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[aprssig] Kenwood TH-D7 GPS connector grounding

Greg D. ko6th_greg at hotmail.com
Sat Sep 13 05:42:42 UTC 2008

Hi all - Dave, Bob, Patrick, Steve,

Wow, this is a bit more complicated than I thought it would be.  A couple of items:

1.  I goofed on my measurement between the battery and GPS connection, assuming (yeah...) that the exposed outside of the GPS plug would be ground.  Nope.  Just mounting, and not connected.  The actual ground side is just a millimeter back inside the throat of the connector.  And, yes, that is connected to chassis ground and the ground side of the power plug.  Glad I goofed, or I wouldn't have asked...

2.  Ok, so I see (thanks for the schematics!) that when you put a power adapter in, the battery is disconnected from the negative side.  So, that certainly would mess things up if I wanted to run the whole contraption from the car adapter or a wall wart.  In my original plan, I'd be re-connecting the two, which would put the full 13.8v across the 7.something volt battery pack, through a diode (D3 on page 8).  Probably fry the diode, and if it didn't, it'd cook the batteries or something else (me!).

3.  So, that diode (D3) also prevents any juice from the battery from being available at the power plug.  But, I think this means that if I can get a wire to the positive side of the battery pack (way down deep inside), I can get juice from either the battery or external power (through the battery pack connection to the radio, since they're always connected), and I can pick up the ground side from the ground side of the GPS plug (NOT the battery pack!).  Yes?  And, at that voltage, I'll need a 5v regulator; a simple 3-terminal one should do.

4.  If I get power from the full pack, that solves the imbalanced discharge (4 of 6 cells) that I proposed earlier.  I did a quick measurement, and the radio only draws about 60ma, just sitting there.  It climbs to over 100ma with the TNC on (I forget the exact value), but yes, the GPS is almost half of the power.  Too much to be ignored, that's for sure.

5.  The diode in the BT-11h pack is already on my list for being reworked.  I'd really like the option of charging the AA cells from the radio, even if it takes days.  And, at 70ma into a 2700 mAh battery (that's C/38, if I did the math right), I can let them overcharge for a while without doing any damage.  So, it appears that the biggest challenge in this whole project is going to be the mechanical parts - getting inside the battery pack to access the positive terminal, and optionally  jumper across the diode.  And I still need a better way to mount the GPS on top of the radio.

Thanks to all for the help!

Greg  KO6TH

> From: bruninga at usna.edu
> To: aprssig at lists.tapr.org
> Date: Fri, 12 Sep 2008 09:05:01 -0400
> Subject: Re: [aprssig] Kenwood TH-D7 GPS connector grounding
> Very interesting!  
> Making your own top-mount GPS for the D7 just like the new
> VX8R...
> If I understand what you are suggesting...
> Using a tiny GPS like the one from Byonics, taping off the D7 +V
> from the existing D7 external power connector center pin, and
> GND via the GPS shield connector (and having a 5v regulator
> somehwere in line..) 
>> So, I'm still working on mounting a GPS unit 
>> on top of my TH-D7.  For the curious, pictures at:
>> http://home.wavecable.com/~ko6th/Ham_Radio-IMG_0232.jpg  
>> 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....
>>... So, I want to tap into the 
>> main battery pack...
> Bob,wb4apr
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