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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.

Marshall 2266 — April 26, 2017

Fixed a Marshall 2266 with KT88s today. This amp had a bunch of little problems and one big problem. It put out only 20W. Low power is always a pain to isolate because the amp is working but just not at the level it should. I verified that the fault was in the output coupling transformer by subing in a generic transformer. Then I had a a discussion with the owner and we decided to install a Mercury Magnetics transfromer rather than go with a factory transformer. In the picture you can see the original transformer held up against the new transformer that is installed. See the new transformer has almost twice as much metal in it. This is excellent. More metal means the transformer will not saturate as fast. I tested the transformer under load to clipping and determined the output power went up from 20W to 45W. This is less then I had hoped but it is a normal output level for an amp with 2 output tubes. It is now time for the owner to try it and tell me what he thinks.

Morley Volume / Wah Pedals — February 28, 2017

Today our topic is Morley Volume/Wah pedals. Anyone who has one of these knows it is useless as a Wah pedal because it hums like a bastard. I tried fixing this a few years ago and gave up. I filtered the power supply with great success. I got the ripple voltage down to nothing. But this would produce no change on the humming and buzziing on the output. Yesterday I tried again. This time I assumed we must have multiple ground currents in the same ground paths. I found the Chassis ground was arbitrarily placed. I move it to the ground at the input jack. Big difference. Then I found that I was getting hum from the emitter leg of the final transistor. So I lifted the emitter resistor and related cap and also grounded them to the input jack ground point. Hum dropped down to almost nothing. I love working miracles. Stay tuned. Next week I plan to walk on water. Who says religion and science have nothing in common?

Music Man 212-65 — January 27, 2017

This is the inside of a Music Man 212-65. These amps tend to sound harsh. But I have a solution. See on the picture there are caps across 3 of the resistors. These are compensation caps. Each one rolls the high end off at 6.5K Hz. This is the top of the functional range of a guitar. Each cap creates a 6db/oct roll off. 3 roll offs in series creates a 18DB/Oct roll off. This gets rid of the harness without loosing the useful frequency range of the guitar. This amp also had another problem. The summing point for the 2 channels and the Reverb showed slew rate limiting when you drove it hard. This presented itself as a triangle wave when we should be seeing a sine wave. I concluded that the cause of problem was that the opamp in this part of the circuit was to slow. I searched Digikey and found an opamp with the same package and pinout that was much faster. Bought it and installed it and the slew rate limiting was gone. You can see the new opamp as the metal can that is standing a little higher than the rest.

Bingo Flashboard — January 11, 2017

This is a web-based Bingo Flashboard application, programmed by Jamie Voth. 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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