Metcal MX500P buzzkill

[Jamieson]

The Metcal MX500P power units have a curious design where the power switch is on the transformer SECONDARY. So even switched off the transformer is energized and humming away and burning a couple of watts. Since the transformer is really solidly bolted to the cast aluminum case the entire thing resonates and makes the mains hum even louder. It's obnoxious, and folks have been known to install these units on a power strip they can switch off just to get some peace and quiet.

My question is, why did they design it with the power switch on the secondary like this? Could there be some high inrush current that the designers were trying to avoid by doing this?

If the switch was moved to the primary side and installed in series with the fuse, could that cause any problems?

[Zucca]

Interesting

1) I did not know Metcal schematics are available
2) Probably a secondary switch is cheaper than a main switch and it can be placed everywhere without worrying about mains cables going back and forth in the unit.

That said, IMHO you can surely mod the unit and put a switch in the primary. You need to know what you are doing because playing with mains can generate a lot of holy smoke or kill you.

[jonpaul]

Bonjour, 


All mains trsf have some natural noise from magnetostriction, but the older the noiser.

I was the designer of the SP-200 500 kHz 30W station of Metcal , for Raychem in 1990..1992.
The MX500P may be similar. The management wanted me to simplify the mains wiring and decided to have the transfomer primary direct to the mains IEC on the PCB, despite my objections.

The buzz is from loose mountng hdw or very old units where the tranaformers varnish had baked out.

Suggest to open the case, see if the mains transformer is loose and tighten the hdw.

Was a Signal/Belfuse PC mount trsf perhaps 24V sec.

I place a  rubber mousepad under the case that dampens the noise.

My protos from 1990s are still running!

ENJOY!

Jon

[Jamieson]

Hi Zucca: It's not official from Metcal, but there is a very nice PDF with schematics, assembly drawings, theory of operation, etc. written by Stepan Novotny circa 2006. I've attached it here.

Hi Jon: First of all, nice work on the SP200 design. We have many of those original units still working perfectly around the lab!

I've tried everything to quiet down the transformer on the MX500P. Tighening down screws. Adding rubber bits between the transformer and the case. Unsoldering the transformer and dipping it in varnish in a vacuum chamber. None of it really made a difference, I can still hear these suckers humming away on the bench.

The mains wiring on the MX500P is quite clever. The IEC plug/filter module is bolted to the case, and the lugs slide into terminals on the PCB when assembled. There are no wires connected to mains power anywhere. In that sense it's a pretty clean design, but still, keeping that transformer + rectifier + caps energized all the time is just... not right. Moving the switch to the primary side would be pretty straightforward: remove fuse F1 from the PCB, replace it with an inline fuse holder and some wires to the existing switch. Install a jumper on the secondary side.

Humming sounds aside, these things are total workhorses and built like a tank. I've been to many assembly shops and seen them all over the place, still kicking after 25+ years of abuse. Plus I just like the minimalist industrial design and the soldering irons work amazingly well. A few months ago I purchased a lot of 5 AS-IS MX500P-11 power units on eBay. They were all beat to crap with names carved in them, bad switches, and holes drilled into them. I gave them the full restoration treatment and they look and work like new.

[Jamieson]

adding pics of the restoration

[jonpaul]

Rebonjour Monsieur Jamieson!! Mille  mercis pour les notes....Many thanks for the kudos.

About  my design SP-200  at Metcal (as consultant)  I am  100% in agreement with you  about the poor practice of an always ON 24/7 energized transformer primary.
It was indeed the unique mechanical of the line IEC and PCB with the case that led to their decision.
The transformer magnetizing current (even when off) results in a power loss 24/7.
The magntising current/core losses  ~ 2W heat generntated,  bakes out the transformer varnish over the decades.

1/ We placed our own Metcal SP-200 stations on a switched plugbar, and turn the bar OFF when not is use. The Metcal station life, power and noise are thus controlled.

2/ The magnetizing current, flux  and hence the sound will be at mains freq and harmonics (50 or 60 Hz) and a nonlinear function of the mains voltage, increasing faster
at high line eg 128V and decreasing fast at low line eg 105V . You can observe that with a Variac.

3/ The transformers can be replaced, a new one may be quieter.   Signal Transformer 14A-56-36 PC mount series.
 
4/ My 1992  protos and first production SP-200 and Metcal tips are STILL in use, and like new.
OK Industries and others sold rebranded  SP-200. I think 100,000s have been made.

5/ The SmartHeat (TM) Curie point heater is especially good for large area soldering and rework , eg power supplies and power transformers.

Have an absolutely fantastic day!

Jon in Paris

[Jamieson]

On the MX500P-11 I moved the switch to the mains side and have been using it quite a bit this past week. No issues at all, continues to work normally. No more mains 60hz hum 24/7 is a welcome change 

[thm_w]

Thats good.

I still recommend the power bar method, use an automatic power saving power bar and have the iron power on only when your soldering light or fume extractor is on. This way there is no chance you will forget that its on.

[ststefanov13]

I have a question about the compatibility of the Metcal Smartheat MX-500P-21 station with the HAKKO soldering irons and tips specifically the FX-1001, FX-1002 and FX-1003 they are for the FX-100 Station, has anyone tried this combination and does it work?

[jonpaul]

mechanical and electrical differ. Only way to confoirm  is to just  try it

j

j

[artag]

I have a couple of MX-500 and, although I'm fanatical about getting rid of fan noise, I have never noticed them humming. I do, however, note their effect on adjacent CRTs. This wouldn't be too much of a problem if I could turn the Metcal off when using the CRT but that doesn't work .

However it does indicate that they're not all noisy and maybe a fresh dip in varnish would help.
 

[mtwieg]

Thanks for sharing this schematic!

Not only is the transformer always energized, but so are those two switchmode DCDC converters. The only thing the switch controls is a 78xx linear regulator, which only powers some supervisory circuits, not the RF amplifier circuitry. There's probably a lot of failure modes which would heat the tip regardless of the switch state.

But it's a schematic worth studying. Might give a crack at simulating it. It even describes the RF transformers in the BOM!