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.


VE9KK said...

Good morning Bas, very nice repair and a nice steady hand to get the soldering done. I am like you in that I find stepping away gives me time to think things over and come up with a solution.

Bert, PA1B said...

Hallo Bas, Mooie foto. Inderdaad je zoekt je kleurenblind. Hi. Maar het is gelukt en zeker de moeite waard. 73, Bert

PE4BAS, Bas said...

@VE9KK tnx for the comment Mike. Yes, over the years I noticed a good nights sleep is better to think about a problem or in other words a challenge. In my early years I would have been in panic but not anymore. Although sometimes I feel like that after something broke. I'm happy I was able to repair this meter.

@PA1B Hallo Bert, ja zonder vergrootglas is het niet te doen. Vroeger was mijn zicht echt veel en veel beter. Maar als je de 50 gepasseerd bent gaat het snel achteruit.

73, Bas