OK, back to the beginning for a minute. When I started playing with the Metcal, I put together an adapter to fit an STTC soldering tip to some coax fitted to a connector that will plug into the Metcal. What I found was that for the RFG-30, the tip heated up, but the RF waveform was an oscillating mess. I also have a Metcal PS2E-01 that was in an unknown condition. When I put my makeshift tip on it, the tip got so hot that it oxidized the end and it would no longer tin with solder. I thought both of the Metcal power supplies were bad and took apart the RFG-30 first.
With the help of the EEVblog forum and my investigations I now know what happened. From disassembly pictures of the MX-RM3E wand it is clear that there is some impedance matching in the wand. My hookup without matching caused the problems that I was seeing and both power supply units that I have are actually working OK. When I finally realized this, rather than buy a MX-RM3E (too cheap), I decided to determine the required matching and build a useful wand.
Here is a picture of a disassembled wand from poster "Burning Tantalum" that shows a paralleled inductor and capacitor.
Warmup Reflection.7z (1013.99 kB - downloaded 89 times.)
Several people have identified the 22 uH inductor, but I have not found anyone who identified the capacitor. When I realized that the inductor has an impedance of about j1900 at 13.65 MHz. it was clear that that was not doing any matching. It is just there to provide a DC path to the tip for the later power supplies that can sense the presence of the tip. The capacitor is the matching element that series resonates with the heating coil in the tip. The Metcal patent of the wand also clearly shows a series capacitor. Impedance measurements of a tip and wand are shown in a website at
http://randomfunprojects.co.uk/metcal.html that I found by reading one of the posts on this forum. The essential information in the measurements is that as the tip heats up, the impedance briefly becomes an almost perfect 50 Ohms.
Since I could find no one who listed the capacitor value, I had to determine the value by trial and error. I wish that I had kept some old variable tuning capaciators, but no such luck. So I just kept trying different values until I found the value that worked. My method was to insert my directional coupler in the line and watch for a null in the reflected power as the tip heats up. This took some time but in the end I found that 180 pF gave a warmup that goes through 2 resonances and finally settles with reduced forward power and high reflected power at idle. During warmup and at idle the RFG-30 has no oscillating resets like it did without the matching capacitor. Adding a 22 uH choke across the capacitor had no effect on the warmup measurement. I tried all the different tiips that I have and all showed the same warmup response. The tips do not seem to be overheating.
I made a short video of the scope during the warmup cycle. The reflected voltage is the top trace and the forward voltage is the bottom trace. Remember that the forward power is adjusted by the feedback circuit to be reduced at idle and this is seen in the video. Warmup from cold takes only about 20 seconds. Since I cannot post an mp4 file, I put it in a 7z. It is only about 1 MB.
![Metcal STSS-001 RFG-30 Disassembly and Measurements 1205016-1](https://www.eevblog.com/forum/index.php?action=dlattach;topic=247278.0;attach=1205018;image)
This pretty much concludes what I have to post. Questions are welcome. Since I have machine tools, I will try to make a usable copy of the MX-RM3E wand for my own use. I could post a few pictures if that works out. Thanks for reading my posts.