I just got my 1978 S200DL back from DMI, it had been fried by the PO and also needed a new LCD display. Cost $250, but I figure I would have paid twice as much for a replacement of the same quality. Works fine now. They have replied to many e-mails from me, always within 24 hours.
They charge $75 minimum for diagnosis. Most repairs cost up to $150 but they won't go above without your OK. Mine was extensively damaged.
If you want to keep your unit I'd recommend them-- since you are at least getting a readout most of the electronics must be working. You might describe the problem to them so they can help you decide whether it might be the transducer.