capacitive) load of the 4 additional chips. 

After replacing the driver chips all was fine.

IF YOU HAVE 2 CHIPS (on the right side of the ROMs) LS373 MANUFACTURED BY SGS
REPLACE THEM BY EITHER ALS373 OR LS373 OF A DIFFERENT MANUFACTURER.

Be careful with (de-)soldering, take the neccessary precautions against static
discharge and do not overheat the board. It is better to cut out the chips
and then remove the pins afterwards. I also recommend the use of sockets for 
the replaced chips. 

There is no problem with replacing the 2 big ROMs by 6 smaller ones. But you
must make three simple modifications to the address decoding logic.

First: There is a 74LS11 near the front left corner of the board. Most of its
pins are grounded. This chip must be removed. It is sufficient to cut off 
pin 12. This is the only used output of the LS11.
Second: Left of it is a symbol of a resistor marked W4. This is a bridge you 
must install. Just use some piece of wire and solder it in where the symbol 
showes. 
Third: Near the front edge of the board you find 2 jumperblocks, marked W2 
and W3. They are jumpered between pins 2 and 3. Change this so that the 
connection is 1-2 for both.

DISCLAIMER: The above described procedure worked for me. I do not take ANY
responsibility whatsoever for any damages to your system or data or ego!

---markus---

    I read on USENET that you could just drop a functioning LS373 driver
    right on top of the SGS units and solder... no need to muck up your
    board.
    				- hbm

>       <<< Note 733.1 by FORTSC::MESSENGER "Late in the 20th Century" >>>
>
>    I read on USENET that you could just drop a functioning LS373 driver
>    right on top of the SGS units and solder... no need to muck up your
>    board.
>    				- hbm

    While this may work in the short term, it is not a recommended techinque.
Bipolar logic does not share current well. The weaker device will end up
having current injected into its output, and will burn up. Imagine how hard
it would be to replace such a mess!

							Rob

Mine were socketed, so it was easy.

I recommend desoldering the 74LS11 and putting a socket in its place.  Then
you can easily switch back to the 2 chip version if you want.  I also used
berg-style jumpers where jumpers needed to be inserted, so converting back
and forth is quite easy.

Desoldering is a delicate operation with that much ground on the pins.  I
ended up buying a cheap desoldering iron, use that on the bottom of the
board, and at the same time touch a regular soldering iron to the top next
to the pin to be desoldered.  The problem is the copper ground sucks the heat
away unless you use a fairly high wattage iron.

							Enjoy,
							-Dave