An easy hop off the shame shelf

I was fiddling with an op amp circuit on the new breadboard - using positive and negative supplies.  It wasn't working so I tried looking at the supply voltages, the negative supply is less than -17 volts (unregulated) - it should be more like -12 volts:

a 20th century effort at a home built power supply

So I tried the easy thing, re-seating the op amp in the socket, no help.  Then I went into the parts box and found three 741 op amp NOS parts from DigiKey - I tried swapping out the original 741 and put in a "new" 741 and voila:

the marking on the replaced 741 is easier to see than on the bad 741

I always tell kit builders with non-working kits that it's never the chip but maybe after over 30 years it is the chip, at least this time :)

This supply is derived from an old National Semiconductor app note as I recall.  The positive supply uses a 3 terminal 12 volt regulator.  But the negative supply uses the 741 op amp to drive a series pass transistor.  The output is sampled with a 50% voltage divider (with a pot in the middle).  The regulation comes from driving the pass transistor until the negative output voltage is the same as the positive output voltage (in magnitude) - this drives the middle of the voltage divider to ground.  Unfortunately I can't find my schematic or references, believe it was in my mythical spiral notebook with the Star Wars robots on the cover.  Here is a measurement of the positive supply:

I slapped the cover back on - however now that I'm thinking about it I should probably add some kind of insulation over the fuse holders, they are AC power line fuses.

Best Regards,
Chuck, WB9KZY
http://wb9kzy.com/ham.htm