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REVIEW: Metcal SP200 Other Review by Tom §. at Audio Asylum

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What the hell? A review for a soldering iron?

Well, considering there are reviews for blue butter and other associated tweaks, I think taking up some review space for a high quality iron is worth it. There’s a fair amount of folks on the AA boards that sling solder on a regular basis. A few of these slingers spend thousands of dollars on parts and equipment while possibly spending only $30-$40 on an iron.

A common misconception:
“As long as it’s hot enough there’s no difference in soldering irons.”

There’s a huge difference in soldering irons. I’m not talking about a $9 rat shack iron and a $50 weller either. I’m talking about fundamental differences in heating technology. Commonly, there are 2 types of irons that use stored thermal energy at their tips: Open-Loop and Closed-Loop systems. The tehnology used by Metcal is a 3rd type and totally different from these systems. I’ll go through and explain the first 2 and talk about the Metcal system last for dramatic effect.

Open loop irons are just irons where the heater is directly plugged into the wall socket. There are no temperature sensing circuits or any type of thermal controls. All you have is a resistance across the AC line and the heat generated by that resistance. Essentially, it’s just a constant power system. The temperature at the tip will vary greatly while doing work with it. If you use the tip to solder 3-5 decent sized joints, you drain the tip of the stored thermal energy and you have to wait for the thermal energy to store up again. It’s kind of like having a bucket under a faucet where the faucet is fixed at one flow setting (where water from the faucet represents the heat generated by the fixed resistance and the bucket is the iron’s tip). If you take out all the water in the bucket at once, you have to wait for the bucket to fill up again. If the water output from the faucet is slow (as is the case with these type of irons) and you’re taking water from the bucket too quickly, you are then stuck waiting on every refill. This type of system is what you’ll get in the $5-$80 range of irons. It’s ok if you’ve got time to wait for the heat up and you only have to do one or two joints. Usually the temperature at the tip can vary by 100°F or more.

Closed loop systems are irons often advertised as being “temperature controlled.” Are they though? Let’s look at how they are “temperature controlled.” To start off with, these irons use the same technology to generate heat at the tip as open loop systems. It’s just electrical current going through a fixed resistance generating thermal energy to heat a chunk of metal known as the tip. The tip stores the thermal energy just like the faucet and bucket analogy. Only this time the electrical current through the heater is varied. This time you have control over how much water you put in the bucket. This is accomplished by using a thermal sensor near the heater/tip junction. If it senses less temperature it will give more juice to the heater to achieve a higher temp. If it senses too much temp it will cut the heater power to cool it down. This is better than an open loop system as you can vary the temp to meet the heating needs of the application. The downfall though is that the heater performance varies with age as it’s electrical resistance changes over time. This requires calibration/adjustements etc. Also, you still can only do a couple joints at a time as you still have to wait for the thermal energy to store up in the tip (although not as long of a wait as open loop systems). Closed loop systems usually have a tolerance of 60°F or so.
Closed loop systems are not that cheap to begin with and prices are all over the map depending on built quality, accesories, etc. You could easily spend $400 on a “high quality” temperature controlled soldering iron station.

But is that money well spent? Time to talk about the Metcal SP200. This is Metcal’s bare bones model. It’s $300 normally but you can probably find it cheaper if you look hard enough. What’s so damn special about it? It’s the technology used. It doesn’t rely on storing thermal energy in a chunk of metal. You’re essentially taking thermal energy from the heater itself. That’s right. You now got a garden hose connected to the faucet going through a high pressure washer with multiple attachments. You are using the heat in a more efficient and efficacious manner.

How does it work at the more technical level? This is the cool part. Skin effect. The skin effect is the basis of this technology. That’s right. No banal debates here about the skin effect possibly affecting your audio signal. The skin effect is in full use and undeniably kicking ass right here in this product. The tip is heated by a signal in the Mhz range where the skin effect is put to use. A wire is wrapped around metal core. The metal core is made out of solid copper with a nickel based alloy anodized on the surface. The wire is fed with the Mhz signal and a current goes through the wire. Since it is a coil of wire, this energy is inductively transferred to the core. The energy from the signal heats up the outer coating of the nickel alloy. At a very specific temperature, this alloy becomes non-magnetic and the signal no longer travels on the skin of the nickel alloy. It then travels to the copper core where the core handles any excess energy. There is a sensing circuit that detects this crossover point and cuts power immediately. Once the skin on the core cools, it becomes magnetic again and the cycle continues. The stability of this system is in the 2° range. That is very tight temperature control. Much tighter than the other two types of irons.

But why do you want tight temperature control? You want to get the junction you are heated to just above the liquidation point of the solder. Plus, you want the entire joint to be evenly heated so the solder flows evenly around and through the joint. Even heat also prevents the solder pad from warping due to heat differentials. The metcal allows you to evenly heat a joint better than what you’d be able to accomplish with indirect heating methods described earlier. What this means is better solder connections on IC cables, PC boards, and general hard wiring. It allows a better physical and electrical connection. It allows you to build a better product consistently. No more finding cold solder joints 3 days into testing. No more breaks in the joint after a couple jiggles on a cord. No more redoing connections because you do it right the first time.

The reason why I bought a Metcal was because I used one at work for 4 years. I just got so spoiled that I had to get one in my own home and never fuss about this aspect of the hobby again. If you just solder cables every now and then, $300 is too much to ask. However, if you tinker around a lot and have thousands of dollars vested in high quality parts for your amp, multi-thousand dollar speakers, and thousands in your front end, then don’t you think $300 is OK to spend on getting and keeping it together?

Just a thought....

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Topic - REVIEW: Metcal SP200 Other Review by Tom §. at Audio Asylum - Tom §. 11:25:10 03/18/01 ( 4)