The DOD FX-17 -- Troubleshooting & Repair

By Paul Marossy
Last updated 11/27/09

I recently repaired an FX-17 which exhibited some strange behaviour, and worked intermittently or not at all. Basically, it wouldn't pass a signal or it would only pass a signal for a few minutes and then stop working. The owner of the pedal had tried to figure out the problem and gave up on it. So he sent it to me to see if I could fix it after a series of emails about the problem.

After I had this pedal in hand, I came up with a systematic approach to troubleshoot one of these things. The following is an outline of what I did to to eventually find the problem and repair it.

1. Diassemble the unit to allow for testing. This will require the removal of the foot paddle, and the larger of the two pieces of sheet metal which cover the lower half of the base.

2. If you don't have one already, the first thing to be done after this is to make an
audio probe to follow the signal path to see what parts of the circuit the signal is traveling to, or if it is even getting where it should be. I typically use an audio oscillator to feed a continuous tone to a device under test to help trace the signal through the circuit.

3. Verify that the 9V & 5V power supply is OK. Measure the voltage at Pin 8 of U1 (LM358), there should be 9V present. Check voltage at Pin 7 of U3 (CA3080 chip), there should be 9V present. Verify voltage at Pin 11 of U2 (13600), 9V should be present. Also check voltage at Pin 14 of U4 (CD4007), you should measure 9V. The CD4007 IC chip on the side mounted PCB should have 5V at Pin 14. If all these voltages check out, proceed to Step 4. If you measure no voltage anywhere, then either wiring harness with the power connection has been jarred loose or possibly the contacts in the DC jack have become corroded and a good contact is not being made (I bought a "dead" FX-17 off of ebay to find that it was not working due to corroded contacts in the DC jack which caused the unit to not get any power, an easy fix).

4. Verify that U5 (78L05 voltage regulator) is supplying 5V by measuring the voltage present at the output of the device. If this device is faulty, then any part of the circuit using 5V will not be functioning properly.

5. Verify that the Vref power supply is at the correct voltage. To verify this, measure the voltage present at the positive terminal of C5 (47uF). You should measure approximately 4.5V at this point. Any problems with the Vref supply will cause problems throughout the entire circuit as many of the active devices will malfunction as a result.

6. If everything checks out up to this point, then it is time for the audio probe to be put to use. The U-shaped metal piece that attaches to the underside of the foot paddle will need to be removed so that it is free to move around by hand in the base of the unit while testing. Don't forget to attach the grey wire to it. Now it is time to trace the signal path with the audio probe. There should be signal present at Pins 5 & 7 of U1 (LM358). Next, check for signal at Pin 3 and Pin 6 of U3 (CA3080). The amount of signal at Pin 6 should vary with how the U-shaped piece of metal is positioned. When in the toe down position, you should have maximum signal here when the pedal is in volume mode. If you don't, then there is either a problem with U3 (CA3080) or possibly U2 (13600).

7. Most likely by this point you will have found something wrong at U3 (CA3080) if you have an FX-17 that seems to be non-functional even though all of the voltages check out throughout the circuit. Since U2 (13600) and U3 (CA3080) appear to affect the operation of eachother, the easiest way to verify if they are functioning properly is to desolder them from the PCB and install an IC socket in their place so you can try new IC chips. Both chips are not real easy to find, so ebay is the easiest way to get some replacements. I have found that if U2 (13600) is faulty, it can affect the operation of U3 (CA3080) and as a result the pedal will not pass a signal at all. Also, it should be noted that D2 (red LED) and D5 (green LED) should light up in proportion to the strength of the signal going thru the circuit, meaning that with a low signal the LEDs will be dim and vice versa.

8. Hopefully, by this point you will have found the problem and the pedal will be working reliably again. If you are still having trouble, then there may be a problem on the side mounted PCB which makes contact with the metal U-shape or possibly a problem with Q1 and/or Q2 (J113s).

I have repaired various older effect pedals and have found that sometimes IC chips just go bad, for whatever reason. The shotgun approach of replacing all the active components usually works, too - but that can be expensive, unnecessary and not a guarantee of success. While you have the pedal apart, consider replacing all of the electrolytic capacitors as a preventative maintanence measure as they are most likely around 20 years old, or older.

Other things to note are that you need to be really careful when desoldering any components because the PCB is easily damaged. The solder used seems to be a bit difficult to remove, so be patient, especially at the IC chips where all of the pins are folded down. Also, make sure that the U-shaped metal piece makes a very firm contact with the side mounted PCB because if there is too much space between the contact points, the unit can also malfunction as a result. In some units, this seems to be really critical and in other units it's not. But it is definitely something to keep in mind while you are testing and putting the unit back together.

I hope this page will help anyone with an FX-17 that they want to repair. Please contact me if you find other things that you feel might be helpful info to add to this page!

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