87-I, is that right Series One? That is nearly 20 years old.
It sounds like it got zapped in its lifetime or its got major dirt inside.
Disassemble and clean the rotary switch and check the PCB for shorts.
Hello Joe , I will agree here with my friend @saturation
There is no need to get in panic, and even cause an damage to it ..
So I propose too , to Disassemble it , and wash all of it with white pure alcohol and one small brash.
Wash well the PCB from the display area and under , let it dry for few minutes, and even use some hot air on it , so any trapped humidity to move away ...
Do the same with the face plate , wash it with soap and hot watter , plus brush .
If you have compressed air , use it to dry the terminals holes..
Do all this first , before we go to the next levels ... of suggestions.
Oh yes they are some...
Do that and report back ... and if you do all the above in less time than two hours ,
then you must do it again " properly this time "