Join Date: Mar 2006
Location: Cape Fear, NC, USA
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(1) Seems unlikely to me (wild guess on my part) that a 1970's era depth finder would be multi frequency.
(2) Get an O-scope and crack the case on the old sounder and hook to the leads that feed the transducer and look at freq/period of the wave form.
(3) What makes you think the old depth sounder is multi-freq and more than that is a 50/200kHz?
(4) Call tech support of the new depth sounder company and ask them what they think/know.