What do you mean by piggy back on top of each unit? Don't I have to desolder each chip I am trying to replace?
This is a tech trick but you have to be really careful and have some way to see the video screen or have an assistant. It is just like it sounds, you piggy back good ram chip right on the suspect ram chip and when garbage clears up, you found the bad chip.
EDIT:
Copied this from a search and cut it down to this and explains method much better.
The piggyback method requires that you install the test chip
over a board RAM chip so all pins make good contact and no pins
short together. The test chip must be oriented correctly as well.
There is a notch or dot (or both) on one end of those ICs. Those
markings must line up because if the test chip is installed
backwards, it will likely be damaged when power is applied. You
can press the test chip pins against a tabletop to push all pins
inward slightly, and that will provide some spring tension when
the test chip is inserted over the board chip and can hold it in
place. It's OK if you want to hold the chip with your fingers and
then power up the computer. There is only five volts present and
therefore absolutely no danger of electrical shock. Again, if the
bytes free number changes, that board IC is probably bad and
should be changed. If there is no change when all RAM is checked,
suspect the two RAM control logic ICs mentioned above. They must
be replaced to diagnose a problem there.
here is the page this came from
http://personalpages.tds.net/~rcarlsen/cbm/memory.txt