[Elecraft] Re W2 Wattmeter failure

[Roger Crofts]

Thanks Alan for your thoughts. I appreciate your interest. I too was surprised that 50 watts could damage the op-amps and I was looking for other causes. The W2 was operating OK before the 630m adventure and it certainly was not after. The W2 power supply is a series regulator type and the output is in parallel with a 12V car battery, which also powers the K3. Not much chance of transients getting through that. I do have another theory which is untested. If the input pick-up coils, in conjunction with C1 or C2, were to form a resonant circuit on, or near, 475 KHz, you could have the possibility of much higher RF voltages appearing in unexpected places. I need to check the inductance of the coils to see if that is a possibility.
Roger, VK4YB

On 13 May 2015 Alan N1AL wrote:

I'm looking at the W2 kilowatt sensor schematic and scratching my head 
trying to figure out how that could have happened.  There are 49.9k 
resistors in series with all the op amp inputs, so there would have to 
be hundreds of volts coming out of the detectors to damage the devices. 
  Plus the fact that all 4 op amps (actually two dual op amps) were 
destroyed argues against that as well.

I would suspect the 5V op-amp power supply.  Perhaps there was a failure 
in the W2 power supply or maybe a spike on the supply.  A large voltage 
transient causing a current spike in the sensor cable might do it, 
although it is hard to imagine 50W at 475 kHz doing that.

Alan N1AL


On 05/12/2015 11:21 PM, Roger Crofts wrote:
> I made a rare QSO on 630 metres the other day. My W2 wattmeter was
> in-line between the 630m transmitter and the 630m tuner. I was not
> using the W2, because my 630m transmitter has its own internal SWR
> meter. When I returned to normal HF operation, I noticed the W2 was
> not working. I discovered all four unity gain op-amps in the W2
> directional coupler were destroyed. I replaced the op-amps and the W2
> operated normally again. The W2 wattmeter is not intended to operate
> below 1.8 MHz, but it does have a 2KW rating, so one could be
> forgiven for assuming it could handle 50 watts at 475 KHz. Next time
> I operate on 630 metres, I will by-pass the W2. Roger, VK4YB