Garrett Oval Crossfire Repair

[Merc]

My mom picked up a GTA1000 at a yard sale last year for $5. It worked somewhat. After re-soldering the coil connector on the PCB it worked better, but still didn't seem ground balanced and the target ID was way off.
I borrowed a coil from a friend and confirmed it was a bad coil. At the time I just bought a used 8" round Crossfire from ebay and it worked fine. Today I finally had a chance to open up the original oval coil and I
thought I would show you what I found. The coils are sealed in epoxy except for the final nulling loop. It was just held down by masking tape. It appeared that moisture had made its way into the coil and the tape
may have shifted because of it. I'm going to attempt to put it back as close as possible and use some hot glue to tack it down. It would have to be better than masking tape.

Merc

[Merc]

Just to update my own old thread, don't use hot glue to secure windings. Apparently it contracts/expands quite a bit with temperature. Who would have thought a product called "hot glue" would be so temperature sensitive.(Sarcasm). The coil works flawlessly indoors or outside in Spring/Summer temperatures. In colder weather, below about 45F, the coil tuning changes dramatically. Everything IDs as a nickel. Everything. So the plan is to open it back up and secure the tuning loop with perhaps some gel type super glue.

Merc

[Merc]

I opened the coil up again. I removed the hot glue and used super glue to secure the tuning winding. I put it outside where the temperature is near freezing. Left it there for an hour and tested again. The target ID was way off again so it was not the glue causing an issue. The only thing i can think of that might cause this is the capacitor built into the coil. It is embedded in epoxy but I can see it. I will try to dig down to it to test.

[6666]

The Temperature Coefficient of a capacitor is the maximum change in its capacitance over a specified temperature range. The temperature coefficient of a capacitor is generally expressed linearly as parts per million per degree centigrade (PPM/°C), or as a percent change over a particular range of temperatures. Some capacitors are non linear (Class 2 capacitors) and increase their value as the temperature rises giving them a temperature coefficient that is expressed as a positive “P”.
Some capacitors decrease their value as the temperature rises giving them a temperature coefficient that is expressed as a negative “N”. For example “P100” is +100 ppm/°C or “N200”, which is -200 ppm/°C etc. However, some capacitors do not change their value and remain constant over a certain temperature range, such capacitors have a zero temperature coefficient or “NPO”. These types of capacitors such as Mica or Polyester are generally referred to as Class 1 capacitors.

[Merc]

I checked the value of the capacitor in the coil. Measured about .4968uF before and after chilling in the refrigerator so I don't think the capacitor is drifting. This could just be an issue with these coils. Perhaps the epoxy they used. Not sure. I think I will just seal it back up and use it only in warm weather.

Merc