Problems with Regulator Decoupling

On two separate occasions recently equipment was not performing quite correctly, both faults were later identified as the regulator decoupling capacitors being open circuit on 78 and 79 series devices.

Firstly a Kenwood TS430 HF rig which was working perfectly except an unstable carrier swished about the receiver in the low megahertz range (like 3-5 megs).  After much thought about what could be wrong initially thinking the PLL was not locking up the fault was traced to an open circuit one microfarad decoupling cap on the output of a 7808.. Changing this cleared all the unstable carriers.

On another occasion I had needed to introduce a video distribution amplifier to provide several feeds from one video source.  The unit I used was ex equipment and previously untested by me.  I was pleased that it did what I needed to do but there was slight patterning on the picture when viewed after the DA  I tried several things like multiple earth loops but could not remove the slight interference to the picture.  The DA had previously been in use commercially so I guessed it was capable of perfect pictures. 

I first tested the rail voltages for +/- 5v from 7805 and 7905 regulators which checked out ok.  I then tried adding more smoothing to the main unregulated rails but no improvement.  I then noticed 10 microfarad caps on each regulated output.  I connected another capacitor at the output of the negative regulator and completely cleared the fault.  Double checking with a scope, although the rail was a perfect -5v it had a huge self oscillation modulated on the rail.  Replacing the decoupling cap it was then completely smooth.

Donít forget these caps are often very close to regulators that get hot and over time may go open circuit causing strange  effects. Always try a scope first on supply rails to check for self oscillation .

Denis G3UVR

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