I recently built a simple signal-strength meter for my Hallicrafters S-40B shortwave, tube-type receiver. Essentially, an S-meter responds to the AVC voltage, much like a tuning eye.

With AVC "on," it's difficult to peak an AM signal by ear, but it's relatively easy with an S-meter.

This is important to me because I have also added a digital frequency counter to my set. To accurately mesure the frequency, one needs to get the RF signal peaked precisely.

My S-40B has, in addition to the S-meter and the digital frequency counter, an antenna matching tuner. For what I've spent, I could have two digital communications receivers. But I persist.
Doug

I have a couple 50 DC microamp old Micronta meters, and a 1 Ma as well.
If I wanted to covert one of these to a 150Ma meter, is it as simple a using ohms law and running a resistor in series with it?

Frank, no - what you need to do is add a small value resistor in parallel with the meter, so that its effective shunt has a lower resistance. This will increase the meter's maximum scale.

Putting a resistor in series with an ammeter won't change its response. It'll still display the current current through it as before.

You can measure the resistance of the original shunt, but the ohmmeter will peg the ammeter, which might very well be ruined. Better to install another ammeter and load resistor in series with the 1-mA meter and play with additional shunts until the 1-mA meter reads the way you want. Or, you can disassemble the meter, and disconnect the meter from its shunt before measuring the shunt's resistance.

For my S-meter, I'm using a 1-mA DC meter with an added shunt to make it behave like a 5-mA meter, per the Hallicrafters' S-meter service bulletin on BAMA.
Doug

:I have a couple 50 DC microamp old Micronta meters, and a 1 Ma as well.
:If I wanted to covert one of these to a 150Ma meter, is it as simple a using ohms law and running a resistor in series with it?

Deux parte:

Sir Douglas:
I have been exclusively using AVC line monitoring for my alignments of communications and ham receivers
dating waaaay back to the forties. Just a tapping onto the AVC way back right at the detectors output
with a high Z metering device, such as to have minimal interference or any loading down of that inherently high Z line itself. Then the tuneup is optimized up to the point of no further differentiation, THENNNNN you can kick in the afterburners, by going forward on the AVC line to the point where the typical 1-4 meg filter/dropping/isolation resistor is and then go towards its front end ...IF,mixer, RF supply end....... and ground that point.
Then , the resultant gain enhancement will shift the developed AVC voltage upwards of another decade.
Thus making further alignment adjustments being.... enhanced /as discriminating as... less than a slots
width in a tuning capacitors standard screw head. Or even trickier in rocking back and forth in a slotted
slug adjustment of the pot core of an IF transformer....touchier yet on a hex slot incorporating a diddle stick being used for its adjustment..
Sounds like your outboard goodies just might be as massive as your '40's propers dimensions !

Sir Franklin
Your described manner would be making a voltmeter, to be making a Ammeter/ milliameter/microammeter
(of lesser sensitivity) you would be wanting to shunt the meter with the appropriate resistance. In that
manner the current passage would then be proportionally/ratiometrically divided between the two units.
And with that pricier...more desirable 50 "micropeter elephant" it would certainly be hoarded by me until
an application that required that degree of sensitivity of movement. Utilize one of your available other "current hogs" in the make up of a developed 150 ma rating.....e,g, 100 ma, 50 ma, 10 ma, 5 ma, 1 ma 500 ua...whatchagot ?

73's de Edd

OK, Edd, responding to your comment, here is a link to a photo of my Hallicrafters S-40B, with all of its accoutrements: http://img408.imageshack.us/img408/4853/hallis40bandperipheralsqd8.jpg

On top of the receiver, left to right, are: MFJ antenna tuner, S-meter, and DD-103 digital frequency counter. It's a bit like putting a saddle on a sow.

The antenna tuner really does help. I'm using about a 20' outdoor longwire. The MFJ tuner has two adjusments, capacitance and inductance.

The S-meter is homemade, per the Hallicrafters' tech bulletin on BAMA. It doesn't measure AVC voltage, per se - instead it measures the plate current of the 1st RF stage (which is, itself, controlled by AVC voltage). I mounted the ammeter upside down since an S-meter reads backwards. I penciled in S-units on the dial card - in the photo, it's reading about S-5.

The digital freq meter in the photo is displaying 10000.9 kHz. I had WWV (at 4:00pm CST, in Chicago), and my meter was off only 0.9 kHz. The frequency meter holds tight on all four bands of the S-40.
Doug

:Sir Douglas:
:
:Sounds like your outboard goodies just might be as massive as your '40's propers dimensions !
:
:73's de Edd

:OK, Edd, responding to your comment, here is a link to a photo of my Hallicrafters S-40B, with all of its accoutrements: http://img408.imageshack.us/img408/4853/hallis40bandperipheralsqd8.jpg
:
:On top of the receiver, left to right, are: MFJ antenna tuner, S-meter, and DD-103 digital frequency counter. It's a bit like putting a saddle on a sow.
:
:The antenna tuner really does help. I'm using about a 20' outdoor longwire. The MFJ tuner has two adjusments, capacitance and inductance.
:
:The S-meter is homemade, per the Hallicrafters' tech bulletin on BAMA. It doesn't measure AVC voltage, per se - instead it measures the plate current of the 1st RF stage (which is, itself, controlled by AVC voltage). I mounted the ammeter upside down since an S-meter reads backwards. I penciled in S-units on the dial card - in the photo, it's reading about S-5.
:
:The digital freq meter in the photo is displaying 10000.9 kHz. I had WWV (at 4:00pm CST, in Chicago), and my meter was off only 0.9 kHz. The frequency meter holds tight on all four bands of the S-40.
:Doug
:
::Sir Douglas:
::
::Sounds like your outboard goodies just might be as massive as your '40's propers dimensions !
::
::73's de Edd

Sir Douglas, Esq:
While we are on the subject of your S-40B, what was the source of the CIA broadcasts you were geting on it? I am sure you have identified the source by now, but we've heard zip from you. Did you find something you didn't want us to know about? Hmmmmmmm...?
Lewis L.
P. S. I have a S-77 someone gave me, it is in the basement. I think I will bring it upstairs and see what I can find out there in radio land.

LL

Lewis: The mysterious RF source is somewhere in my basement, where I have printers, copiers, radios, stereo, computers, modems, fax machine, fluorescents, multiple wall-warts, etc. (When I trip the basement ckt breaker, the weird signal disappears, but I haven't pinpointed the source beyond that.)

Once I determined that the source was not the CIA or other clandestine organization, I lost interest. You must be very disappointed, just like I was.
Doug

:While we are on the subject of your S-40B, what was the source of the CIA broadcasts you were geting on it? I am sure you have identified the source by now, but we've heard zip from you. Did you find something you didn't want us to know about? Hmmmmmmm...?
:Lewis L.

:Lewis: The mysterious RF source is somewhere in my basement, where I have printers, copiers, radios, stereo, computers, modems, fax machine, fluorescents, multiple wall-warts, etc. (When I trip the basement ckt breaker, the weird signal disappears, but I haven't pinpointed the source beyond that.)
:
:Once I determined that the source was not the CIA or other clandestine organization, I lost interest. You must be very disappointed, just like I was.
:Doug

I had a similar problem, except mine blocked a station I like on the AM band, all over the house. Turned out it was a touch-dimmer screwed into my bedside reading lamp. I meant to take the thing apart and see why it put out a carrier on 940, I think it was, but I never got one of those "round tuit"s, I guess you know what i mean.

Lewis

:
:
::While we are on the subject of your S-40B, what was the source of the CIA broadcasts you were geting on it? I am sure you have identified the source by now, but we've heard zip from you. Did you find something you didn't want us to know about? Hmmmmmmm...?
::Lewis L.
: