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MD-7020C underwater detector, Tuning, increased volume but ran into some trouble
Hello guys,
First for the good news before the not so good one. I posted a thread a couple of weeks ago about how to go about increasing the performance and increasing the audio volume for this detector. When I got the detector I immediately went about disassembling the detector and inspecting the PCB, enclosure and the seals. As you might already know this detector is based (cloned) from a fisher 1280x,albeit a bit cheap and lacking in overall quality. I found that the discrimination external potentiometer was not even glued very well, so the detector would have flooded the first time out. Thankfully I inspected both potentiometers and sealed them with marine epoxy. I went ahead and replaced all the cheap Chinese internal pots with quality replacements with integrated miniature knobs for easier tuning on the field. I went ahead and replaced the threshold(sensitivity) internal pot with an external high quality 10 turn 10k external potentiometer.
Initial testing after carefully following the DetectorPro Wader tuning guide, yielded an increase in air detection distance for a U.S. quarter from 4.5" to aroeund 6". I could not increase the sensitivity past a certain point due to excessive external interference or noise. I know the original fisher 1280x would give over 8" for a U.S. quarter in air tests. I figured that maybe if I used the detector's metal enclosure as an electrostatic shield I might be able to increase the sensitivity even more. I went ahead and grounded the PCB to the metal enclosure with a piece of enamel wire. Incredibly enough this simple mod allowed me to minimize outside external interference and increase detection distance to over 8.5" at par with the original.
I solved the weak audio issue by replacing the original high impedance headphones which sounded very muted and low in volume. I used some affordable Maxxel low impedance headphones(which I waterproofed) and added an internal in-line 10 k mini pot on the headphones to control volume. At half volume this headphones allowed me to hear even the whispers under water. I field tested the mods at the beach with great success and recover some quarters deeper than 8 inches on wet sand.
Now for the problem; while adjusting the ground balance internal pot at the beach (highly mineralized sand), I managed to get a drop of sweat from my forehead in the PCB which I thought had missed it. When I got home, I tested the detector after cleaning and the detector basically stopped working It will only detect metal at maximum 2-3 inches from the coil. There are no faint audio signals anymore, or midrange detection of metal, only a loud audio bang sound when sweeping any metal target 1-2 inches from the coil. After carefully inspecting the potentiometers for correct functioning just in case they got wet since I did submerge the unit n shallow water, I found them to be in perfect working order.
I inspected all the new pots I soldered to the PCB for any loose or broken soldering points. All of them were still good as well as the other components. I was almost done inspecting the board when I found the culprit or culprits of the problem. I found a darkened brownish spot around the 78l06 transistor which upon closer inspection, it seemed to be a drop of stray sweat fell on the PCB with some sand that probably fell from my forehead too, lodged underneath the 3 pins of the voltage regulator. I suspect the drop originally fell on the 221n capacitor besides the 78l06 regulator and dripped below to the PCB and the regulator right besides it. On my visual inspection of the PCB, I also found a miniature piece of metal shaving on the board that fell on it when I was drilling the hole on the metal case for the ground. I don't know how I missed it. Although I did use the detector for over 6 hours before the in incident with the sweat drop happened.
Ok guys; I really need help from the electronic gurus of the board. Do you think that I damaged the 78l06 transistor enough to lose all detector sensitivity?
Although it still detects metal with a loud audio response when held close to coil.
Do you think the stray metal caused a short somewhere else which damaged some other component?
What do you guys think?
Thanks sentella4d
First for the good news before the not so good one. I posted a thread a couple of weeks ago about how to go about increasing the performance and increasing the audio volume for this detector. When I got the detector I immediately went about disassembling the detector and inspecting the PCB, enclosure and the seals. As you might already know this detector is based (cloned) from a fisher 1280x,albeit a bit cheap and lacking in overall quality. I found that the discrimination external potentiometer was not even glued very well, so the detector would have flooded the first time out. Thankfully I inspected both potentiometers and sealed them with marine epoxy. I went ahead and replaced all the cheap Chinese internal pots with quality replacements with integrated miniature knobs for easier tuning on the field. I went ahead and replaced the threshold(sensitivity) internal pot with an external high quality 10 turn 10k external potentiometer.
Initial testing after carefully following the DetectorPro Wader tuning guide, yielded an increase in air detection distance for a U.S. quarter from 4.5" to aroeund 6". I could not increase the sensitivity past a certain point due to excessive external interference or noise. I know the original fisher 1280x would give over 8" for a U.S. quarter in air tests. I figured that maybe if I used the detector's metal enclosure as an electrostatic shield I might be able to increase the sensitivity even more. I went ahead and grounded the PCB to the metal enclosure with a piece of enamel wire. Incredibly enough this simple mod allowed me to minimize outside external interference and increase detection distance to over 8.5" at par with the original.
I solved the weak audio issue by replacing the original high impedance headphones which sounded very muted and low in volume. I used some affordable Maxxel low impedance headphones(which I waterproofed) and added an internal in-line 10 k mini pot on the headphones to control volume. At half volume this headphones allowed me to hear even the whispers under water. I field tested the mods at the beach with great success and recover some quarters deeper than 8 inches on wet sand.
Now for the problem; while adjusting the ground balance internal pot at the beach (highly mineralized sand), I managed to get a drop of sweat from my forehead in the PCB which I thought had missed it. When I got home, I tested the detector after cleaning and the detector basically stopped working It will only detect metal at maximum 2-3 inches from the coil. There are no faint audio signals anymore, or midrange detection of metal, only a loud audio bang sound when sweeping any metal target 1-2 inches from the coil. After carefully inspecting the potentiometers for correct functioning just in case they got wet since I did submerge the unit n shallow water, I found them to be in perfect working order.
I inspected all the new pots I soldered to the PCB for any loose or broken soldering points. All of them were still good as well as the other components. I was almost done inspecting the board when I found the culprit or culprits of the problem. I found a darkened brownish spot around the 78l06 transistor which upon closer inspection, it seemed to be a drop of stray sweat fell on the PCB with some sand that probably fell from my forehead too, lodged underneath the 3 pins of the voltage regulator. I suspect the drop originally fell on the 221n capacitor besides the 78l06 regulator and dripped below to the PCB and the regulator right besides it. On my visual inspection of the PCB, I also found a miniature piece of metal shaving on the board that fell on it when I was drilling the hole on the metal case for the ground. I don't know how I missed it. Although I did use the detector for over 6 hours before the in incident with the sweat drop happened.
Ok guys; I really need help from the electronic gurus of the board. Do you think that I damaged the 78l06 transistor enough to lose all detector sensitivity?
Although it still detects metal with a loud audio response when held close to coil.
Do you think the stray metal caused a short somewhere else which damaged some other component?
What do you guys think?
Thanks sentella4d
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