HUMBROL Computer Logic Lab

I was given this electronic kit for either my birthday or Christmas in 1986. It’s prettty basic by today’s standards, the only IC is a TTL 7400 Quad NAND Gate. The only inputs and outputs are three Morse-key style switches for inputs, two red LEDs make up the visual output, and a high-impedance crystal earpiece is used for audio output from oscilator circuits.

Circuits are built by connecting individual components together with flying leads pushed into the springs that form terminals on each component. It’s basic, but it works. I quickly worked though all the example circuits in the accompanying book (I really must scan it and upload it to the Internet Archive), then used the kit for a few more years building my own circuits on it and connecting it to other kits and circuits on my bench.

Since the early 1990s, the kit has sat unused in the attic. A recent thread on social media reminded me of it, so I got it down from the attic and was surprised to see it was still in very good condition. I was very pleased to see I removed the batteries before I put the kit away - no horrific 30 year old leaks to clean up! The LEDs still work (I have a vague memory of burning these out, so I must have replaced them at some point), the IC still functions, and the earpiece is still painful to wear in the ear - something never change. The only other parts that might have suffered the years badly are the electrolytic capacitors of which there are four, 2 x 220 𝝁F and 2 x 2.2 𝝁F.

I grabbed a handy component tester, nothing fancy, just a cheap ebay LCR & Transistor tester, and measured the capacitors, the results are show in the table.

The 220 𝝁F capacitors seem pretty much on-spec, they were low-end parts at the time and not used in any power supply circuits, so I’d ot expect them to be low-ESR types. The 2.2 𝝁F capacitors seem to have gained capacitance and their ESR does seem a little bit high - I’ll probably replace them.

A final test of all the resistors using my Keithley 178 bench multi-meter and my UNI-T UT171C multi-meter shows all the resistors to be within tolerance.