And so to the next instalment of trying to get my Epson MX-80 F/T III dot matrix printer back up and working.
Previously on Epson MX-80… Just over a year ago, lots of smoke came out accompanied by a crackling noise. I’ve removed the main smoothing capacitors from the PCB and tested them. They seem fine. I tested the transformer. It seems fine. WTF.
Nothing shows any sign of damage. The only indication that there was an incident is some smoke residue on the underside of the upper part of the case. This is just above the transformer but could, conceivably, have come from the caps too. I’ve also taken a close look at everything on the PCB that might possibly have caused the smoke stain. Nothing to report.
Naturally, I hit the forums. One person mentioned the possibility of the bridge rectifiers having let go. There are three of these, for the 25V, 15V and 9V AC supplies coming from the transformer. But all three look in tip-top condition.
Another comment was that capacitors might test fine on a multimeter but fail when taken up to their working voltage. So I took each capacitor, strapped a 100k resistor across the terminals as a load and powered up from my bench supply. I also monitored the voltages with the oscilloscope (although, to be honest, the DMM would have done just as well). I took each cap up to its rated voltage. No smoke, no heat – nothing out of the ordinary. When I cut the power I watched the voltage decay exactly as you’d expect a cap to behave. In other words, these caps are acting as if nothing’s wrong.
Using a 100k load might have been a tad over-cautious as it kept the current very low. But I wasn’t really in the mood to have a capacitor catch fire on my workbench.
I’m still not entirely sure about the transformer. There is that slight bulgy-ness (yes, that’s a word). It’s the same on both sides. But like I said, it’s working. Last night I had it powered up for over an hour. No smoke, no heat.
So at this point I’ve come to an astonishing conclusion:
Sometimes the magic smoke can make its way back inside.
It’s going to be a little while before the replacement caps arrive on their slow boat from China. But when they do, my plan is to solder them to the board, put everything back together, power up and see what happens. I mean, this is a 35 year-old printer. What have I got to lose?