How to stop BFO drift?

[SVEN1]

A detectorist just bought an old PNI Bounty Hunter BFO from the 1970's.
He went out hunting with it and found about 50 coins.
It was bit bothersome as he had to keep adjusting the tuner to threshold drift.
Just wondering if it is possible to control the drift with some part changes?

[kt315]

thats why BFO had dead as the class.
two main ways.
1. hard stabilization of power supply voltage. an impulse converter.
2. hard stabilization of support generator frequency. quarth stabilization.

[Skippy]

There's too many things to drift, some in unknown ways, that's the problem. Two oscillators ... ideally you would want them both temperature stable, but if they tracked each other in drift, that may be a solution, except the coil is not inside the control box with the other bits.
It's probably worth replacing electrolytic caps, eg on power rails.
If you're happy to tinker, maybe replacing polyester caps with polypropylene types would help. Polypropylene have the opposite tempco to polyester, but only 10 or 20% the slope.

[kt315]

maybe replacing polyester caps
---
replacing whole board on bara, surf of mirage. you can not remake raritet in something new modern without full radical exchange of board.

[Teleno]

Try this PLL controlled oscillator and tell us how it performs: https://www.geotech1.com/forums/show...756#post242756

[Skippy]

There's little point making the reference oscillator stable, if the oscillator based on the search-coil is drifting. It would be better if they both drifted in the same way, for example if they both used the same type of capacitor in their oscillator.
Without a schematic, and a good understanding of how things drift with temperature, there's not a lot you can do. Maybe the machine was never that stable when it was new ? Maybe the instability is caused by components ageing over the decades ? Both of these may be fixable, but a different approach is needed for these two cases.
It's worth trying to stabilise the power supply, though. Example if it runs on a PP3 battery, make an LDO 8 volt regulator, and run it from a 7-cell or 8-cell NiMH

[kt315]

[kt315]

[Teleno]

There's little point making the reference oscillator stable, if the oscillator based on the search-coil is drifting.

Since drifting is slow, you can use a stable, controllable reference oscillator that lowly adjusts its frequency to the unstable one, for example via an integrating control loop with a slow time constant (1s or more). For example, a PLL with a slow loop filter.

[SVEN1]

I don't think he is going to worry about the drift.

"...dig all targets --- today I took out the old BFO detector which has no disc. --- coins are easy to tell , they are really really loud --so I decided to dig only the smallest sounds --- junk after junk targets --- dug 4 nickels , 1 dime, and 1 penny that produced small signals --- then, boom , a very very nice gold ring popped out --- 25 diamonds --- goes to show what we are passing up when we use too much disc. !"

[dbanner]

Excellent!
Tokyo drift!
Carl had posted scheme for Garrett bfo.