[FoxHunt] HTX202 S-meter

Sat, 22 Jun 2002 12:46:32 -0700 (PDT)

Hi Dave,
 Sorry I couldn't respond sooner.
--- Dave Willey <[email protected]> wrote:
> Hi Bill,
> 
> Thank you for putting up the details of your S-meter
> tap point.
> It looks like this is the approach I'll take for my
> audio S-meter.
> 
> I do have a few questions.......
> Hee Hee, I know enough electronics to be dangerous.
> So bare with me if they seem elementary.
> 
> 
> > Date: Fri, 7 Jun 2002 13:59:19 -0700 (PDT)
> > From: Bill PIENUPS <[email protected]>
> > Subject: Re: [FoxHunt] HTX-202 Audio S-Meter Mod 
> 
> 
> > I have been using the HTX-202 for T-hunting,I take
> > the s-meter signal from pin 13 of U4.The output at
> > this point is a current source that is converted
> to a
> > voltage by R33 (33k).The voltage is about +.47
> volts
> > with no signal and about +1.5 volts with max
> signal.
> 
> I assuming that you mean the voltage is across R33,
> rather than the hot side of R33 & ground?

One side of r33 is gnd. The voltage is the hot side of
r33 (pin 13 of IC4) referenced to gnd.

> 
> > use a single op amp to null out the +.47 volt
> offset
> > at no signal,and then the amp drives a meter for
> the
> > signal strength indicator.You could use the op amp
> to
> > drive an other input instead of the meter. It has
> > about 70 db of useful dynamic range. 
> 
> How did you null the +.47V? I'm thinking the op amp
> is isolated from the radio ground.  Or is there a 
> voltage divider  between gnd & the +DC & your 
> tapping the +.47 V to the negative input of  the 
> op amp.  What am I missing?

The op amp is a single supply amp. Vee=gnd and
Vcc=+5V.

>  
> So what op amp did you use?

LM324

> 
> Is it a single DC supply off of the radio's DC
> supply?
> Or is it a dual +- supply to the op amp.

Single supply from radio's regulated +5V.

> 
> > When the signal
> > gets strong I attenuate the signal by cutting the
> > voltage to the receiver front end (Q8,Q9,and
> Q10),from
> > 4.9volts to .62volts.
> 
> By what method? A pot, or a fixed value resistor, 
> or some sort of L-pad?

I switch in a 1N749A and 1N4711 diode in series with
the +V to the rcvr front end. The necessary drop was
determined experimentally to give me some overlap on
the two meter ranges when in the attenuater mode.

> 
> > The mike jack became my output
> > jack and I used the Hi-Lo button for my attenuator
> > switch. Digging into the ht to cut
> track,solder,etc
> > wasn't a lot of fun, but the finished product has
> been
> > very effective. I hope this helps.
> 
> > Bill Pienups K1BP
> 
> Thanks for taking the time to answer these
> questions!
> 
> 73 & Happy Hunting, 
> 
> Dave Willey - KD6KWM
> 
> 

Dave, I don't know if this list allows attaching
pictures, I don't remember seeing any. I will attemp
to describe the meter circuit: LM324 pin4=+5V.   
pin11=gnd.    pin3=input from pin 13 of IC4.   
pin2=47k res to pin1,47k res to wiper of "zero pot"
(5K pot).   pin1=39k to wiper of "full scale pot" (20K
pot).   cw end of fs pot=+side of 50ua meter.   -side
of meter=gnd.   cw end of zero pot=gnd.   ccw end of
zero pot=12k to +5V.
A messy way to send a schematic. HI. 

My df unit is actually configured as a combination
TDOA or wide range field strenght meter, with the
electronic box bolted to the case of the HTX-202. 
 On my unit the meter starts to indicate at -125dbm,
full scale is -59dbm. In the attenuate mode the meter
range is -67dbm to -7dbm. My bar graph graph worked
from -113dbm to -98dbm. This will give you an idea of
what to expect. Your results may vary. HI. If you have
any more questions feel free to ask.

      73   Bill Pienups K1BP   

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