If you are of a sensitive disposition, get bored easily, or have a short attention span....Look away now......It's going to get technical.... And I'll try to address Minx and Fruitcake's concerns while I'm at it....

Not happy with altering the value of the potentiometer, I decided to continue down the route of the heating up of the resistor affair...
A good way to have a visual look inside an electronic circuit is to connect a milliamp meter in series with the battery and study the varying power consumption. It would certainly tell if the circuit was experiencing 'Thermal Runaway'....

The milliamp meter showed an ambient consumption of 21.7 mA...But continued to rise after a short period of time to 23.5 mA. This rise in current required adjustment of the potentiometer which brought the consumption back to 21.7 mA. Over the period of ten or fifteen minutes the consumption had risen so much that there was no longer any more adjustment on the potentiometer, and I was no longer able to stabilise the circuit.

What to do....?
It was obvious that 'thermal runaway' was the culprit, but after the heatsink failed to show any results I removed the circuit board from the unit, and replaced the 680k ohm ( what should have been a 2 watt resistor) with three large resistors (the only heavy duty ones I had) in series, and making up the 680k ohms required. A bit untidy, but practical......

After reassembly of the unit, it was ready to test.
I ran the unit under lab conditions for several minutes, where an initial rise in consumption was counteracted by adjustment, until it stabilised. Good news, it maintained the 21.7 mA for over twenty minutes, and following a search of the front and rear lawns, did not require further adjustment of the potentiometer. No gold coins though....

It had passed it's test with flying colours using the inbuilt 8 ohm speaker, however, when I repeated the test around the garden using the headphones the sensitivity became meteoric! and I found a few anomalies that had laid undiscovered while using the speaker.
Some underground cables connecting the summerhouse to the garage, and even some metal shuttering I had used while concreting the drive.....
So it's all systems go folks.......

Minx! Even with a fully operating metal detector I'm not expecting to exceed the remaining space in my rucksack with artifacts.... I must point out also, that with an ambient current of 21.7 mA, six AA batteries should be quite sufficient for a days detecting. But I admire your optimism....

Fruitcake! Unfortunately (or fortunately as the case may be) I don't have any access to Sky. The only dish in my house is the one that I eat my porridge out of....
I watch very little TV, I find it terribly counter productive, however....After suffering a couple of heart attacks I tend to get very tired in the evenings, and pretending to watch TV while resting stops Mrs Fox from worrying and getting concerned about my health....

Very interesting, OGF, and encouraging that you've got it to work well.
Headphones sound the better option.

I'd like to ask one question: how deep a metal object can it detect, say a water pipe? Presumably, a plastic water pipe would be less easy to detect.

Thanks JB, yes, the headphones are brilliant, especially if there is background noise or wind.
The cable I located to the summerhouse was about two feet deep under some paving slabs, but I'm wondering if the power in the cable assisted the locating....I'll give it a try later with a fifty pence coin and report back JB... I seem to have a surplus of change since nobody wants to take cash anymore....

Thanks, OGF.
If they're of any use, I can let you have lots of 5p, 2p and 1p coins.

How's it going with the great treasure hunt, OGF?

I read the technical details and had to do so s l o w l y, as I have but a small understanding of electronics. Always eager to learn though, so thank you for the elaboration.

Aww... Thanks for looking in Sufermom, it's good to see you....
Unfortunately lockdown has been getting in the way of treasure hunting, but there's a rumour going around that by the end of this month I might be able to visit a beach somewhere and run the detector through it's paces....

Are there any good beaches along the River Don then? My, how things have changed since I lived there.

If you are of a sensitive disposition, get bored easily, or have a short attention span....Look away now......It's going to get technical.... And I'll try to address Minx and Fruitcake's concerns while I'm at it....

Not happy with altering the value of the potentiometer, I decided to continue down the route of the heating up of the resistor affair...
A good way to have a visual look inside an electronic circuit is to connect a milliamp meter in series with the battery and study the varying power consumption. It would certainly tell if the circuit was experiencing 'Thermal Runaway'....

The milliamp meter showed an ambient consumption of 21.7 mA...But continued to rise after a short period of time to 23.5 mA. This rise in current required adjustment of the potentiometer which brought the consumption back to 21.7 mA. Over the period of ten or fifteen minutes the consumption had risen so much that there was no longer any more adjustment on the potentiometer, and I was no longer able to stabilise the circuit.

What to do....?
It was obvious that 'thermal runaway' was the culprit, but after the heatsink failed to show any results I removed the circuit board from the unit, and replaced the 680k ohm ( what should have been a 2 watt resistor) with three large resistors (the only heavy duty ones I had) in series, and making up the 680k ohms required. A bit untidy, but practical......

After reassembly of the unit, it was ready to test.
I ran the unit under lab conditions for several minutes, where an initial rise in consumption was counteracted by adjustment, until it stabilised. Good news, it maintained the 21.7 mA for over twenty minutes, and following a search of the front and rear lawns, did not require further adjustment of the potentiometer. No gold coins though....

It had passed it's test with flying colours using the inbuilt 8 ohm speaker, however, when I repeated the test around the garden using the headphones the sensitivity became meteoric! and I found a few anomalies that had laid undiscovered while using the speaker.
Some underground cables connecting the summerhouse to the garage, and even some metal shuttering I had used while concreting the drive.....
So it's all systems go folks.......

Minx! Even with a fully operating metal detector I'm not expecting to exceed the remaining space in my rucksack with artifacts.... I must point out also, that with an ambient current of 21.7 mA, six AA batteries should be quite sufficient for a days detecting. But I admire your optimism....

Fruitcake! Unfortunately (or fortunately as the case may be) I don't have any access to Sky. The only dish in my house is the one that I eat my porridge out of....
I watch very little TV, I find it terribly counter productive, however....After suffering a couple of heart attacks I tend to get very tired in the evenings, and pretending to watch TV while resting stops Mrs Fox from worrying and getting concerned about my health....

It doesn’t have to look pretty Sly old grey fox. It just has to work! Seems like it’s working well! When are you going to try test it out for real? Your riches awaits! So excited!