On Mar 5, 3:26 pm, (Chris Suslowicz) wrote:
In article .com,

"terryS" wrote:
Thanks Chris and others. Part of what I was wondering is a seemingly
'dud' RF ammeter. The movement might be OK and could be, say, 1 or 5
milliamp etc. fsd.

Ah, right. In that case:

The ex-RAF meters (marked A (crown) M for Air Ministry) have the resistance
wire soldered directly to the back of the terminals on the case. The
thermocouple is then spotwelded to the middle of the resistance wire,
and one lead goes to one side of the movement directly, the other goes
via a "coil spring" of nichrome resistance wire (length cut to get the
thermocouple calibration correct) to the other side of the movement.

If it's open circuit across the terminals, then the resistance element
has blown. The thermocouple and movement should still be perfectly OK.

(I've considered trying to replace the resistance wire but calibrating
it will be the difficult part.)

Somewhere I have a box of dead meters....

I may be some time.

Chris.

--
Most uses of "spider" have noninsect meanings. -- Christine Malcom-Dept.

Back in the old marine electronics days we had those 410 to 512 Kcs MF
transmitters. They were all required to have built in RF Ammeters. The
transmitters had to meet minimum SOLAS radiation requirements by
multiplying the RF current by the antenna height above the deepest
water line. Usually they were 15 amps or 5 amps fsd (15 for the main
tx and 5 for the emergency tx). These meters were pretty accurate on
mf but some of the transmitters used them on hf also. On hf they were
only accurate depending on the length of antenna/frequency, where the
current node is. We are talking a single wire type feeder. At mf the
current is pretty much the same anywhere on the antenna due to the
long wavelength. I still see some of these meters being sold at
hamfests. Virtually every RF Ammeter used had a built-in shunt. Randy,
wb5kcm