Commodore 128D User Manual download pdf (Page 9)

MOS 6526 CIA MC 6821 PIA

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PA0 PA0

PA1 PA1

PA2 PA2

PA3 PA3

PA4 PA4

PA5 PA5

PA6 PA6

PA7 PA7

PB0 PB0

PB1 PB1

PB2 PB2

PB3 PB3

PB4 PB4

PB5 PB5

PB6 PB6

PB7 PB7

TOD

CB1

CB2

IRQ

R/W

FLAG

R/W

CS0

CS2

CS1

D7 D7

D6 D6

D5 D5

D4 D4

D3 D3

D2 D2

D1 D1

D0 D0

RESET RESET

RS3

RS2

RS1

RS0

CNT

RS1

RS0

IRQB

IRQA

CA2

CA1

Figure 5: The CIA and the PIA

2.3.1 Removing the old memory chips

First you have to remove the memory chips U38 through U53. If you look at the mother board from the front of the

computer as if you were typing, the chips are on the front left in two rows of eight chips. They are of type 4164 (or

3164 or 6665 or 6664 or 8064 or. . . ). You could install the new chips into sockets, but I thought that it is a waste of

money.

If these memory chips are already on sockets, the most of the work is done for you. It helps a lot, if you remove

the bypass capacitors before removing the chips. Removing the components is easiest with a desoldering pump. It

becomes even easier, when you ﬁrst solder the pins with fresh solder, so that the hartz from it makes the removal of

old solder easier.

Using much power is questionable, as the copper folio comes off the board in a surprisingly easy way. As usual,

I used a screwdriver like a crowbar, and the through-coppering got lost from several places. This was not crucial, as

those pins were connected only to the down side of the board. However, three or four routes broke on the top side also.

This made it far more difﬁcult (and slower) to solder the new chips in, but I succeeded on the ﬁrst try.

After you have removed the 4164s, you can solder the 16-pin sockets (or the 41256 memory chips) into their

places. You can solder the capacitors back as well, if you removed them.

2.3.2 Adding the new address line

You must connect the pin 1 of each memory chip (or socket). It is the extra address line (MA8) to the switcher. The

best way is to solder a Wire-Wrap wire to each contact under the mother board, but any thin and pliable uni-strand

wire should do. The wire does not affect in any way the computer’s operation with 64 kb chips.

After the pins have been connected together, they must be temporarily connected to +5 V, which is in the pin 8 of

the memory chips. Comparing to TTL chips, the operating voltages are ‘reversed’ in dynamic memories.

Now the new 256 kb memory chips can be installed to the sockets (preferably right-side forward), and you can try

switching the power on. You do not have to connect anything except the power cable and the cable to the TV set or

monitor. Ensure that the “40/80 display” key is up, and that the monitor is set to display the 40 column screen, too. It

is a good idea to turn on the monitor ﬁrst and let it warm up, so that it will show the picture from the very beginning.

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Commodore 128D User Manual Page 9

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