Friday, November 25, 2022

The QRP(p) powermeter revisited and more meter repairs


  8 years ago I did need a QRPp powermeter to measure an output of approx 10mW to use on WSPR. The original article on my blog showed a foggy photo and it was a job in a rush. In my current SWR/power meter repair I needed some resistors that were luckely still in this meter. So I decided to use them for the broken Stabo SWR/Power meter and rebuild the QRP(p) meter a bit. I also made better photos. I disabled the SWR meter function and only use the power meter part. To calibrate I use my FT-817 at 1W and adjust with the potmeter to full scale. Then I adjust audio to 100mW output and adjust the potmeter to fullscale again. After that I adjust audio till 10mW can be read on the meter. I realize this is an indication but I don't want to use attenuators because it would limit my receive as well.  At the moment I'm researching the possebility to experiment on 40MHz WSPR/FT8 with 10mW ERP without a license after some blogposts from G3XBM about this matter. 

  Back to the repairs. I bought a nice Stabo SWR/Power meter on the last radiorally I visited. Unfortunately this device was defective, I could read forward power but the pointer didn't move when I wanted to see my actual SWR or reflected power. At first I measured all diodes, all did show correct readings. Next I measured the switch, it was ok. Next the potmeters, I noticed on didn't have any resistance at all. After removing the thing I noticed why, it was broken. Now searching for 200 Ohm potmeters in my pile of electronic waste I could not find any good ones, most are in the K ohm range. So that is the point I decided to get them from my QRPp meter.

  This is the SWR bridge from the QRPp meter. It contains two 130 Ohm resistors. However the Stabo meter use 2 potmeters of 200 Ohm. I removed the resistors and soldered them in the Stabo meter. Readings are not correct. With a 50 Ohm load I read a SWR of 1,8:1. I reverse the input/output leads (TX signal at antenna side) to see of it would change but same reading there as well. 

  I think that is is a matter of a poorly designed pick-up transformer, actually no transformer as well. The signal pick-up is just 2 wires wrapped around the tube that is connected to the inner side of the connectors. You see this often in cheap SWR meters. Wish I could exchange the bridge for the one from the QRPp meter because that one looks a lot better and I know it is very accurate. Anyway in the end I replaced the resistor at the side of the reflected power pick up with the 200 Ohm potmeter that was still ok. 

  This time I could calibrate SWR almost 1:1 with a 50 Ohm load and 1:3 with a 150 Ohm load. Besides that I had to calibrate the power meter as well. It showed 3,5W when 5W was applied. Still it is not very accurate, even not after the calibration. I know the output of my FT-817 is pretty right what is should do. If I measure it with the Welz powermeter it shows right 0,5 - 1 -2,5 and 5W. With the Stabo it shows 0,6 - 1,2 - 2 and 4,8W.

  But at least it is repaired and I could use or sell it now.

Here the picture from both meters. Well, analogue meters might be a bit outdated. But it was fun to be able to repair something.

Then I remembered I had another broken meter. Well, it is a case of a meter that I never use. It's a Daiwa VHF/UHF SWR meter. Whatever power I put in both meters didn't move. I did a quick check on the pointers of both meters, they moved allright. So, easy as that something blocked the signal. Then I remembered I did a quick check before about 4 years ago and that revealed both switches are just very bad. Probabely corrosion because I actually never use this meter. I decided to use contact cleaner and compressed air to blow any corrosion off as far as it is possible from the out side.

Well, it worked well after some fiddling with the switches/air and contactcleaner. I measured all contacts and it was working again. Connecting the FT-817 showed movement of both meters again. Hopefully it will stay in good condition now. Or may be I should leave a note inside in case I have to open this meter again in about 10 years ;-)

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