I had recently come across an old function generator manufactured by Southwest Technical Products Corporation (SWTPC) on eBay for a decent price, and being in need of a function generator, I decided to get it. It was manufactured starting around 1973, and is based off the Intersil ICL8038 chip, a monolithic function generator. It was sold as a sort of "DIY kit" where you got all the parts and put it together yourself. It is capable of generating a sine wave, triangle wave, square wave, sawtooth wave, or pulse train as an output over the frequency range of 0.1[Hz] to 100[kHz], which is more than good enough for my purposes for now. All-in-all, a pretty decent hobbyist function generator for the time, and even now if you don't need too much fancy functionality. Below in Figure 1 is an advertisement in SWTPC's newsletter for the device that was sent out in 1975.
There's just one thing about this particular device, however, and it's that it was being sold as "for parts" and that it was untested, meaning the device is likely broken. But hey, for the total price of $45.94, with shipping included, I feel like that's a bargain whether I have to fix it or not! A picture of the unit is shown below in Figure 2.
Right off the bat, looking at the front of the unit it is clearly missing the 0.1[Hz] frequency range selector button, which is not ideal. It connects into the switch board inside the unit, so that will most likely have to be opened if I wanted to be able to use that frequency range, and I do, though admittedly I can't think of any proper uses for the range of 0.1[Hz] to 1[Hz] off the top of my head. Besides the missing switch, there really aren't a whole lot of other problems from the outside it seems, other than the obvious wear over time with the scratches on the main frequency knob, some of the stickers on the button peeling, or just general dirt. Before we crack it open to get a look at the insides, let's just turn it on and see if the thing works in any capacity first. Since it's untested, it might turn out that it works, and the only real thing that'd need fixing is the 0.1[Hz] switch button. I got the oscilloscope connected to the function generator's output and...
IT WORKS!! Oh, oh no, wait hold on, I misspoke... It doesn't. Cycling through all of the functions available on the unit keeping the frequency constant at 1[kHz] to test, each one seems to work as intended. That is, until I tried changing the frequency. So the keen amongst you may have noticed in the picture above that the frequency that the oscilloscope is measuring is about 2.5[kHz], which is already wrong. Using the scope's cursors I measured over once cycle of the sine wave and came up with a frequency of just about 1[Hz]. Now, this may just seem to be some sort of oscilloscope issue, but I don't think so. I know this scope works and is accurate enough to measure basic waves like this, as I've used it countless times for just that purpose. And to top it off, changing the frequency with the main knob, or with the frequency range buttons had NO effect on the waveform. I think this is definitely a problem with the function generator itself, which means it's time to crack it open and get acquainted with it.