[Icom] icom ps-125 schematic

Sun, 2 Jun 2002 16:25:01 -0400

Kevin:

The PS-125 Power Supply's fan is controlled by Q14 (2SC945) which is a black
plastic (TO92) NPN transistor.  This black (-) lead of the Fan is connected
to the collector of Q14.  The emitter of this transistor is connected to
ground (common).  The red (+) lead of the Fan is connected through a 15 ohm
resistor to 13.8 VDC.  The Fan interfaces to the PC Board via connector
CN15.  You will find Q14 just to the lower right of CN15 when looking at
this connector with the front of the PS-125 towards you.

Transistor Q14 turns ON to ground the Fan's black (-) lead.  This is when
you hear the Fan wind up to MACK 2.

You can add a 6.8 V, 1 Watt Zener diode across Q14 (cathode of the Zener
Diode to Q14 Collector) so that the Fans runs continuously at low speed
(with approximate 5 VDC across it).  Then if the Power Supply heats up
during transmit it will ramp up its speed, but it will only do this
occasionally during longer periods of transmit (unless air flow is choked
off to the Power Supply, or the ambient temperature is high).

If you don't want to remove the Power Supply PC Board to add a Zener diode
across Q14, you can simply run the Zener Diode between the Fan's (-) black
lead (cathode) to ground (anode).

Note:  I have seen a modification for the PS-125 Fan circuit that uses a
resistor in series with the Fan so that when it comes ON it will runs at a
low speed.  I highly recommend against this, because when the Fan is
signaled to come on, it is truly required to cool the Power Supply, and this
just isn't sufficient at high ambient temperature. I'm sure that you would
find times when it just runs continuously even in receive mode under these
circumstances, and then the Power Supply ends up running too warm or hot,
especially during transmit.

Please contact me if you have any questions.

73, Ted, ([email protected]) Rochester, NY

----- Original Message -----
From: "Kevin Schuchmann" <[email protected]>
To: <[email protected]>
Sent: Saturday, June 01, 2002 10:37 AM
Subject: [Icom] icom ps-125 schematic

Hi All,
  has anyone run across a schematic of the ps-125?  I want to go inside and
modify the fan circuit so the fan will run all the time
at a very low and quiet setting, and then when it gets hot it will come up
too full speed like normal...  I find the fan switching
on and off  just receiving to be a little distracting...

Thanks
Kevin WA6FWF

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