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Repair of Yanmar Tach-Hour meter.


Hour meter display is erratic, faint or flashing numerals, or no display.Hour counter is still operating but display is unreliable, unreadable or simply out.



Remove failed/defective silicone rubber LCD substrate passivation whereribbon cable and display integrated circuit are attached to the LCD (Liquid CrystalDisplay) substrate. This material was introducing spurious conduction paths.

Root cause:

Defective silicone rubber applied over the IC, ribbon connection andLCD substrate became conductive or trapped contaminants and/or humidity anddisrupted circuit interconnections on the LCD display substrate which includestransparent Indium Tin Oxide conductors and a surface mounted integrated circuit

Access to the trouble area requires careful lifting of the face display
which is glued down. I used a razor blade and exacto knife to separate the face plate
from the mounting face.

Having lifted this face plate, a wooden tooth pick, chisel shaped, is used to dislodge
and remove the failed/defective silicone rubber which crumbled away from the
substrate, ribbon cable connection area and integrated circuit. A wooden tooth pick
is recommended because it will not scratch the LCD substrate, ribbon cable or IC.
The LCD substrate has transparent conductive oxide electrodes made of the
transparent conductor Indium Tin Oxide (ITO).
The silicone compound covering this area flaked off and crumbled readily. It must be
scraped away from all surfaces to which it was applied. A can of air is helpful to blow
the crumbs away. A small stiff brush can also be helpful. Once the majority of the
material is removed, a careful going over of the surfaces with a fresh, sharpened
chisel flat, wooden toothpick will remove the last bit of residue. This last step is very
very important because the defective silicone rubber on the surface of the LCD
substrate must be fully removed to eliminate spurious conductive paths.

1. Tach-Hour Meter assembly lifted out of housing showing display ribbon cable and






2. Tach-Hour meter assembly showing display ribbon cable and connector. Note
connector collar down in the capture position.
























3. Circuit board end of display ribbon cable out of socket. Note socket collar lifted up
in the release position. This end of the display cable and its circuit board connector
were originally thought to be the source of the problem. After cleaning these
connections and application of silicone grease to create a gas tight fitting, the display
continued to fail.





















4. Silicone grease used on connector end of ribbon cable to create gas tight
connection between ribbon connector cable and connector.


























5. Thumb holding face plate up to expose ribbon connection area including LCD and
IC at the right of the LDC display.




























6. Thumb holding face plate up to expose ribbon connection area including LCD and
IC at the right of the LDC display. A close-up of this area in the next photo.




























7. Close up of LCD substrate area where ribbon cable connects and IC is connected.
This entire area had the failed/defective silicone rubber compound applied. Residue
is still present on the substrate surface. Not done yet.
Ribbon cable to
Silicone rubber
residue and























8. Reassembled Tach-Hour unit with persistent, bright, well defined numerals in hour
display. Tach shows 3000 RPM with 510 Hertz square wave applied to tach input.
: I expected the problem to be mechanical via connector corrosion but
instead found a failed and defective silicone rubber compound covering conductors
on the LCD display substrate. The purpose of this compound is apparently to
prevent humidity and contaminants on the LCD substrate and its exposed
transparent conductors of Indium Tin Oxide. After removing all of the silicone
compound and its residue, the spurious conduction paths were eliminated and the
display began to work normally. No damage was discovered or caused in the
disassembly. The unit is returned to its original operating condition.
The unit was placed in the freezer for an hour then removed and powered up. No
problems were seen with condensing moisture on/in the unit. However this remains
a concern since the removed silicone compound was originally applied for a reason.
I tried to measure the conductivity of a crumb of the silicone rubber I removed but
was unable to make a reliable contact to the very small crumbs on which I attempted
this measurement.