[sebhc] H89 strange problems....

[Lee Hart]

Dwight Elvey wrote:
> If the connectors haven't degraded too much, there is
> another solution that I've been using for years on high
> current connectors. I put silicon grease on the pins.

This is a tried and true solution for high current connections.
Basically, it keeps air and water away from the contact surfaces so they
don't get dirty or corrode, and provides a little cooling (anything
conducts heat better than air).

But in this case, the fundamental problem is that the Molex pins that
Heath used are only rated at 5 amps RMS, and that assumes good cooling.
When you have a fully-loaded H89 with 64k memory, two disk controller,
serial board, etc. the current in this connector is over 5 amps. And,
the location is poorly cooled, exacerbating the problem.

While we're on the subject, let me mention some things I did to improve
H89 power supply reliability. The hottest spot in the H89s was that
metal plate beside the power supply that held the bridge rectifier and
regulators.

1. The fans were randomly oriented; some blow up, some blow down.
   The bridge and regulators ran cooler if the fan was oriented to
   blow air DOWN.

2. The Z89-37 soft sector board came with extra finned heatsinks to
   go on the three regulators on the power supply plate. Be sure to
   add them any time you have 3 or more accessory boards plugged
   onto the CPU board.

3. Don't run the flat cables for the disk drives across this power
   supply plate; it blocks airflow. Route these cables low and away
   from this plate.

4. The metal plate has a 90 degree angle at the bottom. Bend it a
   little sharper, so the top of the plate leans over the power
   supply PC board a little bit. This puts it *in* the airflow from
   the fan rather than beside it.

H89s with these changes ran considerably cooler, and never had any
failures of the regulators or bridge rectifier.
-- 
If you would not be forgotten
When your body's dead and rotten
Then write of great deeds worth the reading
Or do the great deeds worth repeating
	-- Ben Franklin, Poor Richard's Almanac
--
Lee A. Hart, 814 8th Ave N, Sartell MN 56377, leeahart_at_earthlink.net
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