SMD Hot-Air Rework Station Repair

 

Ya Xun Hot-Air Rework Station cost just RM209 (less than USD50) 

Last year I bought a cheap hot air rework station. It was probably not a good idea - I have never used one before, and this being pandemic season, I would be on my own. My eyesight is not getting any sharper but the electronic components are definitely getting smaller. On the other hand, all the fun stuff nowadays, like ESP8266, Aduino and Raspberry Pi all seem to use Surface Mount Devices. And most compellingly, SMD parts are much cheaper than their through-hole equivalents. 

Some SMD components are just about manageable using old-school soldering irons

I found myself buying soldering irons with smaller and smaller tips, right down to 0.5mm. With discrete SMD I simply used two irons (I would have used more but I ran out of hands): you can often heat up the entire thing and lift it clean off with two irons. To solder a new SMD part in I used the 0.5mm iron. SMD ICs were a problem: sometimes you just could not heat up all the leads with two irons. But if I had new ICs on hand I would simply cut the IC off the printed circuit board. You then removed each soldered lead one by one. 

You need a small and really sharp pair of micro-cutters. Anything less and the leads tend to get ripped off along with the tiny PCB pads. That cutter you never lend out or use for anything else. You guarded it jealously and threatened anyone with immediate bodily harm if he tried to take it.

Duratool's micro cutter: don't leave home without it. 

But QFN and BGA SMD packages were quite another thing. The contacts and PCB pads are underneath the body, and only a hot air gun will get them off without damaging the PCB. Well, I could sneak the PCB into the wife's oven in the kitchen, but this tends to remove all the parts. To inspect the soldering, one would need nothing less than an X-ray machine. 

 

QFN (Quad Flat No-lead) IC

BGA (Ball Grid Array) IC

Even when there are no BGA or QFN parts, sometimes there is simply not enough room to place the irons, and neighboring parts may get moved, burnt or worse. But then came a China siren to lead me into temptation: at only RM209 (less than USD50) the Ya Xun 850A+ cost about as much as a Raspberry Pi 4. The clunky box may even be an advantage - it is less likely to use SMD parts, which makes it repairable with soldering irons.

China temptress

In about a year it failed completely: it blew its fuse. After replacement, its temperature LED no longer lit up and the hot air flow is no longer adjustable. I whipped off the cover and got the PCB out. Usually I could figure put most PCBs but looking at the parts mounted. But only if I am already familiar with them. There is just one problem: I had never worked with triacs before and this board had two, in essentially AC power "dimmer" type circuits. This means to make any sense of it I would need to first trace out the schematic. Oops.

YX850A+ Controller PCB: old-school single-sided PCB with through-hole parts

Warning: this repair deals with lethal voltages. Do not attempt unless you have been properly trained!

The PCB used old-fashioned through-hole components as expected. Even better,  the PCB is single-sided: it only has traces on the bottom (ie solder) side. This makes it easy to trace the leads; PCB traces o the component (ie top) side will run under the parts and be obscured. Single-sided PCBs are also easy to desolder. 

Tracing the PCB was not so bad, but having worked with a human PCB designer for over 10 years, hand-drawing schematics felt lame so it seemed like a good idea to install geda again. Two weeks later, armed with the schematics and a wobbly knowledge of triacs and diacs, the repair work began.

The first problem was the internal wiring was badly crimped; the wires were the wrong sizes and the resulting crimped joints were loose, in particular those at the front panel switch.

Loose crimp connectors at the panel switch (top) and temperature sensor wiring (bottom)

The front panel main switch switches in both mains AC and 12V DC to the PCB and loose joints here cannot be healthy for the triacs. And sure enough the triac Q2 BT136-600 is shorted Gate to T2. The LED D1 which indicates airflow was also shorted, together with its series rectifier D2 (1N4007). Unusually, the LED appeared to be driven directly by the triac output AC and is probably more sensitive to line surges. The PCB trace from D2 to Neutral was melted a good 4mm. 

BT136 triac and associated components

The gate drive diac, D4 (DB3) and its series capacitor C5 (120nF polyfilm) were also shorted. Now triacs are turned on using the gate, but can only be turned off by letting its load current drop to zero, which means interrupting its input mains. This is done using a relay K1 (HK4100F-DC12V-SHG an Omron G5V-1 equivalent). The relay is powered from 12V output from U3, an LM7812 linear regulator which was also shorted input to output.

The other triac, a BTA12-600 supplying the heater seems to be fine. The heater LED failure was due to a loose temperature sensor wire. The heater is probably a lot less inductive than the air pump motor. It also helps it is triggered from another optically-isolated triac U2, an MOC3023. Interestingly the MOC3023 drives the main triac's gate using a series rectifier D4 (1N4007) which means it only puts out half the AC waveform and pretty much guarantees the BTA12 turns off during the other half cycle. The MOC3023 is in turn driven by the temperature sensor via U1, an HA17358 (probably an LM338 workalike) which seems none the worse for wear despite being exposed to unregulated 25Vdc from the shorting LM7812. The 40V maximum of the LM338 probably helps.

Repair was easy. The parts easily dismounted and replaced. The dodgy crimp connectors were soldered to their wires. Replacement parts were cheap and arrived quickly despite the pandemic-induced parts shortage. The Malaysian branch of Element14 in particular offered free delivery, in contrast Digikey wanted USD89 to deliver a USD2 part.

To test, I used a 300VA isolation transformer dialed down to 220Vac (China boards are rated for 220V; Malaysian grid is 230V but I am at the very end of the mains power line at a feisty 240V). For additional safety I also used a portable ELCB (RCD to Americans). Since some semiconductors (usually mains bridge rectifiers) are connected directly to mains AC sometimes there is a short from  Live to DC ground, and an ELCB will pick this up.

Portable ELCB

An isolation transformer is a two-coil transformer (ie not an autotransformer) putting out the same voltage level as its input. If the output is shorted, the current is limited by the magnetic flux saturating in its core, limiting power delivered to its VA rating. 

110Vac Isolation Transformer. Note the Earth line passes through but Live and Neutral are isolated. 

It often lets you diagnose faulty boards before it completely burns out. 60VA, or even 30VA are very handy ratings, and the idea is to progressively work up to the full device rating. I always make my own; a handy way is to use two regular (ie step-down) transformers connected back to back. To dial down the output voltage you will need to use a variable transformer to drive the isolation transformer.

2000VA Isolation transformer from Carroll & Meynell

Testing was straightforward except for one thing: the air pump continued to run despite the panel power switch being turned off. Despite having used it for a whole year, I did not really notice if it did that before. My excuse was I was struggling with tiny SMD parts. Having the air pump run meant the BT136 is on. Was the relay turning off?

Air pump relay circuit

I needed to make some live measurements. This is never a good idea when dealing with stuff connected to mains AC. The isolation transformer protects the repair item but will easily deliver a fatal shock to a human (50mA or 15VA will ruin your day, probably your life) . The mounting wires are short and the board has to be mounted vertically which means reaching past exposed live bits to probe.  

Thus far I had looked for defective parts the safe and preferred way: offline, unpowered and using a digital multimeter. If you suspected something you took it out and make some more measurements. When testing I powered up at low VA, operated the panel knobs and switches and avoided touching exposed parts. 

NPN transistor Q6 (SS8050) was an immediate suspect - with the panel switch off, its collector was 6V, way lower than the 12V if it were off. Maybe it was leaking? It tested OK dismounted: both semiconductor diode junctions seemed OK, but maybe it was partially leaking? I replaced it with a generic 2N2222 (watch it: ON Semi produces a PN2222 with reversed leads unlike my KSP2222A), but there was no change; the air pump kept running even though the panel switch is off.

An examination of the circuit provided a clue. In addition to switching in mains voltage, it also switched on 12V to Q6 (and the opamp). The capacitor C6 is 220uF and will store a walloping charge and R19_2 measures about 100K in-situ but is probably higher (the color codes are faded with heat). When switched off, it will ensured it discharged slowly into the base of Q6. The time constant is in the order of 22 seconds. And always fearing a defect I had been quick to cut off power to  the isolation transformer while testing.

Using this formula, my Change is 95% (12V to 0.6V) at time constant 22 will result in a delay time of 64.8s. I assembled and tested it again; and sure enough after a long minute the air pump went off. But why would it want do that? It is somewhat misleading to have the power on even after the panel switch is off.  The manual for a similar system yielded this clue:

Ah, that was unexpected, but at least it was not broken. I would recommend you spend that minute, set the airflow to maximum and wait for the air pump to finish doing its thing, and then turn the YX850A+ off at the power socket, for the manual goes on to say:

An auto-disconnect would be nice, maybe a few seconds after the air pump turns off. So would a thermal cutoff switch. And the triac heatsinks looked puny with the cooling fins horizontal instead of vertically where they would do most good. But that is a project for another day. 

There you have it: Hot-Air SMD Rework Station Repair.

Happy Trails.