Whites TDI SL 5 Battery 20 Volt Pack. What Have I Done...?

[CaliGold]

I built a 5 battery 20v pack for my whites TDI SL. Got info from this forum.  I think I blew a resistor maybe more. No obvious visual damage but the detector behaves strangely now and signal is delayed on targets. If I put a target to the coil (miner john 8x12 stock) then the signal is delayed from sounding off for maybe 1/4 second or so. Odd... The miner john was handling the 19.8 volts fine but when I put the Coiltek 14" "TDI Series" coil on my troubles began. Wild threshold swings, wont pick up small targets (the 14" wasnt good with them anyhow) and now the MJ 8x12 has same delayed signal sound and struggles to  pick up a 1.5 gram gold cross I used to test it with. The detector "works" it's just wonky and no use for prospecting now... Any help or thoughts greatly appreciated as well if anyone knows the wattage on the small 2512 size resistor at R9 area and around there I'd greatly appreciate the knowledge.. I've used the detector before in all areas it acted strangely in so please rule out EMI... I'm sort of in the country and it's relatively low signals around here despite the housing and especially in the river bed near my house. I think the Coiltek jacked up my PCB parts somehow. 

 

Thanks and have fun laughing at my rookie mistake! 

[CaliGold]

Please note I reduced the battery back to 4-18650 cells and even at around 15.8 volts the behavior is  Still strange with the detector.... 

[CaliGold]

This is the board AFTER the coiltek install and issues began. I pulled it to look for any burned or bulging components. Looks identical to the full board before I ran the coiltek with the 19.8v battery. 

[Geotech]

3 components are subject to overheating: Q4, R9, and R14. R9 should be 1000Ω and R14 = 680Ω, I think they are both 1W. Most likely Q4 has been partially damaged, enough to alter performance but not enough to kill it. When jacking up the battery voltage these 3 components really should be upgraded for add'l power handling.

[CaliGold]

14 minutes ago, Geotech said:

3 components are subject to overheating: Q4, R9, and R14. R9 should be 1000Ω and R14 = 680Ω, I think they are both 1W. Most likely Q4 has been partially damaged, enough to alter performance but not enough to kill it. When jacking up the battery voltage these 3 components really should be upgraded for add'l power handling.

Thanks for the insight. You're the man! I'm assuming by upgrading you mean more like 2-3 watts and higher temp range on the resistors. ...and I really wouldn't know for the Q4 IRF740S mosfet...

[geof_junk]

There is a section in this ....LINK....click here.... about Mosfet in metal detectors.

 

 

https://www.lammertbies.nl/electronics/pi-metal-detector

[Geotech]

Yes, 2-3W on the resistors and at least a heat sink on Q4.

[CaliGold]

13 hours ago, Geotech said:

Yes, 2-3W on the resistors and at least a heat sink on Q4.

            Size 

Many thanks.

10x10mm heatsink, 2512 size 2 watt resistors for R9, R14, and two options for replacement of Q4 D2PAK Mosfet coming Tues to try to restore to normal function...  One mosfet is of the same spec and one of what I hope I'm not erroneously assuming is higher function. I put pics of the specs on the 2 inbound mosfets. Thanks for all your help again everyone, especially @Geotech

[CaliGold]

Welp. I need to practice soldering.... I was able to install the parts and the detector works well now. The stock miner john 8x12 coild is behaving nicely and targets seem to be as they should be. May melt some sluiced fine gold into .25 and .10 gram gold pieces to test detector as I dont have any small gold pieces or bullion below 2 grams to test. I will be installing a sadie coil 8x6 asap so excited to test that on the bedrock with 16 volts. Not going back to 20volts. Will probably test the 14"coiltek tdi series coild with the 16v and if it doesnt kill the mosfet again I'll just stay at 16v until further notice. Thanks everyone for rehashing all this with me. Learned a lot the past week about soldering, flux(and cleaning after re-work), solder pads on pcb's and all kinds of stuff. I have spair resistors and mosfet for aforementioned replacements so if my handy work doesn't hold up I'll replace them again with nicer solder work. I had to splice mosfet smaller pad to the trace as it broke during removal. Fun Fun... Thank God it works for now... More testing to come hopefully I can post some scenery. 

[CaliGold]

Thanks again to @Geotech with the advice. My machine is working good for now. Tested a 20 swiss francs buried at 6 or so inches and worked with both coils no problem on 15.5V battery (drained a bit past day or so). Will probably run right at 16 volts. And bring it drained or pack a energizer Lithium AA pack for the Coiltek for  is. Picked that metal fence scrap up about the same distance / was screaming. So glad its back in working order. I forgot the pin for the coiltek to fasten on to shaft but it worked and ground balanced by hand. I live near coast but I'll be back at Motherlode, Kern, or Angeles area soon and I'll post some Au finds. We're getting into late Summer here and Fall is gonna be NICE! Water a little high right now in some of my favorite bedrock...

(yes I took care not to get dirt in that Coiltek as it is in mid-retaping right now... 

Take Care ALL!