Probably A Simple Fix

EHX Micro Metal Muff

EHX Micro Metal Muff

A true Metalhead's delight; as this time I'll be taking a look the Micro Metal Muff from Electro-Harmonix.

 

More Metal

The Metal Muff series of pedals was released by Electro-Harmonix back in 2008 and featured three different models. At the top end of the range is the Metal Muff with Top Boost featuring gain and volume controls as well as a full three-band EQ with a foot-switchable Top Boost that allowed you to dial in to provide some extra bite across a narrow band of frequencies in the highs.

At the opposite end, is the stripped back Pocket Metal Muff featuring just a gain and volume control and a three-way switch to attenuate the mids. The Micro Metal Muff (the subject of this blog entry), sits somewhere in between with gain and volume controls and the addition of single tone control that boosts or cuts the mids by ±15 dB and a three-position switchable Top Boost.

The Metal Muff Family from Electro-Harmonix. The Micro Metal Muff (left); the Metal Muff with Top Boost (center), and the Pocket Metal Muff (right).

The Metal Muff Family
The Micro Metal Muff on the repair bench.

The Patient
Another acquisition from Trade Me, the issue with this pedal was described as ‘showing no signs of life’. This was confirmed upon arrival and due to the buffered input, it wouldn’t pass any sort of audio signal whatsoever. Even when bypassed.

My intuition told me it was probably an issue in the power supply with the worst-case scenario being (again) the dreaded the wrong polarity power supply being used and frying everything inside.

 

The Micro Metal Muff on the repair bench.

Upon removing the backplate, I quickly spotted a burnt-out resistor nestled between two electrolytic capacitors. This resistor, located in the power supply section, seemed to be a dam stopping the flow of power to the rest of the circuit as everything voltage wise checked out up until hitting this resistor. Whereas thereafter, the voltage was measured as virtually zero at key point around the PCB.

The Micro Metal Muff with the backplate removed. Note the combination of through-hole and surface-mounted components used in its construction.

The Micro Metal Muff with the backplate removed.
The burnt-out resistor nestled between the two electrolytic capacitors.

This resistor obviously needed to be replaced before any further troubleshoot could commence. However, removing the PCB from the enclosure proved much more challenging than expected.

My initial thoughts were that I’d need to unsolder some of the components in order to do so as the ends of the input and output jacks extended into the sides of the enclosure (à la Tech 21 NYC) but with little gentle encouragement, I was able to carefully pry the PCB out the enclosure.

The burnt-out resistor nestled between the two electrolytic capacitors.

With the PCB now free from its confines, I was able to desolder the resistor ready for its replacement.

I was then able to determine its value, 10 ohms, tracking down a schematic online of Micro Metal Muff's big brother, the Metal Muff with Top Boost (the one with the three-band EQ) as a quick trace of this section of the PCB proved virtually identical.

The burnt-out resistor successfully removed from the PCB.

The burnt-out resistor successfully removed from the PCB.
The silkscreen showing the intended location of the resistor R8.

But just to be sure, I reached out to the manufacturer who were kind enough to confirm the value. They also recommended installing a higher wattage resistor (the one installed was rated at a ¼ watt) than what was previously installed.
 

Armed with this information, I tracked down a 10-ohm metal film resistor rated at a ½ watt from my ‘Bit Box’ and prepared to install it. Interestingly enough, the silkscreen (R8) showing the component location is actually on the opposite side of the PCB as opposed to the component side. It was a bit of a tight fit between those two electrolytic capacitors anyway so I decided to install the new resistor where it was intended it to go and where there was a little more space to work.

The silkscreen showing the intended location of the resistor R8.

This particular example was a ‘rev.D’ of the circuit; the images I found online of the Micro Metal Muff's PCB were mostly rev.C and either didn’t have this resistor installed or if it was installed it was on the potentiometer side of the PCB. So the resistor in this particular pedal could well have been retro-fitted after the initial manufacturing process. Only Electro-Harmonix knows for sure.

The revision 'rev.D' number shown on the PCB.

The revision 'rev.D' number shown on the PCB.
The new 10-ohm resistor installed.

With the new resistor installed, it was time to begin the next stage of the troubleshooting process. Powering it up this time, I re-checked the voltages and things were looking much better (that's a good sign). But I wasn't out of the woods yet. Will it pass an audio signal?

Plugging a guitar into the pedal this time resulted in a clean signal being passed through to the amplifier in bypass mode (that's an even better sign) and when engaged, the status LED lite up, and a metallurgist's dream poured from the amplifier's speaker.


The new 10-ohm resistor installed.

So Was It An Easy Fix?

Yes; another pretty straight forward repair. The most difficult part was getting the PCB out of the enclosure. It did have the potential to be more complex via the 'whack-a-mole' principle; when fixing one problem then reveals another.

In this case, restoring power to the rest of the circuit by replacing the burnt-out 10-ohm resistor returned the pedal to 100% of its functionality.

 

Ready to rock again.

Ready To Rock Again.