[Elecraft] Radio Shack Switching Power Supply

Sat Nov 30 13:06:01 2002

I've been wanting to raise the output voltage of my RS switcher also so 
I searched the archives for info on RS 22-503.  The following posts came 
from QRP-L and Elecraft, April 1999.   Please note the warning about 
hazardous voltages.  In their entirety ......

Randy   KD8G, W8KEY

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Date: Thu, 22 Apr 1999 04:16:05 -0700
From: David Fifield <[email protected]>
To: Low Power Amateur Radio Discussion <[email protected]>
Subject: [38692] RS 22-503 Switching Power Supply
Message-ID: <[email protected]>
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Guys,

First - a warning - DO NOT open your 22-503 power supply with
it plugged into an AC outlet and don't attempt the mod I
suggest below unless you absolutely *KNOW* what you are doing.
There are lethal voltages inside this unit - be very careful!

I had a look inside my new 13.8V 3A Radio Shack PSU tonight.
It uses a Power Integrations TOPS II switch device, the TOP226Y.
The circuit is very simple. I traced out the schematic a bit
and compared it to the Power Integrations datasheet on the web.
I found the circuit to be very similar to the one on page 7
of the datasheet at:

http://www.powerint.com/PDFFiles/top221-227vc.pdf

If you replace R1 with a 500Ohm ten turn precision trimpot,
you can vary the output voltage from about 13.5V to over 16.5V
(don't go too high!). Certainly, you can control the output
voltage very accurately around 14V, which is where I know all
you K2 builders using internal batteries want to be.

You opened up your unit and couldn't see R1? That's because
it's a surface mount 390Ohm resistor on the underside of the
PCB - you have to more or less completely dismantle the
power supply to get the board out to have at R1.

It's not a bad design. I don't like the way the power output
wires track all the way to the back of the unit over the
switcher again (they are bound to pick up noise doing this).
Screened leads or repositioning the output terminals might
be cures for this....YMMV.

BTW, I *DO* have a vested interest in Power Integrations, so
my opinion here is biased - I have a substantial block of
POWI shares in my retirement portfolio!

72, Dave Fifield, AD6A

From: Jerry Henshaw <[email protected]>
Date: Sat, 24 Apr 1999 20:51:08 -0400
Subject: [Elecraft] R/S Switching Power Supply Variable Output Mod
Message-ID: <[email protected]>

Hi Gang,

Thanks to Dave F. for sluething out the Radio Shack switching power

supply.  I never thought about looking on the bottom of the pc board for

SMDs.  I removed resistor R1 (390 ohms) and replaced it with a R/S 1K

15-turn trim pot.  I super glued the trim pot the output transistor 
heatsink

and aligned it with one of the ventilation holes in the top cover... I 
ran a short

length of hook up wire from the R1 pads to the pot.   I pre-adjusted the pot

to 390 ohms (just to get close).   I can adjust the output voltage 
through the

top cover vent hole.  Mine is now 14.05 volts.  The adjustment range is 
quite

large .... mine was over 16 volts when I first installed the pot.  The mod

works great!!  Thanks again Dave.

72's

Jerry Henshaw
    KR5L/4
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