Metcal PS-900 Soldering station heats up to over 400 °C

[Everbrave]

Hi,

I've a Metcal PS-900 with the same symptom. I think it may be due to a missing turn on L3.

We've a few of the PS-900's at work, an older OKI branded one from about nine years ago and a more recent batch of about 15 including the problem one. The older one has been rock solid and never caused any problems. The newer ones seem prone to burning out elements with a noticeable number of failures - that may be related but I've not had a poke at any of them yet.

The PS-900 schematic (attached) is very similar to the older SP-200, the higher power output appears to be enabled by a reduction in the number of turns on L3. On the older (reliable) PS-900 L3 has 41T while the problem unit only had 40T. (The SP-200 has 46T for the equivalent part).

I'm not sure if this is a production error or a factory modification that doesn't quite work as expected (unless they're trying to sell more spare parts!), I'd need to open up some more of them to check.

Running the iron from a bench power supply, rather than the internal 48V switch mode power supply to make it easier to measure power/current (and to save me electrocuting myself), shows an input power in excess of 72W at power up when the tip is cold. That's the current limit for my supply, it's obvious the circuit is trying to draw more. Adding an additional turn to L3 (41 turns) reduces the peak power demand to slightly over 60W; the circuit is regulating power rather than the bench supply. It's nominally a 60W iron, although Metcal's not very clear about this.

The change does seem to work although I should mention that I've only got a tip cartridge from my SP-200 to test with - it's effectively the same thing in a different package. I've used a big heatsink and a blob of solder to check the maximum power output when the tip is up to temperature.

L3 is the one that's mounted flat on the PCB with a blob of silicone underneath.

Hope that's of interest  :-)

this makes me think, on the other hand, that the power delivery of SP-200 can be increased by decreasing the number of turns of L3 provided the power transformer is replaced, e.g. with a higher power toroidal one! The power MOSFET is capable of delivering the current.
Is this a viable idea? Any thoughts?

[I2R]

You might need a tip that could also cope with the higher power, the problem with the PS-900 caused the element to burn out. I doubt the transformer is the limiting part.
My testing with the SP200 tip was to get the PS-900 working, it's not something I used long term.

I think the next most expensive model they offer (MFR?) may use the same/compatible tips as the SP200 but I've never seen one of those and I'm not sure what power they run at. Might be better to dig up some info on those and see if it's the same inside as the PS-900 but with a bigger price tag - the board has lots of missing bits which suggests either unimplemented features or cost cutting.

[Everbrave]

You might need a tip that could also cope with the higher power, the problem with the PS-900 caused the element to burn out. I doubt the transformer is the limiting part.
My testing with the SP200 tip was to get the PS-900 working, it's not something I used long term.

I think the next most expensive model they offer (MFR?) may use the same/compatible tips as the SP200 but I've never seen one of those and I'm not sure what power they run at. Might be better to dig up some info on those and see if it's the same inside as the PS-900 but with a bigger price tag - the board has lots of missing bits which suggests either unimplemented features or cost cutting.

I have both the SP-200 and the MFR-1110 which also can have a hand-piece that can accommodate the SSC cartridges used with the SP-200. The MFR-1110 itself comes with another hand-piece for the SxP cartridges. The MFR runs at 60W which means the SSC cartridge can also run on 60W.
May be it is worth trying reducing the inductance of L3 a little, perhaps making a new inductor instead of modifying the original one, and see how it behaves
Any suggestions?
Thanks

[I2R]

May be it is worth trying reducing the inductance of L3 a little, perhaps making a new inductor instead of modifying the original one, and see how it behaves

If you make a new inductor to replace the original you'd need to know what the core material is. Easier to get some enamel copper wire the same diameter and trim/rewind the existing choke. If you've not done that before measure the wire (from the original) and cut to length before you start winding, and make sure the new turns are tight and evenly spaced - you need to maintain the tesion on the wire as you're winding. Not too hard and easier if you're reducing the number of turns as you can cut the wire to remove a turn(s), strip the enamel from the end, refit the choke and do some measurements to see what's changed.

[Everbrave]

May be it is worth trying reducing the inductance of L3 a little, perhaps making a new inductor instead of modifying the original one, and see how it behaves

If you make a new inductor to replace the original you'd need to know what the core material is. Easier to get some enamel copper wire the same diameter and trim/rewind the existing choke. If you've not done that before measure the wire (from the original) and cut to length before you start winding, and make sure the new turns are tight and evenly spaced - you need to maintain the tesion on the wire as you're winding. Not too hard and easier if you're reducing the number of turns as you can cut the wire to remove a turn(s), strip the enamel from the end, refit the choke and do some measurements to see what's changed.

No worries, I have wound inductors of all kinds many times. I thought you have tried reducing the inductance and measuring the power in this particular case of SP-200. In my experience, using the same or close number of turns with a lower core inductance is preferable to reducing the turns since the inductance varies proportional to the square of number of turns (linearity). I have a LCR meter to measure the inductance in advance.
Thanks