#2000 only has 1/2 ram
8 messages in this thread
I just recieved a used B2000-CR Rev 4.3, the previous owner had installed
the 1 meg agnus chip (8372A) and he had informed me before I agreed to
purchase it that he had been unable to get the chip to recognize a full
1meg chip ram. However, what he neglected to tell me was that it wouldn't
recognize the system as 1/2 chip and 1/2 fast, only as 1/2 chip.
I would like to either get the 1/2 fast ram working or configure it for
1meg chip ram. I had never seen the inside of an A2000 before and don't
know what the procedure is in either case. There is a jumper (J500) that
appears to have the entire jumper block removed (there is solder iron
burns on the MB), would this have anything to do with it? I looked for
other modifications on the board and I didn't notice any. There are 5
other jumpers on the board:
C302, J301, J300, J200, R105
Would these have anything to do with this problem? If it makes any
difference, the KickStart chip is KS V2.04 37.175, the wire that jumps
between two of the pins has been removed, I remembered there was this wire
on the KS upgrade I did on my old A500, and it is obvious it has been
removed from this KS chip, you can still see some of the solder left on
the two pins.
Any suggestions?
=Rick=
Richard,
J500 and J101 are the two jumpers you need to worry about. The older 4.x
motherboards frequently had no jumper block at J500. They just had the two
pads connected by a thin trace. When (whoever) did the upgrade, they had
to cut the trace with a hobby knife. If they messed with it, they might
have tried to re-solder the connection, explaining the burn marks. Double
check with an ohmmeter that the trace is actually *open*. There will be
some resistance, but not a dead short.
J101 is just to the lower right of the motherboard power connector. It
should be set to pins 2 & 3 for the 1 meg chip RAM option.
For the 1/2 & 1/2 mode, J500 should be shorted, and J101 would be on 1 &
2.
The missing wire from the Kickstart ROM isn't a problem. The wire was only
there for 500's and very old 2000s.
-Jim
Jim,
When looking at the motherboard from the front of the machine, is pin 1 on
the on the left or right of J101? J500 is pretty scared up, it looks like
instead of just cutting the trace the previous owner tried to remove the
entire solder pad. There appears to be a trace right behind J500, but I
haven't determined if it is damaged, if it was would it cause a problem?
Thanks for your help.
=Rick=
Rick,
There should be a '1' silkscreened on the board next to pin 1 of J101. In
any case, pin 1 is to the right as viewed from the front. I don't know
what the trace behind j500 does, but if it's damaged it could cause
problems…. but since the system comes up, the trace is probably OK.
-Jim
Jim
I don't know how much you know about the wiring of the MB on this unit,
but I removed the MB and spent some time looking at both sides of it at
J500. It dawned on me that maybe the problem is on the underside since
the trace that runs from the Gary chip to the one pad of J500 on the
underside is broken. Below is an attempt at drawing how I think it should
be on the board:
J500
A —–[] []– B
| |
| |
| C ?
|
_________|_____
| |
] Gary |
|_______________|
'A' is a thin trace on the board that runs for a few inches. 'B' is a
fatter trace that is only about a quarter inch long and does not appear to
be connected to anything else. 'C' is a fatter trace that appears to be
connected to one of the very wide traces that connect to the power
connector
The traces labeled 'A' & 'B' are on the top side of the MB, the trace to
the Gary chip is on the bottom side of the MB. With J500 shorted, the
trace from the Gary chip is connected to the 'B' & 'A' trace, when the
J500 is cut, the trace from the Gary chip only goes to where trace 'A'
goes. At least this is my impression of how things should be from
studying the mother board.
In reality, this MB has been modified in such a manner that the trace from
the Gary chip is not connected to any of the other traces, regardless if
J500 is shorted or not.
Also, on the underside of the MB, on the side of J500 the the above 'B'
trace is mentioned, there is a square solder pad, should it be soldered in
such a way that it is connected to the trace I have called 'B' on the top
side? Currently it is not.
I hope I haven't lost you on this <G>.
=Rick=
Rick,
Without tearing open a 2000, I couldn't say what the layout of the
motherboard should be. My own 2000 is a Rev 6, which is somewhat different
although the jumpers are labeled the same way. I can say that the only
purpose of J500 is to supply a 'ground' signal to the Gary chip for
configuration purposes. Your picture then looks right.. The heavy trace on
B would be the ground bus. C probably goes to some other place using a
ground, so I wouldn't sweat it. The trace to the Gary from 'A' is another
matter. With it disconnected (did you ohm it out?), it's the same as if
the jumper is out regardless. Trace A probably goes to the pull-up
resistor for the signal on the gary pin.
GND—–< >——-/\/\/\—5 volts
j500 |
|
GARY
So, if the trace from Gary to point A is broken, the Pin will electrically
"float" to some voltage, instead of the 5v supply level through the
resistor. I forget whether MOS chips float true, but it's not good
practice to let pins float. It might work, but be flakey…
I don't know what to tell you about the bottom of the MB. Without seeing
it, it's hard to tell. You know that the 2000 is an (at least) 4 layer
board? Even if it looks like a trace isn't connected anywhere, it may be
routed inside the board. Traces can go from one side to the other or one
layer to another through via holes that look like small solder pads. If
the trace doesn't look fried, I'd leave it alone.
-Jim
Jim,
Thanks for your help, it WAS the trace from the Gary chip, it was not
passing through the MB at J500 properly and thus didn't terminate at R500.
I soldered a wire between the appropriate Gary pin and R500 on the
underside of the MB and set the other jumper you mentioned on pins 2 & 3
and now the system boots with 1 meg chip ram.
Thanks again,
=Rick=
Rick,
Glad to have been of help.
Jim