depth sounder not working after new antifoul

[aquapore]

I have a very faithful Stowe depth sounder giving 25 years of perfect service (housed in a wet well) which developed an issue after I applied a new band of anti-foul.
The instrument fails to consistently register a reply pulse. The instrument blinks with the last good measurement.
Works perfectly when I put the sensor over the side.
I have cleaned out the wet well and replaced the water.
Has anyone else experienced similar problems.
Regards

Aquapore

[JEN-et-ROSS]

Ours is an even older Autohelm, though it's a 'through hull' unit, so the face of the transponder gets a liberal coating of antifouling each season....
And it doesn't seem to affect it's performance..... Maybe after 25 years a fault has occurred (water penetration?).....
If you can find what the specified resistance (ohms) should be, it might indicate that it's time is up.................Bill.