C-SCOPE VLF TR 770D battery question

[Topfmine]

I have a vintage C-Scope VLF TR 770D metal detector which has a discriminator and a visual meter which once set up moves to the left for rusty iron and to the right for brass copper etc which i prefer in a detector. I have had this for years and still works well but it takes 4 x 9 volt batteries which is quite and expense, it seems that all these old detectors use multiple batteries. I was wondering if there was better way to run this apart from using 9 volt rechargeable batteries. Any way of using a lower voltage rather than the 36 volts it needs from these 4 batteries or a smaller rechargeable lithium battery that would be much better, what do guys suggest..

[F117]

The batteries are 2 x 2 in parallel to provide + and - 9V. 2 batteries in parallel for each voltage for longer usage.
I modified mine with a voltage inverter to generate -9V from +9V.
This saves you at least 2 batteries.
Have a look at the 7660 voltage inverter, eg:
https://www.ti.com/lit/ds/symlink/tl7660.pdf

[F117]

While changing the battery layout I also put the PCB in another box and attached it to a more ergonomic stem:



The original configuration always gave me blisters after a couple of hours swinging it.

[Skippy]

Use high-capacity rechargeable PP3 batteries. EBL Lithium ones have an impressive 600mAh capacity.

[Topfmine]

I do like your set up. Thats how i have put the electronic in a similar box and arranged the dial the same way.

[F117]

I only kept the tuning potmeter, the null pushbutton and the meter.
The "disc" potmeter was replaced by fixed resistors. Don't remember the value but probably in the middle.

Just to be clear again: the batteries are 2 x 2 in parallel. If you measure between + of one group and - of another, you will have 18V.
The point where + of one group is connected to - of the other is the virtual ground. Measuring between this point and + of one group will give +9V. At the - of the other group you will have -9V.

[Xtrem]

I only kept the tuning potmeter, the null pushbutton and the meter.
The "disc" potmeter was replaced by fixed resistors. Don't remember the value but probably in the middle.

This "disc" poti is very usefull, its more a ground balance poti, than a discrimination.
about the 770 ,there is no schematic on the net, but its exact the same circuit with the 660.
(660 has no meter,and BATT test)
the whole circuit is working with +5 and GND, -9 is needed only for the reset circuit and for the audio amp.

[F117]

Here is the schematic of a 660 - as indicated by Xtrem: very similar to the 770:
cs660_2.pdf


CS770 ergonomic mod:

before:


after:

[Topfmine]

I need to lengthen the coil cable which is a 4 cored cable with a earth wire total 5. The 4 colored cables are sheathed with a foil. Does the cable need to be the same or can i use a similar 5 cored cable without affecting performance.

[F117]

You can use an old USB cable for this. It has 4 wires inside and is typically shielded with foil and a bare conductor. Make sure that you do not only connect the 4 wires, but also the shield.

[Xtrem]

Make your tests, if all is working ok , and if you want a little more gain , here is what you can do.

but the DISC pot , is necessary.