NTSC Flat C-128 IEC serial problem

Hi all,

Years ago I bought an NTSC Flat C-128 off ebay it powers up just fine but
it has this issue and error with the IEC when any DOS command is issued in
basic:

DEVICE NOT PRESENT ERROR?

I cleaned the userport edge connectors
and the cartridge port connectors  they seemed to be corroded and dirty.

Just last fall I bought a new C64/C128 Diagnostic harness out of Canada
so i hooked that up but had an issue with the keyboard dongle and Im sure I
plugged that in the right way, but when I powered the C128 on it went
straight into the Monitor.
So I just didnt plug in the keyboard dongle and then the Diagnostic would
start.
So I tested the following:

1-CIA that handles the IEC serial work---Tested okay and I have working CIA
spares.

2- TTL chip that works inline with the CIA--- desoldered and replaced IC,
no difference same IEC serial error.

3- Diodes in all of the IEC circuit---- Replaced--- Result same serial
error.

4- Replaced DIN serial socket-- I admit I messed up 2 traces so I added
bodge wires, and I tested the continuity with a Multi meter and all seem
okay then.

So with all of that still the same Device Not Present Error? comes up.
 Yeah all of my disk drives work fine with all my other Commodore computers.

  Would both the 8502 processor or the PLA be a culprit but they didnt test
faulty in the
Diagnostic?

  Suggestions appreciated I just dont know what else to do as I stated the
Diagnostic cartridge says all other chips are OK and I have run that dozens
of times with the same test results so Im really stumped.

The Diagnostic harness is awesome it did point me in the right directions
and I love these tools so theres no guess work, kind of when it still
leaves you with nothing to find is what is puzzling so just the experience
of just what to look for is what I dont have I admit.

Terry Raymond