In message , Jeff Liebermann
writes



One problem with the thermocouple ammeter. It's slow.

The other problem with the thermocouple ammeter is that it's easy to
burn out the thermocouple.

If you come across one in a sale, before you part with your money first
twist the meter enthusiastically from side to side, and note how the
needle swings. It should be reluctant to move - ie it's well damped by
the almost short circuit of the thermocouple. If it swings freely, it
almost certainly means that the thermocouple is open-circuit.

But if you've been unfortunate to buy a duffer, don't totally despair.
With a simple internal rewire, at least you'll have a usable* 500uA or
1mA FSD moving coil meter.
*Convert to diode type?

.




--
Ian

On Mon, 6 Jul 2015 20:33:29 +0100, Ian Jackson
wrote:

In message , Jeff Liebermann
writes



One problem with the thermocouple ammeter. It's slow.

The other problem with the thermocouple ammeter is that it's easy to
burn out the thermocouple.

Yep. All mine were guaranteed to be fried when I obtained them for
free. However, they can be repaired. I've only done one successfully
so far:
http://www.tuberadio.com/robinson/Thermocouples/

But if you've been unfortunate to buy a duffer, don't totally despair.
With a simple internal rewire, at least you'll have a usable* 500uA or
1mA FSD moving coil meter.
*Convert to diode type?

That's the most common solution:
http://k4che.com/GO-9%20Transmitter/RF%20Ammeters/RF%20Ammeters.htm
I had problems with RF leakage being rectified by the diode resulting
in inaccurate readings. At least the typical WWII ammeter is well
shielded. Calibrating the scale is impossible but replacing the meter
scale is easy:
http://www.tonnesoftware.com/meter2.html

--
Jeff Liebermann
150 Felker St #D http://www.LearnByDestroying.com
Santa Cruz CA 95060 http://802.11junk.com
Skype: JeffLiebermann AE6KS 831-336-2558