Fuse blowing printer

Heathkit had a couple printers they made in kit form. One was the H-14, a seven pin dot matrix which had some interesting design features, but it dated to 1977, and it showed in many ways. While I would love to have one for historic purposes now, then, it was best avoided.

However, they also had the H-25 (or in H-100 colors, renamed the H-125, in preassembled form, either WH-25 or Z-125), which was a mighty workhorse printer. It had four tractors -- two pulling paper into the printer, two others pulling it out. H-25 printers had paper jams only when something external to the printer was very wrong.

But since it was a kit, things could go wrong that most printers COULD never experience -- like builder error. And there were a lot of very expensive parts that you didn't want builder error to break, so rather than having one fuse in the power supply of the printer, there were something like 25 fuses, one for just about everything the printer could do (one for each print head pin (9), one for each winding on each stepper motor (at least eight). The idea was if something went bad, the fuses would blow before the print head or stepper motors fried.

It turned out there were some building errors that could cause bad things to happen, but saved by blowing fuses ... ALL the fuses, and sometimes really a trick to diagnose. For example, if you pinched the speaker wire against the metal chasis of the printer, the printer would power up and test print fine, but when you attached it to the computer, it would freak out and fire all the drivers at once ... blowing all the fuses.

Well, one came in dead. Opened it up, and sure enough, all the fuses were blown. So, we started by checking the speaker wire -- fine. We replaced the fuses, applied power...and it worked fine.

We attached it to a computer, it worked fine. We printed out a big chunk of a box of paper...worked fine. Called customer, "couldn't reproduce", they picked it up, and called us back to say it immediately died upon firing it up at their office.

"Bring it back with the computer" -- we figured a flaw in the computer might fake the speaker problem. But no, attached to their computer, it still worked great.

"Bring in everything associated with the computer, please". This time, they brought in a Constant Voltage Transformer -- a nifty device that cleans up dirty power very nicely. However, it was rated only 400va, which was, I thought too small for their computer (a Z-100) and this printer together. Sure enough, if we attached the computer and printer to the CVT, the printer immediately popped all its fuses on power on. Apparently, the limited capacity of the CVT caused the voltage to rise too slowly in the printer, causing the logic circuits to go spastic and fire all the heads and windings at the same time, popping all the fuses.

Troubleshooting can be fun...

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since 2/14/2021
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