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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.

Bingo Flashboard — January 11, 2017

This is a web-based Bingo Flashboard application, programmed by Jamie Voth. Contact Don if you'd like to purchase this application for $50.00. For a fee, the application may be modified to suit your needs. Features include:

Moog Prodigy — December 21, 2016

This is a 5 Meg Log taper Pot out of a Moog Prodigy with a broken control shaft. The shaft is the white plastic part. You can not buy a 5Meg Log pot anywhere. They are made of Unobtainium. So what could I do? I drilled a hole into the shaft and tapped 6-32 thread into the hole. Then I screwed in a 6-32 screw and cut off the head. That is what you see in the first picture. Then I found some copper pipe and I glued it on to the screw with epoxy. Epoxy is the stuff my wife uses to make puffed wheat squares. (I think?) Then I drilled out the knob you see in the picture so that it would fit the new shaft. I put it all back together and we have a working glide control again! It is not perfectly straight, but it is very close. This is how I like to give my customers the shaft. "Oh Don your so punny. Don't give up your day job."

Marantz Turntable — December 9, 2016

HA HA! Another factory defect fixed!

This Marantz Turn table made a popping noise when it turned off at the end of an LP play. Not a huge problem in the old days of two-speaker stereos. But add some subs and a ton of gain and now you have subs going nuts at the end of the record.

After I found the schematic for this baby on the web, I spent a lot of time staring at the drawings trying to figure out how the motor control circuit worked. Got that figured out and what did I find? The motor control circuit was floating with respect to ground. Not the best idea! So I grounded the control circuit. Sure enough, the popping went away but motor hum was introduced instead. Not acceptable. So what did I do?

I coupled the motor control circuit to ground through a cap and then earth grounded the Chassis with a 3 prong AC cord. Eureka! All the problems were gone. Sometimes I am so smart.

Mesa Boogie 50Cal — Dec 9, 2016

I just created the quietest Mesa Boogie 50Cal in the world.

This amp came in with a very noisy preamp section. First thing: isolate the noisy tube. That's easy. DONE. Then, there was hum as we increased the level of the input volume control. I traced that problem to an unshielded wire between the volume control and the input to the tube. In the picture, you can see the black shielded wire soldered to the pot. With that, no more hum was coming from the volume control. But there was still too much hum coming from the master volume control. I isolated this hum to be coming from V2 (the tube we just changed). I tried to filter the plate voltage to V2. No change. I tried to hum balance V2. No change. Put DC filament on V2. No change. I was running out of ideas! Then, I added a filter cap on the cathode of V2 with the ground of the cap at the controls ground buss. The hum went away. WHAT? ...I lifted the cap and resistor on the cathode of V2 from its ground traced and ran a jumper to the ground buss of the controls. Hum is 90% gone.

Why did this happen?

This is a classic example of "single point grounding" at work. When you do a PC-board layout you have to pay close attention that every circuit section has its own highway back to the central ground point. If you let ground paths lay on top of each other, then ground currents add up and this produces horrible humming problems. Likely the V2 circuit had ground traces overlapping that should not have been. By moving the ground path of V2 to the control main ground buss, I eliminated the ground loop.

I also had to retube the output section and this meant rebiasing. Mesa's do not have bias pots. In the picture you can see we added a resistor to bring up the bias circuit to 30mA. This mode does not radically change the factory bias circuit. It is important to be the least invasive you can be.

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