On the factory assembled units the 1/8″ jack is wired for earphone output and the 1/4″ jack is wired for speaker output. Both jacks are wired for stereo plugs. If you want to use a mono plug you must cut the jumper between the tip and ring connections. If you do not cut the jumper the mono plug will short out the SCAF-1 output. You can use a wire cutter to cut the jumper between the tip and ring connections. On the phone jack this is a small bare wire, on the speaker jack it is a red wire. If you want to swap the speaker and earphone jacks you can easily do so. Examine the connections on the two jacks. If there is a green wire connected to each jack and no 100 ohm resistor connected between them just swap the green wires on the two jacks. If there is just one green wire and a 100 ohm resistor connected between the two jacks move the green wire to the tip connection on the jack that you want to be the speaker output. Verify that the 100 ohm resistor is connected between the tip connections on the two jacks. This resistor provides the earphone output from the speaker output. If you do not feel comfortable doing any of these changes I will do them for you if you pay shipping both ways.
If you are using the SCAF-1 connected to the speaker output of a tube type radio you may have a distortion problem. This is due to the fact that the 2 kohm input impedance of the SCAF-1 does not provide sufficient load on the speaker output. Adding a low value resistor across the speaker output should fix the problem. Use a resistor in the 3 to 10 ohm range with a power rating sufficient for your radios audio output power. Thanks goes to Parker, WD8JOL for the solution to this problem.
Ron, K0QVF
If your filter does not work properly after assembly the first step should be to triple-check your soldering. Use a bright light and a good magnifying glass, like a jeweler’s 7x loupe. Look for pins with no solder, solder bridges, that kind of thing. Power up the SCAF-1 and put your finger on all the ICs to see if any get hot. A good sharp look with a critical eye is the best troubleshooting device, period.
Next check is the power supply. Verify that there is a minimum of 12 volts between the red and ground wires on the power input jack when the SCAF-1 is turned on.
If you look at the schematic, you’ll notice that there are two ground symbols: one with lines, and one that looks like a triangle. The one with the lines is the actual chassis ground. The triangle is an artificial ground, created by the zener diodes D1 and D2. Most of the ICs need a positive supply voltage and a negative supply voltage. The easiest way to provide that is to create the artificial ground; that way +12 VDC and ground on the chassis can become +5 VDC and -5 VDC relative to the artificial ground.
The artificial ground is the wide trace running down the center of the circuit board. The first thing I do when I’m troubleshooting a SCAF-1 is to solder a scrap lead to two of the via holes in that trace. That gives an easy way to clip one multimeter lead to the artificial ground. Once you’ve done that, then test to see if +5 VDC and -5 VDC are there. Make sure that you use the artificial ground as your reference rather than the chassis ground.
If you have +5 VDC and -5 VDC, then the next thing to check is the square wave coming from the 555. If you have an oscilloscope, check for the square wave coming from pin 3 of the 555. Again, use the artificial ground as a reference. If you don’t have an oscilloscope, measure the AC voltage output coming from pin 3. It should change as you turn the front panel knob. If it’s zero or close to it, then you probably don’t have the square wave.
If the square wave is there, then it’s time to go signal tracing. The easiest way to do that is to rig up a pair of headphones. I take a couple test leads with alligator clips and clip onto the headphone jack. The sleeve contact gets attached by a test lead to — you guessed it — the artificial ground. The test lead connected to the tip contact gets clipped to a probe (any scrap lead can serve as the probe) and then poked around the circuit. You can probe any part of the circuit that uses the artificial ground this way. Power up the SCAF-1, run a signal into the input (the output from an AM radio works great), and put the headphones on your head. Using that method and the schematic, follow the signal through the circuit, and see where the signal goes in and doesn’t come out.
Here are some voltage checks that you can make. All readings should be within + or – 0.5 volts.
Reference the following to the artificial ground as listed above or you can find on U1 pin 6.
| IC | Pin | Voltage |
|---|---|---|
| U1 | 7 | +4.5 |
| U1 | 2 | -4.6 |
| U1 | 5 | 0 |
| U2 | 7 | +4.5 |
| U2 | 2 | -4.6 |
| U2 | 5 | 0 |
| U3 | 7 | +4.9 |
| U3 | 4 | -5.0 |
| U3 | 6 | 0 |
| U3 | 3 | 0 |
| U4 | 7 | +4.9 |
| U4 | 4 | -5.0 |
| U4 | 6 | 0 |
| U4 | 3 | 0 |
| U5 | 4 | +4.9 |
| U5 | 8 | +4.9 |
| U5 | 1 | 0 |
Reference the following to power ground.
| IC | Pin | Voltage |
|---|---|---|
| U6 | 2 | 0 |
| U6 | 4 | 0 |
| U6 | 6 | Power input voltage minus 0.7V |
| U6 | 5 | One half of the reading at U6 pin 6 |
If these voltages are normal then we need to trace the signal to see were it is getting lost. Set R17 and R27 to mid-range. Connect the filter to your radio and get a good signal with it switched out. Switch the filter in and turn the knob fully clockwise. Using an oscilloscope check the signal at the points listed below. If you do not have an oscilloscope connect one side of your phones to ground, connect the other side of the phones to one side of a 0.1 uf capacitor. Then connect the free end of the capacitor to the following pins and listen for the signal. The signal should be about the same on all pins except U6 pin 5 which will be stronger. This will tell you were the signal is getting lost.
RCA jack center pin
U1 pin 8
U1 pin 5
U2 pin 8
U2 pin 5
U2 pin 3
U3 pin 6
U4 pin 6
U6 pin 5
U5 output waveform checks.
The photos below are oscilloscope displays for pin 3 on U5. The horizontal center line is 6 volts. The vertical scale is 2 volts per major division. The horizontal scale is 5 microseconds per major division for CCW and 1 microsecond for CW.
U5 pin 3 output with front panel knob fully CCW
U5 pin 3 output with front panel knob fully CW
If you do not have an oscilloscope then connect a volt meter with plus on pin 1 and minus on pin 6 of U1. With the front panel knob turned fully to the left you should read about 2.5 volts DC. As you turn the knob to the right the voltage should increase to about 4 volts DC.
This page last revised on August 7, 2024