New England QRP Club : The Excitement is Building....

Hi Grant.
I'm sorry to hear about your trouble after applying 17 volts to your NEScaf.

If you look at the schematic for the filter, you will see that the 555 timer IC is powered by the output of the 9v voltage regulator. The LED and the SCAF chip are also powered from the regulator. The only place the "raw" DC power goes is to the regulator and the LM386 audio amp.

Since the LED does not light, It is likely the regulator has been damaged. However, they are usually good up past 30 volts. You can check it's output with a voltmeter to be sure.

I would suspect that there may also be damage to the audio amplifier IC, the LM386. It is generally only good up to about 16 volts or so.

If you touch a metal screwdriver to pin 3 on the audio amp, you should be able to hear AC mains "hum" from the headphones or speaker.

If you could proceed by checking DC voltages at the power pins on the ICs and the in/out of the voltage regulator, I am sure we can help iron it out for you.

Good Luck,
Bruce Beford, N1RX

Bruce,

Found the IC4 on the board. A seniors moment, I think. I have checked all the voltages on the various ICs and as far as I can see, they look good. The regulator shows 8.9v on pin 8. I also get the voltages suggested on ICs 1, 2 and 3 as per Stage 1 of the build instructions, so it seems to me that the ICs are OK. Putting a screwdriver on pin 3 of the audio amp does produce hum, even though it is slight. So tonight I'll try the filter on a qrp rig on 40m and see if I can shift the band at all with the left hand knob. Can't tell if this function is working without a signal coming in - swinging the knob seems to produce no difference.

Grant VK4JAZ

Hi Grant. It is good that you are getting some "action" out of the right hand (dual) pot. That is the bandwidth control, and serves to widen or narrow the filter response. The other pot, with the center detent, is the "center frequency" control. It controls the frequency that the 555 clock generator runs at (in conjunction with the on-board trimmer pot.) If you turn the bandwidth pot to it's extreme position (narrowest setting), you should be able to hear a difference in band noise as you vary the frequency control. The noise should change in pitch as that pot is moved from lower to higher frequencies.
-Bruce N1RX

Grant, there is still a possibility that you had some other problem unrelated to the excessive supply voltage. If, as you said it was working properly once, I would be checking for faulty solder joints, a broken wire to the pot, etc. If you have access to a frequency counter or oscilloscope, you should be able to see the output frequency of the 555 changing as you rotate the freq control.

I can't imagine a failure mode of the SCAF chip that would allow the bandwidth pot to vary the filter, yet the clock frequency change not to vary the center freq.

I would be looking to borrow a freq counter or scope to check the output of the clock generator, after carefully checking all connections.

Also, in your original post, you said the LED had been working, then went out. Did it just "start working again?"

-Bruce

Grant,

if You hear in the headphones noise but no effect when turning the detent pot then try to check if the NEScaf board has proper grounding. Specially to not have grounding loops, long wires etc. I noticed the problem like that when the filter was on the bench with long wiring and some of GND loop between in/out sockets etc.
Try to wiring just single wire from PCB gnd to input socket gnd then continue to output socket gnd...
Let's try if there is noticeable change when detent R10 pot turning cw/ccw.

73 - Petr, OK1RP