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[aprssig] Kenwood TH-D7 GPS connector grounding

Dave Baxter Dave at emv.co.uk
Fri Sep 12 12:46:55 UTC 2008

My collection of TH-D7 schematics and other info can be found at....

ftp://Ham@g8kbv.homeip.net:12021/TH-D7/  no password is needed, if your
browser or client asks for one, just enter a blank one.  You have
read/download only rights.

Schematic8.jpg is the one with the power/battery/charge system...


Dave G0WBX.

> -----Original Message-----
> From: Greg D. [mailto:ko6th_greg at hotmail.com] 
> Sent: 12 September 2008 06:54
> To: 'TAPR APRS Mailing List'
> Subject: [aprssig] Kenwood TH-D7 GPS connector grounding
> Hi folks,
> So, I'm still working on mounting a GPS unit on top of my 
> TH-D7.  I have it all running, somewhat lashed-up, and it's 
> great not having all sorts of little things strung together.  
> Currently the GPS is being powered by 4xAAA 900mAh NiMH 
> cells, and they seem to work great.  For the curious, pictures at:
> http://home.wavecable.com/~ko6th/Ham_Radio-IMG_0232.jpg and 
> http://home.wavecable.com/~ko6th/Ham_Radio-IMG_0233.jpg.  The 
> GPS unit is an EM-406A from Sparkfun.com; runs on 4.5 to 
> 6.5v.  The TTL output is inverted with an NPN transistor, and 
> then direct into the TH-D7.
> But, batteries make the attachment too big, and need to be 
> charged separately from the unit.  So, I want to tap into the 
> main battery pack, which is a Batteries America BT-11h with 
> 6xAA 2700mAh NiMH batteries.  Then I can mount just the GPS 
> part of the board on the radio.
> But, I want to do this safely.
> That's the puzzle.  I took an ohmmeter to the unit, to be 
> sure that I'm not introducing a ground loop by doing this.  I 
> expected that the ground side of the GPS port on the side of 
> the radio would be tied to the negative side of the battery.  
> Instead I get no connection at all.  Infinite resistance.  I 
> guess it's possible that they went to the trouble of 
> isolating the two; perhaps they had to.  But my mental 
> warning alarm is going off...
> Does anyone know if grounding the GPS port common terminal to 
> the battery negative will cause a problem?  My plan is to tap 
> into the BT-11h, picking up the negative from the top of the 
> right-hand column of cells (as you look at the pack with the 
> lid open); that's the same as the negative of the overall 
> pack.  Then, 4 cells-worth up the 6-cell string is right next 
> to it, at the top of the middle column.  Easy to pick that up 
> too.  Drill a small hole, and out come the wires with about 
> 5v on them.  The result will be a little uneven in the 
> discharge, with an extra 50ma load on 4 of the 6 batteries 
> when the GPS is on, but I figure that overall it won't be 
> noticed.  The alternative is to add a regulator to the GPS 
> board, but then I'd need to dig deep into the battery pack to 
> get at the positive terminal.  I like simple.
> But either way, the negative of the battery pack will be 
> connected to the ground side of the GPS port.  Is that going to be ok?
> Thanks,
> Greg  KO6TH
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> re_092008
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