This is a Sony EVO-9500A (old 8mm tape player/recorder). Picture of the PCB in a Sony EVO-9500A showing the mystery component What is the cartridge fuse style component? It is 1/4" by 1 1/8" about the same size as a 3AG fuse. It has a scale on the front going from 0 to 10 and looks like a thermometer. The only markings on it are on the back "FC". The PCB has its slot labeled "FC901" (as can be seen in the last picture).

This thing reads about 3.2k ohms resistance (readings fluctuate quite a bit 10k on the high end down to 2k ohms).

I'm not sure if those reading can even help because I don't know if this component even works. (The tape player is not sending video or audio out, which is what started all this.)

close up of the thing, front and back

slot on the PCB for the thing. labeled "FC901"

So what is that thing and what does it do?

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    \$\begingroup\$ That is an industrial grade unit, not a home recorder, as might be found in cable TV studios or an ENG truck. \$\endgroup\$ Commented Jun 4, 2017 at 3:00
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    \$\begingroup\$ The fuse has an on-board thermometer, which is obviously in the red. You need to replace with an OEM "Thermo-Fuse(TM)." \$\endgroup\$ Commented Jun 6, 2017 at 17:49
  • \$\begingroup\$ Did You replace it finally with something else ? \$\endgroup\$ Commented Feb 7, 2022 at 14:59

3 Answers 3


It is an electrochemical hour meter, historically a thin tube filled with mercury with a drop of an electrolyte, current flowing causes ions of mercury to be transported across the electrolyte moving the bubble of electrolyte along the scale.

It almost certainly still works just fine, but will gradually move back the other way if fitted in reverse....

You don't see them much anymore, counters in non volatile RAM being cheaper and more convenient.

  • \$\begingroup\$ Do you happen to know the scale of the hour meter? Is the 0 - 10 hours, days, years? \$\endgroup\$
    – David
    Commented Jun 2, 2017 at 16:40
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    \$\begingroup\$ Depends on the current level, clearly as the process is ion transfer. I would guess from the board labelling that in that application 0->2000Hrs, giving a reading of about 1200Hrs. Sounds reasonable for something like head drum operating hours? \$\endgroup\$
    – Dan Mills
    Commented Jun 2, 2017 at 16:49
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    \$\begingroup\$ 12k hours? That's a lot of movies. \$\endgroup\$
    – JAB
    Commented Jun 2, 2017 at 19:55
  • \$\begingroup\$ I thought it was mercury based. \$\endgroup\$ Commented Jun 2, 2017 at 20:41

The text beside the mounting clips says "X 200 Hour", so I'd guess that it records operating time of the tape player.

  • \$\begingroup\$ The Red mark is at 6 of 10. \$\endgroup\$ Commented Jun 2, 2017 at 15:25

These were common on Japanese professional video recorders from the 1980s. I believe it's a a capillary tube of mercury and was told back then that they can be reversed. The helical scan heads typically had a life of around 300 hours and various pullies and belts also needed periodic replacement. The counter was non-resettable, so a reliable indicator of the life of the machine. In my experience, the machines were just as likely to become obsolete as to wear out!


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