Keeley Tone Workstation Repair
A client dropped off this guitar pedal for repair. The pedal comprises of a drive section, a drive/boost section and a compressor section. The complaint was that the center boost section wasn’t working at all. I opened up the unit and saw the dark colored PCB material. This makes repair difficult when trying to trace out a circuit for which schematics aren’t available.
I did however contact the manufacturer to see if they might be of any help. Unfortunately their company policy restricts the sharing of schematics.
The board uses mainly SMD components. With a multi meter I was able to trace the faulty section to create a basic schematic.
I removed the switch to better see the traces and via’s as this is a double layer board.
Using my component analyzer I was able to determine that the SOT-23 devices were N channel JFETS. This also revealed that one of the JFETS (Q6) was faulty.
This is however a sensitive circuit and therefore I knew I had to closely match the new JFET to the JFET in the first stage. I did this because they would have probably used transistors form the same batch.
I made up a quick test setup jig that I could use along with my Curve tracer.
I used the curve tracer to match a JFET to the known good one from the circuit. Although not precise I tried to get it in the ballpark as good as possible. Both Jfets gave me the same curve when switching quickly between the two. The curve shown was taken just before complete Saturation/ Turn-on.
After replacement the unit tested fine.
This article was prepared for you by Riaan Diedericks. He runs his own electronics repair shop in Pretoria, South Africa. He specializes in Pro Audio repairs. Please give a support by clicking on the social buttons below. Your feedback on the post is welcome. Please leave it in the comments.
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You can also check his previous repair article below:
September 23, 2019 at 9:31 pm
JFET is sensitive current ..good job ..
I like every one when they work in electronics..
September 24, 2019 at 1:06 am
like the systematic use of test equipment!
Kouaplack augustin David
September 27, 2019 at 7:03 pm
I have always been awhile since the last few years ago . You are doing great job in electronic.
Albert van Bemmelen
September 23, 2019 at 9:56 pm
Repair without any support from the manufacturer is always bad advertisement for their product and name, so good job! I noticed you used the PEAK SOT23 smd adapter but not their DCA75 Pro with the build in easy to use curvetracer software. Your curve tracer obviously works but only shows one curve after setting up the external test circuit which takes time. I also have an old Hameg HZ65 component tester that also works as scope curve tracer like yours. Nowadays I however prefer using the DCA75 Windows PC software curve tracer that enables me to show, print and easily compare the on paper drawn results. The Ic/Vce, Ic/Vbe, Ic/Ib and the hFE/Vce curve graph options plus the option to choose what semiconductor I want to test run is so much easier. And much easier to digest than analyzing a graph on an scope screen without any real scale or values shown. And considering the fact that my Hameg HZ65 at that time did cost about the same as my DCA75 pro costs now, the latter is worth every penny!
September 24, 2019 at 2:33 am
Great job. Thanks for sharing with the community.
September 24, 2019 at 3:17 am
You are one smart cookie Riaan! Now THAT's what I'm Talkin bout! Great job.
Would like to see more detail on the tracing process, but overall very impressive work.
September 24, 2019 at 8:43 am
I would say that company's non-cooperation was a blessing in disguise. Because that aroused a challenge and with the highly professional gadgets and spontaneously innovative tools, you did more than a master's job that could put the company to shame! Excellent! I never heard of or used Curve Tracer! A lot of learning from your article! Many thanks!