Don Voth has been servicing electronics for musicians, professional audio engineers and lighting technicians since 1989. Don started his own business in 2001: the Don Voth Electronics Company. Technology has evolved over the past 25 years, but Don Voth is still coming up with dynamic solutions; finding and fixing the most difficult and bizarre technical defects.
In this video, Don Voth explains how he fixed a Peavey 5150. Check it out, and share with your friends!
I fixed an old Korg Sigma yesterday. This is actually a very cool instrument. It sounds great now. Besides having a lot of intermittent tab switches, this unit did a very weird thing. When you would release a key the note would drop in pitch by about a semitone. It would do this only on the synth voices. Synths create notes by sending out a voltage. The higher the voltage the higher the note. The synth remembers the last note that was played with a "sample and hold" circuit. It samples the voltage from the keybed and then holds that voltage after the key is released. In this unit the "sample and hold circuit" was not working quite right. It would sample OK, but then it failed to hold the voltage to the full value after the key was released. It turned out that some of the voltage was leaking back in to the keybed circuit upon key release. I replaced a 100pf cap in the FET switching circuit and after that it was fine. It was not an easy fault to isolate.
I have a customer who has an old Ampeg SVT300 from the '70s. We had completely overhauled when he got it: new tubes, new caps, the works. It was very quiet, but he hardly used it because he said it sounded sucky. So I put it back on the bench and discovered that the preamp was very short on gain. I modified the preamp to boost the gain. Then, I found the unit would start to clip long before that amp reached 300W. I traced this problem to a set of limiter diodes in the front end of the power amp. I thought, "That is a dumb idea." So I took them out. Now that went right up to 300W cleanly and would only start to clip at 300W. I took the amp to the owner's band practice night. Everyone went crazy for the tone of that bass amp. They could not stop talking about it! This amp went from "never use it" to the go-to amp of choice. Anyone who has an old SVT300 and wants it to come alive, I know what to do.