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.
Leslie amplifiers have a serious problem with crossover distortion when you drive them hard. Here I have replaced the output coupling transformer with a Dynaco Style Transformer from Mercury Magnetics. As you can see, there's a massive improvement in crossover distortion and about 20% more power as well.
ABOVE — Here is the new Mercury Magnetics Transfromer installed with the original transformer shown in front. Big difference in size.
ABOVE — Crossover distortion on a Leslie power amp.
ABOVE — Crossover notch after transformer modification.
This is a Korg Mono/Poly. It was marketed as a "low-cost, computerized Monophonic Synthesizer with the fattest sounds around and polyphonic capabilities too". I discovered four problems with this unit.
First problem: Half the keys would not play. I had to clean the keys twice, but I got them all to play.
Second problem: The 'Effects On' button would work only once after powerup. So, the Effects could be turned on but not off. I verified that the switch was degenerated. It was a huge job to get the switch out so that we could clean it. Again the first cleaning did not seem to take, so we had to do it twice. The switch worked perfectly out of circuit, but for some reason, it is not perfectly consistent in circuit. I suspect there is another problem with the toggling circuit. The problem is not so bad that we need to spend time on this.
Third problem: The unit hums badly when you depress a key. This problem got worse when I installed the AC U Ground. It would go away when I opened the unit up. Turns out the Transformer is too close to the Audio output circuits when the lid is down. So I moved the transformer over to the left next to the Regulator PCB. You can see the original position and you can just see the new position on the right hand side of the 3rd photo. With this change all the hum was gone.
Fourth problem: We needed to install the MIDI upgrade kit. This was the only part of this job that was super easy. I installed it and ran the cables and wires. I tied everything up with tie-wraps and tested. Worked first time. Checked MIDI out and changed the midi Channel to #1. All good. You can see the MIDI Upgrade PCB in the photo below:
Just fixed this Vestax PMC-08 Pro. I am told you can not get these babies anymore. This one got smashed front and back. As a result, the phono inputs hummed. This is very bad because everyone who knows what this unit is uses a turntable. I took everything appart and straightened out the front and back plates. One of the phono inputs was actually broken. Bad news: you can not get these parts anymore. Good news: With a bit of epoxy I fixed the phono input. Also, there were lots of fractured solder joints on the PC-board because of the impact damage. I re-soldered the bad joints. I reassembled and tested the unit. One of the phono inputs is still a little crooked, but it works great.
Don fixes a Crown CTS 4200 that has a power supply fault.
Last week, I went to the Mid-Ocean School of Media Arts to teach students how to make and repair audio cables. It was a great experience.
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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.