Learning how to troubleshoot electronic circuits is one of the handiest skills anyone can have if they are building or modifying amplifiers or effect pedals. At some point you’ll power-up a circuit and find that it doesn’t work, and then what? For this reason we write “The Repair Bench” section of Guitar Kit Builder about our own troubleshooting of amplifiers and other devices, to pass along to the reader the thought process, tips and techniques of troubleshooting electronic equipment.
In this edition of “The Repair Bench” we cover a common repair for us – replacing the input jack on a Roland Micro Cube amplifier. As we’ve mentioned before, when we’re not writing for Guitar Kit Builder we operate rock music schools where we use the Micro Cubes in some lesson rooms. With many lessons a days, and students plugging-in and out all day long, the input jacks on our amps take a beating. And let’s face it, kids are just not always as gentle on equipment as they could be. So on a pretty regular basis we need to replace the input jacks on all of our amps, including the Micro Cubes.
THE ROLAND MICRO CUBE
If you’re not familiar with the Micro Cube, it’s the small fry in Roland’s line of cube amplifiers. It’s an ultra-compact package that runs on batteries or from a 9 volt wall-transformer power supply otherwise used for effect pedals. Weighing in at just a few pounds, the Micro Cube comes with six digital signal processing (DSP) effects (chorus, flanger, phaser, tremolo and separate delay/reverb) and COSM® amp modeling.
COSM is composite object sound modeling, Roland’s proprietary amplifier modeling technology. Once a musical instrument generates sound vibrations, it reaches the human ear through various mediating objects, each of which significantly affects the sound. The material and configuration of the instrument, the electric/magnetic amplifying system, the air and the reverberation of the room all affect the final sound. Sound modeling, the latest DSP technology, “virtually” reconstructs these objects. Roland’s COSM uses the advantages of multiple modeling methods and succeeds in accurately emulating existing sounds, as well as producing sounds new sounds. Seven amplifiers are modeled in the Micro Cube:
- Acoustic – a dynamic acoustic guitar sound from a standard electric guitar.
- JC Clean – models Roland’s famous JC-120 Jazz Chorus guitar amplifier for a smooth, ultra-clean, ultra-flat sound. This is also a good choice if using an external effect.
- Black Panel – models the classic Fender Twin Reverb tone, with rich lows and a bright high end.
- Brit Combo – models the Vox AC-30TB, the rock amplifier that created the Liverpool sound of the 1960s. It produces a broad range of sounds, from clean to overdrive with increased distortion and greater power in the low-midrange when the Cube’s gain is boosted.
- Classic Stack – models the sound and response of a Marshall JMP1987, well suited to classic and hard rock.
- R-Fier Stack – models the super high-gain of the MESA/Boogie Rectifier amp, for slash metal, grunge and other lead tones.
- Mic – used for when a microphone is connected.
This Micro Cube arrived on our bench with the input jack (Photo 1) pushed below the front panel and the gain control stuck and very difficult to turn.
Photo 1 – Micro Cube with Input Jack Pushed Below Front Panel
It appears that at some point the nut for the input jack became loose and eventually fell off. This is a chronic problem with the plastic jacks and nuts used in many modern amplifiers. In this case the amp continued to be used, which eventually pushed the jack inward until it stopped working.
OPENING THE MICRO CUBE
Opening the Micro Cube is pretty straightforward, but we’ll mention one tip here. Since we’re frequently repairing more than one amplifier at a time, usually because we’re waiting for parts to arrive, it’s important to keep all of the parts for one amp together, but separated from the parts for other amps. A simple Ziploc type bag that can be labeled comes in very handy here.
Photo 2 – Keep All Parts Together and Labeled in a Plastic Bag
As to opening the Micro Cube, remove the screws on top and for the plastic bezel, and the larger screws around the perimeter of the back. Next, remove the plastic bezel and lift the amp chassis (Photo 3) out of the cabinet, stopping just before the speaker wires get tight. At that point you’ll need to reach in through the opening and gently remove the connectors from the speaker. Then completely separate the chassis from the cabinet.
Photo 3 – Amp Chassis Lifted Enough to Detach Speaker Wires
There are two small printed circuit boards (PCBs) attached to the control panel (Photo 4). One holds the input jack, gain and volume controls. The other holds the amplifier modeling switch.
Photo 4 – Internal View of Printed Circuit Boards
In our amp, when the input jack was pushed in below the front panel, it caused the gain control potentiometer (pot) to be pulled apart (Photo 5), since the pot shaft is attached to the panel while the body is attached to the PCB.
Photo 5 – Gain Control Pot with Missing Shaft
So the first need we identified was to either fix or replace the potentiometer. After inspecting the shaft and the potentiometer (Photo 6) we realized that they could be re-mated by aligning the guide posts.
Photo 6 – Potentiometer Without Shaft, Showing Guide Holes
After playing with it awhile we were able to get it to seat perfectly (Photo 7) and used needle nose pliers to fold the tabs back in place for attachment.
Photo 7 – Control Shaft Reseated On Potentiometer
Usually when these amps come in for repair the input jack is physically damaged (Photo 8). In this case the jack looked fine, but since we had the chassis open already we decided to replace it with a new one. Our de-soldering tool (shown at right), an inexpensive purchase from Radio Shack, comes in very handy for replacing jacks. This tool has a soldering tip with a small hole in the center of it. The hole is attached to a tube with a rubber bulb at the end of it. To use it you squeeze and hold the rubber bulb while applying the tip to the solder pad. When the solder turns liquid you release the bulb and the solder is sucked up through the tube. You then hold the tip over a suitable area to release the captured solder (not the printed circuit board!) and squeeze the bulb again to blow it clear. We find this tool to be very effective and a real time saver for removing jacks from printed circuit boards.
By the way, the input jacks used in the Micro Cube are made by Jalco, but the only source we found for them is from Roland. You can contact Roland support at http://www.rolandus.com/support/ or call (323) 890-3740. Parts can be ordered by telephone with a credit card. The jack sold by Roland does not come with the nut, so order it as a separate item. It is not a standard size nut, so make sure you have the right one already or order a new one.
Photo 8 – Input Jacks Are Frequently Split Open When Damaged
The underside of the PCB (Photo 9) shows the four pads to be de-soldered for jack removal.
Photo 9 – Solder Pads for Input Jack
With the old jack removed, installation of the new jack is a simple matter of inserting the pins into the PCB and re-soldering. With the jack installed (Photo 10) we can re-attach the controls and jack to the control panel.
Photo 10 – New Jack Installed on PCB
After testing the amp to make sure it is operating normally we just reverse our process of opening the amp, being sure to re-attach the speaker wires and tightening all of the screws.
Photo 11 – Input Jack Attached to Control Panel
This type of repair is about as simple as it gets, but it covers a number of important points:
- Organized – keep parts together and organized with aids such as a clearly labeled plastic bag
- Resourceful – some parts can be repaired without buying a new one, such as the broken potentiometer in this amp.
- Parts – the manufacturer is often a good start when trying to find replacement parts. Some makers provide much better support than others, and supply houses are the best alternative to an unhelpful manufacturer.
- Tools – Some specialty tools, such as the desoldering iron, are invaluable to making the job faster and easier.
Have a question about repairing your Micro Cube? Feel free to post a comment here and we’ll do our best to help.