Hey folks, sorry for the long wait between drinks, a few things have been going on – holiday, work, demolishing a leaking bathroom.
Anyway, after years of telling myself to get a function generator and watching eBay for quite a few months, I finally took the plunge and bought one. A broken one. Yes, I bought a broken function generator from eBay.
Clearly stated in the description it said that the sine wave output was dead. I primarily wanted the function generator for the sine wave output for a few upcoming projects. However, for $65 delivered I was the only bidder and the proud owner of a broken function generator.
Before I purchased, I made some inquiries with the seller and basically I got the impression that it was trying to output a sine wave, but the signal was not getting to the output.
Some quick details of the unit:
- Brand: Matrix (whoever they are)
- Model: MFG-8216A
- Outputs: Square wave, triangle wave, sine wave
- Frequency range: 0.3Hz to 3MHz
- Counter: Internal and external
- Price: $379 from a reputable Australian electronics retailer.
Before I got troubleshooting, I gave it a quick to test to confirm that there was actually no sine wave output.
The green LED above the sine wave button illuminates (agrees with what the eBay seller told me) but no signal output.
Opening the device is straightforward, just a single screw at the back and the top cover slides off.
I’m not going to get all snobby about the build quality, its a no name brand that has obviously been built to meet a target price. On the outside it has a good finish – clear display, nice power button, smooth dials and soft selection buttons. Internally, it looks like some effort has gone into the design, but there have been some late design fixes and manufacturing has been rushed – wonky components, ICs not fully seated in sockets, capacitors with long leads and solder splatter.
Initial troubleshooting began with checking and fixing the obvious manufacturing imperfections. ICs were seated properly in their sockets, some incomplete solder joints were fixed, wonky components were straightened in case their leads were shorting and all the solder splatter was cleaned off. Unfortunately there was still no sine wave output.
Probing around with the oscilloscope identified the main oscillator and triangle wave generator but no sine wave generator. Some previous experience and a few searches for details on the ICs identified that they were mainly generic op-amps and logical chips (AND gates, flip flops, line drivers) expect for one, a MC14066, a bilateral switch. I’ve never used a MC14066 before, but basically it has four analogue channels that you can turn on or off with a digital signal. This is a great clue, because, if I was designing this device, I would need a way to direct each of the wave types to the output of the unit.
Some probing of the MC14066 with the oscilloscope whilst changing wave types showed I got a square wave when the square button was pressed, a triangle wave when the triangle button pressed and no signal when the sine button was pressed. Using a multimeter set to measure resistance, the PCB traces were followed back from the MC14066 to three axial inductors.
Since an inductor is a coil of wire, it should have a resistance. Each of the inductors were measured. Two of them measured 0.1Ω and the third measured open circuit. A bridge wire was installed to bypass the suspect inductor.
Power was applied and the output of the function generator was measured.
Well, the open circuit inductor was the problem.
The open circuit inductor was de-soldered and replaced with a new component for $0.75. De-soldering was a pain because lead free solder had been used. A trick I use it to increase my soldering iron temperature to 400°C, apply some lead solder to the join and remove the excess solder with solder wick. Repeat the process of applying leaded solder and removing with solder wick a couple of times and the component lead should come free. My experience is that the leaded solder can be absorbed by the solder wick more easily than lead free. So applying the leaded solder mixes with the lead-free solder and the leaded solder acts as a carrier to allow the solder wick to soak up all of the joint’s solder.
So, I am pretty pleased that I know what is wrong with my $65 function generator. A bit disappointing that a $0.75 1.5uH inductor had stopped it from working. I expected the problem would be a dodgy transistor or a cooked op-amp.
I won’t tell the eBay seller that a single 1.5uH inductor killed all the value from his function generator.