Learning how to troubleshoot electronic circuits is one of the handiest skills anyone can have if they are building or modifying amplifiers or effect pedals. At some point you’ll power-up a circuit and find that it doesn’t work, and then what? For this reason we write “The Repair Bench” section of Guitar Kit Builder about our own troubleshooting of amplifiers and other devices, to pass along to the reader the thought process, tips and techniques of troubleshooting electronic equipment.
IBANEZ TS9 TUBE SCREAMER
In this edition of The Repair Bench we troubleshoot an Ibanez TS9 Tube Screamer effect pedal. This is one of the most common and well-known effect pedals in use by guitar players, so we won’t describe it further. We received this pedal to repair and then modify. We’ll cover the modification in a later article. The owner reported that the pedal simply stopped working at some point.
Our visual inspection of the exterior showed it to be in typical used condition. The silver label from the back of the pedal was missing, so the serial number could not be identified, but this appears to be what Analogman calls a “2nd version reissue” due to the “Maxon” imprint on the inside of the battery cover.
Opening the case we found the printed circuit board marked “Maxon KU” with a Toshiba TA75558P op amp chip. Viewing the underside of the printed circuit (PC) board showed it to be clean, with no evidence of prior repairs, which is always a welcome finding.
We hooked up the pedal to an AC power supply, plugged in a Fender 1952 Telecaster Reissue, and connected the output to the Carvin SX200 amplifier we recently repaired in a prior edition of The Repair Bench. This test setup confirmed what the owner reported – no output sound with the effect in or out.
We decided to trace the signal with an oscilloscope, but while moving the board around to get it ready, we heard a sudden “bleeep” sound through the amp. By holding the board at a particular angle we were able to momentarily get an output signal. We concluded that we had an intermittent situation. Finding the cause of an intermittent problem can sometimes be difficult. Our procedure is to use an insulated probe or “orange stick” to gently nudge each wire and component to attempt to isolate an intermittent mechanical connection. In this case we quickly found that moving the purple #13 wire from the PC board to the level control caused the sound to cut-in and cut-out. Further inspection revealed that the potentiometer pin connected to the purple wire was touching the metal case of the adjacent tone control (see photo). This can happen when the potentiometer has rotated from its original mounting position, perhaps due to becoming loose from vibration, or perhaps from someone continuing to turn the level control higher when it has already been turned to its maximum.
We loosened the potentiometer mounting nut, rotated the pot to its proper position (photo below) and tightened the nut. Some more testing with the pedal showed it now to be working properly.
As some of our recent repair projects have shown, we don’t always get repair projects that are quite this easy, but they do happen. Our next article on this pedal will cover some modifications for better tone.