Metcal STSS-001 RFG-30 Disassembly and Measurements

[Dan_CA]

While cleaning up my piled high workbench, I came across a Metcal soldering station power supply that has been a sitting there for years.  I also have some of the tips, but no wand.  I decided to see if I could put together a working soldering station.  Since I found some good Metcal information on this forum, I have decided to post my experience here to hopefully help others.  To dispel any suspense, the unit I have is working, but connecting up a tip was trickier than I thought.  I reverse engineered some of the circuitry and took a lot of pictures and waveform measurements and I will be posting what I have.

The power supply unit is old.  The date on the FCC ID application is 1986, which is the same year as the primary patent.  The identification tag says Power Unit RFG-30 for Soldering System STSS-001.  I have found pictures of Power Unit RFG-30 for Soldering System STSS-002 and they are not the same externally.  Of course I don’t know if the internals are different.

I will start with some pictures of the opened up unit as soon as I figure out how to add pictures.

[Dan_CA]

An attempt to add a photo.

The ID tag of the power unit.  It slides in grooves in the back side of the case.

[Dan_CA]

OK.  I'm getting the hang of using this forum.
I'll add some more photos of the RFG-30 that I have been working on.

The case has been opened up and the strips have been removed to show what is under them.  Note that the green LED is my doing to replace the small incandescent bulb that was burned out.  It will have to be changed to a white LED to illuminate the Metcal logo.  The 3 nuts on the left case side fasten the Power Supply Board to the case.  On the  right case side 2 nuts hold the IRF130 against a mica insulator.  The top nut has an insulator behind it because the stud is grounded.  The bottom nut has no insulator because the stud is the FET drain connection.  The other nut holds the driver FET against the heatsink with a mica washer.  The nut holding the RF connector to the case must also be removed to release the RF Board.



Here are the insides of the separated case with the beautiful torroid transformer and two boards.  I call them the Power Supply Board and the RF Board

[Dan_CA]

Some more information and pictures today.

Detail picture of the Power Supply Board.  The red and grey wires at the bottom are from the torroid power transformer secondaries.  Note that the two 1.6K resistors and the white wire at the top are my doing to replace the incandescent bulb with an LED.  The SG3526 IC is the controller for a buck switching converter that controls the power to soldering tip.  The round heatsink at the upper left is on a ZTX749 transistor which is the switch for the buck regulator.  The buck regulator inductor is to it's right.  The pot labeled VR1 is in the feedback loop of the buck regulator and sets the power delivered to the soldering tip at idle.  I will soon post a schematic of this board.



Next is a detail picture of the RF Board.  To remove the board remove the 2 nuts holding the IRF130 FET and remove it (messy heat sink compound).  Be careful of the little insulator under one of the nuts.  The other small nut holds the VN67AF  (TO-202 package) driver FET on the back of the board.  There is also an insulator under the nut.  Finally the nut on the type F output connector is removed and the board comes free.  The round heatsink on the left above the 13.560 MHz crystal is on the 2N3553 oscillator transistor.  The VN67AF is at the top middle under the blue pot.  The little red auxiliary board is interesting and I will have more information about it later.



The final picture is the back of the RF board.  The VN67AF driver FET is at the top.  The mica insulator sheet was left on the heatsink.  You can also see where the IRF130 pins plug into the board.

[Dan_CA]

OK here is the schematic that I traced out for the RFG-30 Power Supply Board.  I think it could be of some use for troubleshooting.  The component reference designators on the schematic match those on the board.  I included component values that I could determine without disassembly.  I drew the schematic using LTspice which I use all the time for circuit analysis.  Of course this schematic has nonfunctional symbols so is of no use for simulation.

 RFG-30 Power Supply Board Final.pdf (66.83 kB - downloaded 276 times.)

Following is my description of the Power Supply Board circuitry:
DC power is provided by a torroidal power transformer and rectifiers using conventional circuits.  The highest voltage and current supply, voltage V1, is provided by the two secondaries in series feeding a bridge rectifier.  V1 provides power to the drain of the IRF130 power amplifier FET on the RF board.  Additionally there are two half wave rectifiers fed from the center connection of the transformer secondaries providing lower voltages.  V2 supplies the input to the buck regulator switching transistor.  The regulated voltage, V4, then supplies power to the 13.56 MHz oscillator and driver FET on the RF board.  V3 supplies power to the incandescent front panel lamp and the SG3526 regulating pulse width modulator which controls the buck regulator.  As can be seen from the board connector, the only voltages supplied to the RF board are V1 and V4.

[Dan_CA]

Next I'll turn to what I have learned about the RF Board.  It has a ground plane on top with the traces mostly on the bottom.  I did not trace it out and make a full schematic, but I did trace out the feedback circuit and I did remove the little red board and trace it out.  I pulled the Metcal patent because someone on another thread said that it was close to the actual board.  Sure enough, it has the same transistors and coils.  Here is a copy of the patent schematic.

 RF Board Schematic.pdf (34.7 kB - downloaded 259 times.)

Notice the 2 trifilar coils in the oscillator and driver and the bifilar coil on the output stage.  These are clearly seen on the actual RF Board.  My tracing of the board, however, shows that the oscillator and driver are powered by V4, the buck regulator output and that the output FET is powered by fixed voltage V1.  The drain of the power stage feeds a conventional LC lowpass filter with 3 coils.  If you simulate it with 50 Ohm source and load using the values in the patent, it works as expected.  The filter coils on the RF Board are the large horizontal coil on the right side, the vertical coil below it and the vertical coil on the read board to the left  The feedback circuit is shown in the patent in box 15, but it clearly has mistakes.  I traced it out and made a schematic.



The little red auxiliary board was a bit puzzling so I removed it to take a better look at it and trace the circuit.  Following are a few pictures and the schematic.

 Metcal Red Board Schematic Final.pdf (55.18 kB - downloaded 140 times.)




I did measure the inductance of one of the secondaries on the torroid as 5.6 uH and the bifilar coil has 10 turns. By experiment I determined that with a proper load on the output the voltage to the base of the 2N3904 is slightly negative and it is off.  However, if the output is open, the 2N3904 is biased on and the SG3526 is reset.  This shuts down the RF for about 20 milliseconds and then the process repeats causing the RF output to oscillate on and off.  This must be to protect the output transistor from failing if the soldering tip is removed.  Judging from the board layout, the red board may have been added after production started.

Can anyone speculate on how the red board actually works?  It doesn't really work in my spice model.

[Dan_CA]

Another picture of the little red board that failed to load on the last post.

[Dan_CA]

After going through the circuitry of the Metcal RFG-30, I decided that I would like to make some RF measurements.  I needed some RF test equipment for this.  My trusty Tektronix 7603 scope with a 7A26 dual trace plugin can measure the 13.56 MHz RF.  I didn't have a high enough power 50 Ohm dummy load so I built one.  Little high power thick film 50 Ohm microwave resistors are available from China on ebay and even on Amazon for cheap.  I bought a pack of 5 100W resistors supposedly good to 6 GHz from Amazon for $13.  I mounted one on a heatsink that I had and fitted it to a length of RG174 50 Ohm coax.  These resistors are probably meant to be interfaced to an RF strip line, but my approach is adequate for my measurements.

Here are a couple of pictures of what I made.





I added a second resistor to be able to have a 25 Ohm load for some tests.

My first measurement was to measure the output power of the RFG-30 with a good 50 Ohm load.  Here is the scope measurement.  I just hold my digital camera up to the scope for the picture



Note that the scale says 2V/div, but I am using a 10X probe so the scale is really 20V/div.  This will be true for all my subsequent scope pictures.  If you call the measurement 120 Vppk then the output power is (60*60)/(2*50) = 36 Watts.  This is about right for the Metcal unit.  The trace is fuzzy around the peaks because the Metcal power supply to the output FET has quite a bit of ripple.  And no, this is not due to a faulty filter capacitor.  The output power can be adjusted with pot VR1 on the Power Supply Board, but I left it at the original setting.

[Dan_CA]

Next I decided to build a directional coupler to get some idea of the impedance of the soldering wand and tips.  There are many articles on HF directional couplers (DC) in the Ham radio community.  Many use ferrite torroid transformers and one design uses a two hole torroid or balun core.  I have some of these cores in my junk box so I gave it a try.  A good article on this form of coupler is at
http://michaelgellis.tripod.com/Directional.pdf
The coupling of the DC depends on the turns ratio and the minimum reflection coefficient that you can see improves as the coupling is decreased.  I could fit 5 turns in one hole of my core, which gives a coupling of -14 dB according to Ellis.  This shows a return loss of about .05 or VSWR of 1.1 with a perfect load on the DC.  This seemed good enough for my measurements.  The 5 turns gives an inductance of 136 uH.  The DC can easily be simulated in LTspice, which I did.

Here is a graphic of the construction and some pictures of my build.







I tested my directional coupler using the Metcal RFG-30 as the signal source and my scope in dual trace mode measuring the voltages across the 50 Ohm resistors of the coupler.  The forward coupled port is the bottom trace and the reverse coupled port is the top trace.  First with no signal to show the scope baselines.



Next the DC terminated with my 50 Ohm load.  The reflection coefficient is the ratio of reverse over forward voltages and appears to be between .05 and 0.1, which is not bad for my quick build.



Next the DC is terminated with my 25 Ohm load, which gives a reflection coefficient of -0.33.  Again the result is not bad.



Finally the output of the DC is left open, which gives a reflection coefficient of 1.  The two traces are about equal as expected.



I was very happy with the build of the coupler and it has resulted in measurements which made a breakthrough in my understanding of the Metcal unit.

[Dan_CA]

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 74 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.



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.

[t0m]

Nice write up. Did you ever get around to making the wand?

[Dan_CA]

Yes, I did make a wand and it works well and I use it often.  It also works on a newer PS2E-01 supply that senses when the wand is disconnected.  I did not post any construction details because it was fairly complicated and requires a lathe and machinist skills.  Here is a picture of the completed wand.

[quadtech]

That's great, Dan. This is probably the first "recorded" fully diy wand - details/pics would be very helpful.
I guess the tip plugs in into a SMB straight connector inside the wand?

Other posts elsewhere suggest the capacitor is 270pf - so you used a 180pf in parallel with a 22uH?

[Dan_CA]

Yes I used a 180pf in parallel with a 22uH as explained above.

There is no SMB connector and I do not believe that the Metcal tips mate to SMB.

[Dan_CA]

Several viewers have noticed that some of my pictures have become missing or are in the wrong place.  This happened after the forum went down last year.  I don't have the time to fix this.  Here are the missing files.





 RF Board Schematic.pdf (34.7 kB - downloaded 183 times.)

 Metcal Red Board Schematic Final.pdf (55.18 kB - downloaded 138 times.)

[H713]

Out of curiosity, what connector do the cartridges use? I'm intrigued by that DIY wand.

[Jamieson]

Sorry to necro this thread... but how the heck do you remove the top plastic cover on the power unit? The older (STSS-001) RFG-30 unit does not have any exposed screws on the top cover. I've tried prying it up but it sure feels like it is screwed down in some way.

[Dan_CA]

Jamieson

If I remember correctly there is a thin plastic sheet glued to the top cover which covers the screws.  The screws are in the corners so you can cut the sheet there and lift the corners off.  Looks like I just removed the whole sheet and it could not be put back on.

[Jamieson]

There it is. Thanks Dan!

[Jamieson]

I have two STSS-001/RFG-30 power units.

The first one is a dumpster find and looks like the one Dan showed in this thread. It is serial number 4007910. It has quite a few kludges and bodge wires, with the smaller red daughterboard. That to me looks like they were still trying to add on a protection circuit and kludged it up to make it work. On this board I believe the VTX749 on the power supply board is blown; waiting on replacement parts.

The other STSS-001/RFG-30 I just got today. I bought it super cheap AS-IS and it not surprisingly it doesn't work. It has serial number 2006685 looks like an earlier model, shown below. Both the RF board and power supply board are much cleaner and show no evidence of being kludged/hacked up. This design seems to be older, and much closer to the design from the Metcal patent 4,626,767 and components have a date code from 1987, so this may be the very first generation Metcal. On the RF board I found a few bloated 10uF caps and the RF power amp FET IRF131 has a gate to drain short, which smoked a 100 ohm resistor connected to it.

[Jamieson]

The saved-from-the-trash STSS-001/RFG-30 power supply was fixed yesterday. The problem turned out to be the 5k pot VR1 on the power supply board. Normally this would be used to bias/adjust the FB signal and control the PWM chip. The pot failed open (oxidation?) and the FB signal was no longer in the correct range to drive the PWM controller, so it shut down shortly after power up. I would see V4 momentarily bump up to ~5V then fall back to zero. With a new 5k pot installed it fired right up and I used one of those Metcal inline power meters (also from the trash!) to adjust the idle power to about 5W.