Hi, thank you for contacting JustAnswer.com. My name is Russell. I will do my best to provide the right answer to your question.
In this case you have a problem that is more difficult than you have stated in your question, as it were.
First, it's an aging system. Second, such phone systems, of that vintage, don't take kindly to inexpert or careless board-swapping... they are very susceptible to damage from static discharge, often, and also sensitive to rough handling. So if you do any work on it, be sure you know about, and completely observe, anti-static discharge precautions... and actually wear an anti-static band and grounding strap on one wrist, and also no wool clothing, no glass sheeting, etc.
Since the system is getting old, and is delicate too, I suggest (if you can observe and do what I just mentioned above in the way of careful treatment) that what it needs the most, is to be shut down, turned Off, unplugged, and to have the boards removed (and placed on either sheet aluminum or aluminum foil, or otherwise very much protected against static discharge damage) then have the cabinet and slots that the boards fit into, cleaned out thoroughly with a powerful blower or air compressor, out of doors.
Also use the compressor on the removed boards and esp. their edge connectors, *not* touching them with the compressor or nozzle, just with the air blast.
Then carefully reassemble the system again.
That might do it good.