fixing faulty Garrett Deepseeker

#5


G'day fellow prospectors
I have 2 Deepseekers that work well and powers up every time they are turned on A fellow prospector gave me his unit to fix.
Sure enough turning it on it was dead as a dodo. Putting in new batteries didn't help. After fiddling around managed for
detector spring into life. With the detector ON it seems that by unplugging and plugging in the coil the unit would make it work.
Concluded the the problem as an intermittent re-solder all the joints on the Transmit and Receive sides. The problem is
still there.Looked at the plug and socket pins they were a bit corroded some what this just might be the problem. Replaced them
but the startup problem still persisted. Now did notice the plugging coil in and out trick didn't work all the time!
Looking at the circuit the unit wouldn't if the there was a problem in the Tx side. Hooked up a CRO ( digital one) and
there was no output.Doing the the coil trick the Tx oscillator would work and hey presto it works again. Analyzing the
weird circuit it was clear the LM 358 op-amp was tired. After replacing it the detector would fire up consistently over
number days. But after being off for a couple of weeks the Oscillator would not startup again. Starting once restarting
was not a problem. Checked the passive components they were all good. There are not many components in the oscillator so
the only variable is it a faulty coil? has anyone experienced this issue?

Posted on: February 19, 2022, 11:32:20 pm

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As commented the circuit is a bit unusual. There are 3 components that determine the frequency The coil inductance & 2 capacitors 0.1 uf and 0.474 uf. Tested the capacitors the 0.1 uf is spot on but the .474 uf was .4uf. So order a ceramic 0.474 uf and replaced the out of spec one. Very strange the frequency went up by 50% can only conclude the design requires a capacitor with specific characteristics that also includes inductance

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Generally electrolytics show distress when faulty. Was of the opinion the 3 470uf caps were OK. But in desperation decided to change them. All 3 still showed 470uf on a testing device. Putting in new ones made no difference! After 24hrs being turned off the Oscillator failed to start up. Unplugging the coil turn on the detector then plugging in the coil oscillator runs. Turning the detector on & off the oscillator restarts but fails when off for some hrs. The other problem is the frequency 2 other detectors the frequency is about 37hz but this one runs @ 75hz. Changed all the components in the Oscillator circuit and found 0.47uf cap replacement with new caps the frequency jumps to 135hz put in the old one the frequency falls back to 75hz.....very strange circuit!!! Don't understand why the large green cap has this effect. Other detectors have different caps of 0.47uf one terracotta colour and the other miniature block type. BTW new batteries, all 3 coils inductance are the same so @ loss  

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