July 2016
My vintage JAX “fuzz” guitar pedal (Figure 1) seems to be dead. I’ve put a new battery in it but it doesn’t pass sound at all – even when I press the bypass switch.
FIGURE 1.
Luckily, the schematic (Figure 2) is printed on the inside cover. I would greatly appreciate suggestions on where to start figuring out what’s wrong.
FIGURE 2.
Please log in to post an answer.
The schematic is unnecessary for fixing this problem. Looking at the picture, the wiring of the second switch in the photo tells the whole story. There are only 5 components involved in bypass mode: 2 guitar cords, 2 jacks, and one DPDT switch, plus the wiring. If it were a broken wire you would see it.
First, check both guitar cords by plugging the guitar directly into the amp or doing a continuity check. If one or both are bad, you’re done. If not, remove the battery, plug a cord into each jack, and check continuity from the sleeve of each plug to the sleeve contact on its jack (note that on the right hand jack in the photo, you want the shorter sleeve contact. The longer one controls battery power); repeat from tip to tip contact, if needed. Bad continuity means a bad jack, since you know the cords are good. If all else shows good continuity, the switch is bad.
I would start by replacing all the 10 µF electrolytic capacitors (the gray components standing up with 2 leads exiting the bottom). Electrolytic capacitors age, and in the absence of electric charge can eventually short or develop a high internal resistance. Perhaps only one is defective, but likely all are marginal and others might fail in the future. It's easier and cheaper to replace them all than to attempt using an ohm meter to determine which are good and bad. Jameco P/N 29891 is a suitable replacement part that costs $0.15 each ($1.80 for the 12 you'll need). RadioShack has a suitable part also (2721025), but its price is $1.49 each. Use caution removing and soldering in the new parts. The old single sided PCBs have copper traces that de-laminate easily with heat.
As the fuzz pedal is pretty old, I would suspect a defective capacitor, the electrolytics, 10 µF, is the problem. Looks like the unit has 12 of the caps, and as inexpensive as they are I would just change them all out as you have to take the board out even even to change one. That era of caps tend to 'dry' up or even sometimes go 'short'. Even if they are not totally defective, I am sure new ones will make a marked improvement of operation.
I suspect your DPDT bypass/operate switch is bad. With the battery disconnected, the instrument and amp connected, use a very short jumper wire (perhaps with small alligator clips on each end of the wire) to short out the middle two lugs of the switch shown in your Figure 2. Activate the instrument and see if your audio gets into the amplifier. If it does, your bypass switch should be replaced. If this doesn't help you, try replacing your instrument and amplifier cables from the device with known good cables.
The foot switch is not shown in the schematic. Looking closely at figure 1, it appears that the foot switch is simply bypassing all of the circuitry and connecting the input jack to the output jack. Based on your statement, first I would check the cables, then I would check the foot switch.
FYI, while not shown in the schematic, it appears that the power to the circuit is only applied to the circuit when there is a cable plugged into the jack on the right. The barrel of the plug shorts the ring contact to the barrel thereby connecting the battery “-” to the circuit ground. Good luck.
Most of those old pedals, and even the new ones, used the case as ground. If the jack nut is not tight or if there is corrosion on the jacks they will not work right. The first thing I check when I get one of these in is the jack nut. That usually fixes the problem.
I studied the schematic you provided on the Vintage Fuzz and found that the schematic is not complete. It does not show the bypass switch or how power actually gets applied to the amplifier.
From the picture, it is apparent that the amplifier only receives power when a mono phono plug is plugged into the jack directly below the battery. Please verify that the phono jack you are using is mono, not stereo. It appears that the switch located on the top center of the enclosure is the bypass switch. If I am correct, then it stands to reason that your problem is caused by a stereo phono jack because the center connections on the bypass switch are tied to the input and output phone jacks and completely bypass the electronics, but a long phono plug must be used in both jacks for the signal to pass from input to output. Note that the tip on the phono plug carries the signal and the middle contact on the phono jack below the battery provides power to the amplifier through the ground (barrel) of the mono phono plug.
If a stereo phono plug is used no power will be supplied to the amplifier. This same problem would occur if you have used a stereo phono plug to RCA jacks or some other connector and accidentally chose the wrong stereo channel because no audio signal connection exists on the middle ring of stereo plugs when plugged into this fuzz box.
Assuming you have proved you have an output from the guitar, also the amplifier and both cables are OK, the fault must be either the FUZZ boxes bypass switch is faulty or a break in the internal wiring between the switch and one or both 1/4” jack sockets.
A quick continuity test with a multimeter will prove where the fault is.
