Reg Mods Update

[CaliGold]

7 hours ago, Melano87 said:

The jumper is made with the Test Points 2 and 31.

When I made this modification, I soldered a wire on the TP31, a wire on the TP2, I installed a switch (2 positions) next to the speaker (only place really available), I twisted the two wires up to the switch.

Why a switch? Because as the gain increases you have to be able to “control” it and there are places where it is difficult. So when I need it and possible I put it in the ON position.

Don't touch R9 - R75 - C42

On TDI SLs manufactured after 2015, D20, D21 and C56 are absent (modification taken into account by Whites). On those before 2015 they are present.

 

I also installed a potentiometer for adjusting the audio volume (like on the Beach Hunter)

Do you think the switch is necc with smaller coild than yours? you have some behemoths there... My biggest coil is a 14" coiltek tdi series round mono....

[Geotech]

1 hour ago, Jim in Idaho said:

Many thanks for the explanation and schematic Karl. I think the mod called for a jumper from test point 31 to test point 2? I'm not sure where TP 2 would be.

Output of the opamp.

55 minutes ago, CaliGold said:

Thanks Karl!  If you have time can you comment on weather a on/off switch for this c42 resistor mod would be helpful or even necessary if one like myself is running mostly a 8x6 sadie with largest coil being 14" Coiltek TDI Series mono. (and the Miner John 8x12 on occasion). I'd rather avoid making the on/off. I would even forego using that 14" coil at all on this machine if the gain mod would make the switch absolutely necessary.

*Also thanks for info on the c21 etc caps, sweep mod. They're hard to source online below 5% or 10% tolerance. Can I get away with say 3% or 5% tolerance on the cap models? I realize the "pair" ultimately used will have to be 1%...

ALSO... Man! you guys are quick with the helpful info! I owe you guys lots of gold found pics and imagery. I promise this fall/early winter will be epic...  I definitely owes you guys one on the DP forum.

The C42 mod should have little to do with coil size. The purpose of C42 is to remove DC offsets before the final gain amp, which has to do with the circuitry and not the size of the coil. As Melano says, as you crank up the gain of the final amp (with the GAIN control) the DC offset gets gained up and can cause noise issues. I've never done this mod so I can't otherwise comment on its effectiveness.

The absolute values of C20 and C21 don't matter much, even 10% caps are fine. Just make sure they match each other within 1%.

[Jim in Idaho]

I have no idea on the switch. I'll definitely install one when I do this mod. As far as the test point location, I was more interested in where TP2 was in the schematic, but appreciate the pic in any case.

Jim

[CaliGold]

On 8/31/2023 at 4:31 PM, Geotech said:

Output of the opamp.

The C42 mod should have little to do with coil size. The purpose of C42 is to remove DC offsets before the final gain amp, which has to do with the circuitry and not the size of the coil. As Melano says, as you crank up the gain of the final amp (with the GAIN control) the DC offset gets gained up and can cause noise issues. I've never done this mod so I can't otherwise comment on its effectiveness.

The absolute values of C20 and C21 don't matter much, even 10% caps are fine. Just make sure they match each other within 1%.

I will have a 10mm mini toggle on/off switch with rubber boot (could only find 125v to ship fast) and a 1kΩ 0805 size resistor tomorrow to go for this C42  gain mod. Do I need to remove the current installed C42 component and replace with the 1kΩ resistor to accompany the test points wired switch or does the on/off toggle switch make the resistor unnecessary?  The .22uf Caps are inbound, but shipping slower. I think that will cover all the possible mods on TDI SL 2018. Someone mentioned in a forum that the c20 c21 sweep mod caps were changed at factory after a certain date but mine are stock .47uf indeed...  

 

SIDE NOTE: I'm getting getting really good side of coil response with sadie coil 8x6  on these gold nugget samples especially see pic below. I buried the 1.1 gram nugget under a fire brick and 5" of Randsburg, California quartz samples and it was a great signal straight over the pile with room to spare. I also Sikaflex 221'd the coil cable inlet on the coil. No bedrock stands a chance now... 

 

Many Thanks! 

[Jim in Idaho]

You want the resistor to be across the cap when the switch is "on". Don't fool with the cap. As Karl said...without the resistor in series with the switch the output of the opamp would be directly shorted to ground.....not good...LOL. I'm looking forward to some comparison tests with the switch on and off.

Jim

[CaliGold]

41 minutes ago, Jim in Idaho said:

You want the resistor to be across the cap when the switch is "on". Don't fool with the cap. As Karl said...without the resistor in series with the switch the output of the opamp would be directly shorted to ground.....not good...LOL. I'm looking forward to some comparison tests with the switch on and off.

Jim

Ok, so solder the resistor right on top of the cap at c42? sounds fun... Then wire from the test points to the switch I assume. Unless I'm imagining this wrong...

Yes, looking forward to some testing too!

Thanks!

Pat

[Jim in Idaho]

No, Pat...the resistor has to be switched into and out of the circuit. So, when the switch is "on" the resistor covers the cap. When the switch is "off" the circuit goes back to the factory setup. In effect your changing the circuit from not allowing DC current through the cap, to allowing it past the cap via the 1k resistor.

Jim

[CaliGold]

3 hours ago, Jim in Idaho said:

No, Pat...the resistor has to be switched into and out of the circuit. So, when the switch is "on" the resistor covers the cap. When the switch is "off" the circuit goes back to the factory setup. In effect your changing the circuit from not allowing DC current through the cap, to allowing it past the cap via the 1k resistor.

Jim

Thanks Jim

Completely lost as to what that means tangibly. I know you guys are trying to educate me, but I think I need direct orders. soooo, Solder a 1k resistor in-line somewhere along one of the two wires going to the toggle switch between the two test points? I'm basically gonna solder one wire to Test Point 2 and another wire to Test Point 31 and  neatly route and connect those two wires to the two different tabs on the bottom of this switch in photo below. I'm clueless as to where to put a resistor in this scheme. I ordered these 0805 resistors and sounds like they are the wrong kind. I'll have more time tomorrow to take crack at it. I just put that resistor on top of the cap in photo to show scale. Why God Why.

 

Thanks

Pat

[CaliGold]

On 8/31/2023 at 3:38 PM, CaliGold said:

Do you think the switch is necc with smaller coild than yours? you have some behemoths there... My biggest coil is a 14" coiltek tdi series round mono....

So you didnt use a resistor? Thanks

[CaliGold]

On 8/31/2023 at 7:43 AM, Melano87 said:

The jumper is made with the Test Points 2 and 31.

When I made this modification, I soldered a wire on the TP31, a wire on the TP2, I installed a switch (2 positions) next to the speaker (only place really available), I twisted the two wires up to the switch.

Why a switch? Because as the gain increases you have to be able to “control” it and there are places where it is difficult. So when I need it and possible I put it in the ON position.

Don't touch R9 - R75 - C42

On TDI SLs manufactured after 2015, D20, D21 and C56 are absent (modification taken into account by Whites). On those before 2015 they are present.

 

I also installed a potentiometer for adjusting the audio volume (like on the Beach Hunter)

You did not use a resistor? This mod is stumping me. I've been told all sorts of tech stuff I don't understand. "across" "covers" "opamp abuse". Thanks