A function generator is one of those basic pieces of equipment that every bench should have. Unfortunately, they’re not always affordable, and when all you need are basic functions, it’s sometimes more palatable to simply slap something together with a 555 or a microcontroller to generate basic trigger signals – which is how I’ve gotten by until now.
Lately I’ve been playing around with a lot of designs that involve sampling various types of signals, and the aforementioned ‘poor mans sig-gen’ solutions were just no longer viable. While I would have certainly loved to drop $300+ on something from, say, the Siglent SDG1000 series; my needs and budget couldn’t even come close to justifying it. Instead I decided to look for something broken that I could repair, which is where most of the equipment I own comes from.
Unfortunately, even when in very rough condition, name-brand equipment still sells for more than I would care to spend on a ‘basic needs’ function generator. After a few weeks of desperate searching, I finally came across a “Global Specialties 2003 Synthesized Function Generator” listed as “not working” for $30 on ebay. While I wasn’t too sure of the brand, it seemed worth a go (especially for the price).
As you can see in one of the pictures, the unit was failing to display anything properly, and the seller couldn’t verify any output. Once it arrived, I wasted no time in opening it up, and the cause of the display problem became immediately apparent (or so I thought).
Now, I don’t know if that was someone’s botched repair, or if that was the solder job from the factory, but either way it’s fucking shameful.
After temporarily bypassing the horribly-soldered flat cable with some kynar, I was greeted with the not-so-fun discovery that the cable wasn’t the sole problem. Half of the display was just plain dead.
At this point things weren’t looking too good for a quick fix. The display was some obscure piece from the mid 90’s, and the only exact-replacement I could find was $70. My next thought was to try and identify the type of protocol it was using, and maybe find a cheaper compatible display. It only took a few minutes of pin counting and tracing before I started to get the feeling that it was an HD44780 compatible display, and I just happened to have one lying around.
It took a bit of messing around to fabricate an appropriate mount and permanent cable solution, but all in all I have $33 invested in this thing, and it does everything I need. Also, while the specs list the max frequency at 1.6MHz, I was able to push it to 2MHz (albeit not very cleanly – but good enough for basic triggering). The interface is also a god-awful nightmare, and if it proves worthwhile I may try and devise an alternative.