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Top Boost in a Can

I recently bought a used Menatone Top Boost in a Can - it is a custom overdrive pedal that is voiced like a Vox AC30 and gets very good reviews from knowledgeable online pedal enthusiasts. The person who sold me the pedal said that he really did not like its sound. I got the pedal for a good price and looked forward to trying it out. When I first plugged it in, I had to agree with the seller. It sounded crappy.

I was curious as to what was going on with it so I popped the back off the Menatone and had a look. Immediately I noticed there were 4 small yellow trimpots. After poking around a few minutes I found that the trimpots adjusted the bias on the transistors. I located the test points to check the bias and a few measurements told me the bias was misadjusted significantly on two of the transistors.

I tweaked the trimpot settings and put the pedal back together. Wow! What a difference! The pedal had come to life after this simple fix and now sounded much better than it had when I first received it.

I do not recommend that you try this tweak unless you have some basic electronics knowledge and can make some simple dc voltage readings. Proceed at your own risk.

If you look at the picture below, you will see that there are 4 small yellow trimpots in a vertical line down the right edge of the perfboard. Each of them has the designation 104 on it which is the code for its resistance value. We will call them TR1 through TR4 starting from the top.

I have added a yellow arrow pointing to the lug that corresponds to a test point for each of the trimmers, therefore TP1 is the test point for TR1. To test, you measure the dc voltage from the test point to ground while adjusting the trimmer with a small screwdriver. A value of 4.5 volts is needed but do not obsess over getting it exactly - any value from 4.5 to 5.0 volts is acceptable. The bottom trimmer (TR4) is a little touchy to adjust because of the resistor values on the transistor.

Do not attempt this tweak unless you are familiar with use of a multimeter. Open up the Menatone and make sure you have a fresh 9v battery in it. Set the meter to measure dc voltage. Insert a plug into the input jack of the pedal to power it up. Place the black (negative) meter probe into one of the screw holes of the case so that it is grounded. The top lug above the TR1 trimmer is connected to +9v so place the red (positive) probe on it to verify the battery voltage. If it is approximately 9v, move the red probe to TP1 and get the reading. Write it down and continue to test the other 3 points indicated in the picture. If they are all in the range of 4.5 to 5 volts, you do not need to touch the trimmers.

If any of the readings are outside the range, adjust the appropriate trimmer while monitoring the voltage until it falls within the specified range. The TR1, TR2 and TR3 trimpots are easy to get close to 4.5v but it may take some finesse to get the last one where you want it.

Once you have all 4 of the test points at 4.5v, put the pedal back together and enjoy the revived sound! (Note that in the photo below, portions of the board that are not relevant to this discussion have been intentionally blurred to obscure the design values.)


2002 Jack Orman
All Rights Reserved

This page last modified on Sunday, 14-May-2006 06:52:44 PDT

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