Corning PC-420D Hotplate/Stirrer repair
I picked up a Corning 420D on eBay, the device was listed as not powering up and was relatively cheap so I figured worth a risk. It turned out to be a very simple repair, but I figured I’d write it up anyway… perhaps the reference images will be of use to someone.
The device indeed showed no power LEDs, but the hotplate was heating up… Taking a look inside you can see that the design is pretty simple. All through-hole, there’s what looks like an 8bit MCU of some description. The stirrer has a optical interrupter sensor on it, which is attached directly to the PCB.
There were no obvious issues on the top side of the PCBs so I removed the main PCB and discovered the ugly mess below… I guess there had been some lab spills…
Cleaned things up with some IPA and touched up some of the joints, but the device still didn’t work (unsurprising). So I started probing around. There’s a 74HCT7541 on the front which was a convenient location to measure digital supply voltage levels… VCC was less than 2V and fluctuating…
I probed the transformer this was putting out ~10v AC and was being rectified… But there was nothing coming out of the regulator (probably a 7805 but I didn’t even get round to checking). Turns out the trace was broken just before the regulator, when I bridged this everything started working… Sorry not a very exciting repair!
You can see the bodge in the picture below (I used a small piece of wirewrap wire). I could do with cleaning the PCB up a bit more, but I’ll leave things as they are for the moment.
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