[sebhc] IC information

[Barry Watzman]

The straight 7905 is a simple 3-terminal device -- input (a voltage at least
2.5 volts "more negative" than -5 volts, that is, -7.5 volts or more),
ground and output.  There should be a small capacitor across the output
(just a couple of uF, and it's probably already there).  The same pins exist
on the 79MGt2C, plus one other one used to change the output voltage.  What
I don't have is a reference as to which pins of the two voltage regulators
are which.  The 7905 should be mounted on a heatsink, you can use the same
heatsink as the other regulators are on.  Verify that the exposed metal back
is ground, if it's not, you will need an insulator.  Run wires to the
appropriate points on the board where the old regulator was.  If the old
regulator is socketed, you can just remove it.  In fact, you could use a dip
header plugged into the IC socket to do this "cleanly".


-----Original Message-----
From: sebhc-bounces at sebhc.org [mailto:sebhc-bounces at sebhc.org] On Behalf Of
Glyn Firth
Sent: Sunday, February 27, 2005 7:07 PM
To: sebhc at sebhc.org
Subject: Re: [sebhc] IC information


Lee,
     I do not have the schematic, would you please advise.
Thanks
Glyn
--- Original Message ----- 
From: "Lee Hart" <leeahart at earthlink.net>
To: <sebhc at sebhc.org>
Sent: Sunday, February 27, 2005 7:26 PM
Subject: Re: [sebhc] IC information


> Glyn Firth wrote:
>>> I have just began to rejuvenate my H8 system. I have discovered
>>> that the -5volt voltage regulator on the CPU board is bad...
>>> The part # is 79MGT2C.
> 
> Barry Watzman wrote:
>> You can probably get one at Radio Shack, it's just a 7905, in an
>> appropriate case (probably TO-220, but 7905's come in lots of
>> different case styles, the letters don't matter). Very, very
>> common and inexpensive part.
> 
> No, Glyn is right. The H8 8080 CPU board uses a *very* unusual part,
> nothing like a generic 7905. The 79MGT2C (Heath 442-618) looks like an
> 8-pin DIP, but with pins 2,3,6,7 replaced by large wide "bat wing" ears.
> It is actually a -2.25v regulator, and uses two external resistors to
> program the output voltage to -5v.
> 
> Finding an exact replacement *will* be a challenge! However, if you just
> want to get the H8 functional, I would replace it with a 7905 (or
> equivalent 3-terminal regulator.
> 
> This is pretty straightforward if you have the schematic. If you need
> instructions to do this, ask.
> -- 
> "Never doubt that the work of a small group of thoughtful, committed
> citizens can change the world. Indeed, it's the only thing that ever
> has!" -- Margaret Mead
> --
> Lee A. Hart  814 8th Ave N  Sartell MN 56377  leeahart_at_earthlink.net
> 
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