Thursday Circuits, Gremlins, Repairs

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Thursday circuits either only work on Thursdays, of fail on Thursdays, during customer demonstrations.
Gremlins are the pesky little bastards the creep out of the wood work and  bite you in the arse, especially with new designs.
Repairs are repairs, but they are often anything but straight forward, especially if you have to deal with the owner too.

I have many stories in this area- but what are yours

(some stories at replies #34 and #37 here: https://www.eevblog.com/forum/projects/circuit-for-mosfets-in-parallel-for-extra-current-capacity/msg2082850/#msg2082850)

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My favourite bug like this was the OpenOffice doesn't print on Tuesday's classic.

The TL;DR:

Print to a postscript file. Observe the line:
   %%CreationDate: (Tue Mar 3 19:47:42 2009)

Change "Tue" to anything else:
   %%CreationDate: (XTue Mar 3 19:47:42 2009)

Save the file and it prints. Tools like evince work because they simply omit the "CreationDate" tag to begin with.

Now something odd happens when my cups script (I am using the Brother MFC420CN) copies the file to a temp file. Some of the code is rearranged, not sure how or why, but it uses a command called "file" to identify the file as "PostScript". This check would work on the original file you printed, but by the time it runs the check on the temp file, it misidentifies. Normally it would return:

   PostScript document text conforming at level 3.0

But there is another check that happens before the PostScript check. If it finds "Tue" at the fourth byte of the file, it identifies it as:

   Jan 22 14:32:44 MET 1991\011Erlang JAM file - version 4.2

So it's not a problem w/ openoffice.org, cups, or the brother printer drivers. It is a bug in the `file` utility

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In the old days, I was a sucker for helping people out when their, TVs, radios, HiFis, cars, etc needed fixing.

One day a neighbor called in an anxious state. He said that he had been moving his TV and had dropped it. He had a large family and was not very well off, so I asked him to bring the set around, which he did. You have never seen such a mess. The veneered ply cabinet was practically split in half as was the main PCB. I said that there was not much I could do with the set because of the extensive damage, but he seemed so distraught at this news that in the end I relented and offered to have a look at it.

The cabinet glued together really well with PVA, so much so, that after a light polish, you could hardly see where the crack had been,  unless you knew where to look. But the electonics were another matter. It took me a week of evenings to Araldite the PCB back together and wire link all the broken traces. Also, some components were damaged and needed to be replaced. Luckily the tube was still intact.

After another three evenings, fault finding, the set finally sprung into life, and after a quick set-up and soak test it was ready to go back to its owner.

On the following Sunday, I took the set over to his house. He was overjoyed that the set was working again and said, "Thanks a lot, that will just tie us over nicely until our new set arrives next week."