Thursday, March 30, 2017

Adventures in cheap tools

Sometimes there's a reason something is cheap. And "cheap" isn't always just inexpensive.

More on that in a minute. First, here's a look at the output of the MAX253 driving a Halo TGM-210NSLF transformer with a 2:1 ratio. The load is the VFD's filament.

The yellow trace is the transformer secondary, showing about 3.2 Vpeak at about 475 KHz. The red trace is the 3.3V input to the circuit. As expected, the output is very close in amplitude to the input.

To the left is a picture of the circuit connected to the VFD. I didn't have the power supplies turned on so none of the segments are illuminated.

The ripple in the input power is because the path between my power supply and the circuit under test was long and convoluted, passing through a digital ammeter, and had only a single 100 nF capacitor nearby. Placing a 10 uF electrolytic across the the 100 nF capacitor pretty much eliminates the ripple. The 10 uF electrolytic can be seen to the right of the circuit board.

Now on to the story of cheap equipment!

About 5 years ago I bought a cheap Chinese knockoff hot-air soldering station from Amazon. It cost a bit over $100, which is a lot less than a truly professional hot-air system from a reputable manufacturer, and it seemed to work well enough for my purposes.

I'm probably going to use Toshiba RN4604 or RN4904 pre-biased PNP/NPN dual transistors for the anode and grid drivers. The only differences between the two are the packaging and price. The RN4604 comes in an SM6 (aka SOT-23) package with 0.95mm lead spacing, while the RN4904 is in a slightly smaller US6 (aka SOT-363) package with 0.65mm lead spacing. I wanted to see if the US6 package was manageable, so I chose to buy 5 RN4904s (and appropriate adapter boards) for my initial experiments.

Why do I do these things to myself? To the right is a photo the RN4904 (SOT-363) on a sheet of 0.1" grid graph paper, with a larger FDV301 (SOT-23) to its right. Good thing I have a microscope!

Clearly the easiest way to solder one of these things is with solder paste and hot air reflow. I put a dab of solder paste on the adapter board pads and positioned the chip on the board as closely as I could, knowing that the surface tension of the melted solder would pull the leads into alignment. I switched on the hot air station and waited for the hot air to flow. And waited.

I could hear the pump running, but no hot air emerged. Then I noticed the metal of the wand was starting to discolor and smoke. The heating element was clearly getting hot, but with no airflow it was overheating. A professional hot air station would cut off at this point, but this is a cheap knockoff. I quickly turned off the station but the wand seemed to get hotter and hotter. Fearing that the control circuitry had failed I unplugged the station, something that you wouldn't normally do as it would cut off the cooling air, but with no apparent airflow I figured it wouldn't matter. Fortunately everything eventually cooled down without the need for the fire extinguisher I keep next to my soldering bench.

The next evening I disassembled the soldering station to see what had gone wrong. The answer was pretty obvious. This is an air reservoir that lives between the pump and the hose feeding the wand. Notice the large crack that is clearly not part of the original design:

I believe the purpose of the reservoir is to act like a capacitor in a power supply filter, changing the bursty output of the air pump into a smooth flow of air through the heating element and onto the work. It was affixed to the back side of the pump with a little glue and two large cable ties. It looks like the cable ties had been pulled too tightly, overstressing the plastic of the reservoir and causing it to crack open. Before I cut the ties the gap was far larger than shown here. That's where all the air was going.

I haven't decided what to do about this yet. New knockoff soldering stations are only about $65 now, but I hate to spend even that much for something I don't use that often. I'll probably try to fix this thing with some epoxy glue if I can get the glue to stick to the slippery plastic. If that doesn't work I may attempt to replace it with some sections of 1" diameter vinyl tubing. And maybe I'll cruise through a store that sells products for large fish tanks in case this came from that world.

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