Oops blew it!
Submitted by nh7wb on November 1, 2008 - 11:41.
NEScaf Discussion
Double oops, got my comment in the wrong place. sri.
Did have a good working NEscaf, loved it, but while changing my power connector, crossed a test lead at the wrong moment and blew the thing. Yes, all was installed. Where do I begin to find what blew, or am I better off building again?
Paul
NEScaf Troubleshooting
Hi Paul.
I don't know if anyone else has gotten back to you with suggestions on how to troubleshoot your NEScaf after reversing the polarity of the power feed.
You can be fairly confident that the 9v regulator is toast. They don't like rev pol at all. You may be lucky, and that is all that is gone.
If you built the filter using IC sockets, then troubleshooting can be simplified. First, remove all the ICs from their sockets: MF-10 (or MF-100), LM555 and LM386. Then check the power input pins on each socket for the proper voltage. (All voltages measured with respect to ground) Pins 6,7 and 8 of the MF-100 and pin 8 of the LM555 should be at about 9 volts (the output voltage of the regulator). If not, then the regulator is blown and will have to be replaced.
Pin 6 of the LM386 (output amplifier) should be at roughly the same as your input voltage (12-14v) It is not powered by the regulator, but rather driven directly from the supplied voltage, via a 10 ohm resistor, R11.
Even if you don't have the ICs in sockets, you can check the above voltages to check the regulator. Next, you can check the audio amp by powering up the filter and use a small metal probe (like a jeweler's screwdriver) to touch pin 3 of the LM386 (or the junction of C9 and C10) this should couple some 60 Hz ac "hum" into the amp, which you should be able to hear in the speaker or 'fones. If you don't hear the hum, then the amp chip is probably blown.
Finally, if you have access to an o'scope or frequency counter, you should be able to measure the clock output of the LM555. This will be a square wave, at 100 times the frequency the filter is set to. If you have your filter set to 600 Hz, the clock frequency is 60 KHz. This will only work once you have the 9v regulator working.
The regulator is most likely gone. The other ICs may or may not be bad as well. These semiconductors are the most likely casualties in the filter, The resistors and caps are probably all fine. . There are some basic ideas to get you started. Depending on what you observe, and have available to work with, I may be able to offer further advice.
GL,
Bruce N1RX