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 ;-)

Monday, November 21, 2022

SWR/PWR meter repair - hold your breath!

 So unfortunate I coincidentally dropped met precious Welz SP-15M SWR/Power meter. From the outside there was no real damage. But unfortunate the meter's pointer wouldn't move when applying power to it. I bought this meter in 2011 on a radio rally and it went with me at several portable occassions. It is a nice and accurate meter excellent for QRP and I don't want to lose it. So searching for a solution I arrived at the TRX lab youtube channel about the repair of a Diamond SX 600. At 4.05 minutes Peter shows a easy way to measure the mechanical voltmeter itself. So, I tried. Between the terminals my meter showed resistance. The meter is not stuck ( you find it on the internet as most common fault). But applying voltage on it with both the multimeter and a 1,5V battery the voltmeter still didn't show any movement. I also measured if the voltage pickup was correct when I was TXing and that showed that there was nothing wrong on that side. Not shure what to do now....I asked Peter and got the answer it could be a mechanical issue. Of course, there are not much electronics in a simple SWR/PWR meter. It consists of some coils, diodes, switches and a voltmeter. What else could it be?

After a good night sleep I was thinking that the fault could only be at the volt meter itself. For that I had to take it out of the housing. First of all I desoldered the wires and the capacitor that was soldered over both terminals, not the easiest job since wires were wrapped around the terminals. After some effort I got the bare meter in my hands and I could finally measure the meter again showed no resistance this time! To research the fault I had to take it apart. A very delicate job with my thick fingers. But as you can see on the photo I managed it.

Next thing was examine the actual inside for any damage with a magnifier since this is really something small. Luckely my phone has a excellent magnifier as well and I managed to picture the problem.

 Can you spot the problem? It is the small spring which is not connected to the terminal anymore. Remember this is very tiny. Personally I can only see it with a magnifier. To solder it you need a steady hand and hold your breath. At first I soldered it but the spring had moved in a wrong position. At a second attempt I got it right. A quick measurement with the multimeter showed resistance again and I saw the meter move as well. Now it was just a case of rebuild the thing and test it.

A quick test with my CB radio which previously showed that it has 3,5W out . After the repair nothing changed so I guess I don't have to calibrate the meter. Now I really need to be more carefull with my equipment. However it is a miracle this didn't happen before since portable use isn't always the best for this kind of meters. I might search for a digital alternative to bring with me in the field and keep this meter in my shack.

Sunday, November 20, 2022

Friese 11 steden contest 2022

 Since this is a primarily local contest I'll write this post in the Dutch language. If interested you're welcome to translate it with google in the language you prefer.

Vorige week was ik de PA-beker contest vergeten. De Friese 11 steden contest wou ik zeker niet missen. Dus toch maar in de agenda gezet en alles voorbereid. Voor degenen die het nog niet weten, de Friese 11 steden contest heeft natuurlijk te maken met de legendarische schaatstocht. Het doel in deze contest is zoveel mogelijk stations te werken waarbij rapport, QSL regio en plaatsnaam als uitwisseling geldt. De multipliers zijn de 11 steden van de beroemde schaatstocht en als 12e multiplier de kluunplaats Bartlehiem. 

Dit jaar werkte ik met de Icom IC-7300, 100W, Heil Proset Elite Ic headset en de antenne de inverted-V doublet.  Dit werkt al jaren prima en het was nu zeker niet anders. Wat mij opviel dit keer was dat er veel amateurs klaagden over veel storing en dan wel zo erg dat het hun ontvangst haast onmogelijk maakte. Zo hoorde ik PA1AT uit Yde die al na een paar QSO moest stoppen omdat hij gewoon niks meer ontving. Daarnaast was het aantal deelnemers gering, waarschijnlijk te wijten aan sport op TV? Het is maar wat je belangrijker vind. Zelf heb ik vrij snel 11 multipliers kunnen werken, alleen Hindelopen ontbrak. Waarschijnlijk was er geen amateur in of bij Hindelopen die aktief kon zijn op 80m, erg jammer, maar volgend jaar beter. Leuk om weer veel bekenden te werken, veel amateurs doen elk jaar mee. Het meest vreemde voorval deze contest was een PD amateur die mij zo nodig tot 2x toe een rapport moest geven. Voor alle duidelijkheid de amateur in kwestie is in overtreding en ik mag volgens de regels niet eens een QSO met hem maken wat ik ook niet gedaan heb. Ik ben niet van agentschap telecom en ook niet van de politie dus ik geef er verder geen commentaar op. Alles bij elkaar was het toch een leuke gezellige contest en kijk ik weer uit naar de volgende keer.