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   IBM JR POWER SUPPLY 5VOLT LINE ADJUSTMENT
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  by Charles L. Terra PhD PE
     chuck1953@bellsouth.net
     12/26/04 
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This mod adjusts the JR power supply 5 volt output.

Please look at all the photos before doing any modification...
Also read this mod completely before starting...


Make sure you have the same type power supply as shown in the 
photos, as the components may be in a different place if it
is not the same model/type.

I noted that the 5V line was really reading 4.85V, which is
really boarder line for TTL components and can cause signal
integrity problems..

I used a 100K 10 turn potentiometer for the modification…
This is a very common POT and can be purchased at Radio Shack etc…


This modification puts a Poteniometer, across the JR power supply
resistor  R15 which allows the 5V reference to be adjusted
This resistor is a 2.15K ohm value....or color code of

RED / BROWN / GREEN   the first three colors on the resistor.
Looking at the component side of the board it is the 4th resistor
down from the top side of the board...


Look at picture 5V_Mod1:
Note the location of the added potentiometer.

Look closely at picture 5V_Mod2:
This shows the POT sitting between the component legs on the back side
of the board…To make this fit, you will have to lift the legs of these 
up away from the board just a little….This allows the POT to sit flush
against the circuit board.

Look at picture 5V_Mod5:
This shows how to solder the legs to the existing pads on the back of the
circuit board.  The POT will have 3 legs on it, you will bend two of them
to the ‘right’ as shown in the picture and they will be soldered together on the
pad to the ‘right’.
The Left most leg of the POT will be bent to the left  and soldered to the
‘left’ most  pad.


Check your solder connections for solder shorts.

This is just for reference of the potentiometer :

	------------------
	|		 |
	|       2	 |
	|	O        |
	|		 |
	|		 |
	|  1 	      3	 |
	|  O	      O  |
	------------------
	BOTTOM VIEW OF POT


	------------------
	|		 |
	|		 |
	|	O        |
	|	 \ 	 |
	|	  \	 |
	|	   \	 |
      --|--O	    -O---|------
	------------------
            BEND LEGS 

Legs 2 and 3 should be connected together as mentioned above


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NOTE:

This is best if you have an OHM meter at this point, but you can do this
with out one.....

With an OHM Meter:

Place the leads across the two soldered legs on the POT.
Adjust the POT until you get the highest reading...this should
vary between 2.15K and  2.10K   relatively....

WithOut an Ohm Meter:

Turn the trim adjustment on the POT in one direction Left or right,
and turn for at least 12 turns...this will ensure the POT is at one END of
it's range.   
NEXT:  turn the pot 5 FULL turns in the opposite direction.

Doing this will center the POT and allow you to adjust it with confidence that is
is not too high or too low.



POWER UP THE JR WITH THE MODIFIED POWER SUPPLY:
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Replace the power supply in your JR.

You WILL need a volt meter at this point !!

Place the leads across the FLOPPY DRIVE connector
the two center pins are GROUND and the outter pins
are 12V and 5V.  OR use any other point that you feel
comfortable with measuring the 5v.


POWER ON THE JR:
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Looking at the Voltmeter, adjust the trim pot until
it reads 5 Volts.

You may have to turn it a few turns to get to 5V.

The Trim limits you should stay with-in are:
 5.10V --  4.90V

these are acceptable limits..but it is always best
to get the Voltage as close to 5.00VOlts as possible...


PROBLEMS:
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Assuming your modification does work...ie...it makes
the 5V line adjust higher.  

Then:

If you seem to be doing every thing right and still can't get the
5V line HIGH enough you may need to use a smaller POT.  You can
try a 75K or 50K pot.

If you seem to be doing every thing right and still the
5V line is TOO High and you cannot seem to get it low enough to the 5V measuement, 
you will have to use a larger POT.  You can try a 150K pot.

Enjoy  Chuck