[HBQRP] Troubleshooting the SWX0+ or "why I hate clip leads"

[Darrel Swenson]

Hi Gang...

After I sent out the Elmer 101 e-mail last evening, I decided to do the
first steps on my SW+ kits.  I had previously inventoried the parts on my
kits, and had also installed the first 4 components, so after a brief read
of the manual, I set to work to see if I could get +8V on my kits.

I found an old RS power supply, hooked it up to my SW30+, and measured the
output voltage.  +15.7 volts is a little high, but from reading the Elmer
101 material I knew U2 is rated at over 20V input, 100mA current.  Since
nothing else is installed, the current draw should be minimal so I should be
OK.  (I planned to open the power supply up and see if there was a voltage
adjustment tomorrow.)

I measured the voltage across D13.  15.7V on the anode and 15.25V on the
cathode... .45V drop.  OK so far.

I measured the voltage on pin 1 of J4...  14.52V... "what tha...?"

OK, maybe the power supply is not a good idea.  I dug a 12V lantern battery
I purchased a couple of years ago Wheeler's went out of business, out of the
junk box.  12.5V... perfect.  I hooked up the battery.

Now I have 12.5V on the anode and 12V on the cathode... perfect.

I measured the voltage on pin 1 of J4 again....  10.9V...  "what tha...?"

OK... now it's getting serious...  I had done this once before 2 or 3 years
ago, so I knew it worked once... what happened?  Maybe my less than proper
handling of the circuit board and IC over the last few years and blow the
IC...  Let's try another one...  I dig out the SW20+, hooked it up, and
measured the voltages...  10.8V on the output of the IC again...  I try the
SW40+... 10.7V... "what tha...?"

They can't all 3 be bad... can they?

Finally I decide to use some basic trouble shooting...  11.9V on the input
(my lantern battery is dying fast!) 2.3V on the ground plane... huh...  0
volts on the battery end of my clip lead and 2.3 V on the PC board end...
hmmmm...

Unhook the clip lead... 850 ohms resistance...  "I hate clip leads..."

Finally, I solder a couple of longer pieces of wire to my power lead... heat
shrink the positive lead... and hook my voltmeter to the PC board ground
plane (instead of the negative terminal on the battery.)

8V where it's supposed to be...  on all three boards...  "I hate clip
leads..."

By now it's now Midnight...

Today I opened up the RS power supply... transformer, diode bridge, and 4700
mF cap.  No regulation!  Think I'll rebuild that somehow... time to crack
the handbook.  In the mean time, I found a bigger, but better supply (only
14.7V dc!)  I'll use it for now...

Re-measured everything... 8.04V on the SW20+, 8.17V on the SW30+, 7.95V on
the SW40+, and 8.18 on the SW40 (which has no polarity diode... think I'll
add one when I wire it into a cabinet.  I measured .61V to .67V across the
others.

By the way... the link for the 78L08 IC data sheet in the Elmer 101 material
has changed over the last 7 years.  Here is the current link:
http://focus.ti.com/lit/ds/symlink/ua78l08.pdf

Darrel ("I hate clip leads") Swenson...
K0AWB