Learning how to troubleshoot electronic circuits is one of the handiest skills to have if you 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 episode of The Repair Bench we are troubleshooting a Roland Cube 100 Bass Amp that we use in one of our music schools. The amp arrived for repair with a report of a loose jack and no sound. If you’ve read The Repair Bench before, you know that this is a common repair for us, since our amps have a lot of kids plugging-in and out throughout the day.
THE ROLAND CUBE 100 BASS AMP
The Roland Cube 100 Bass Amp is a very nice and reliable bass amplifier in a compact package. The main features of this amp are:
- Rated for 100 watts through a heavy magnet coaxial 2 way 12″ speaker.
- 4-Band Equalizer (bass, low-mid, high-mid, treble)
- Peak-light indicator
- 10dB pad switch to lower the input sensitivity
- Compressor to lower high signals while boosting low ones
- Effects – chorus, flanger and T-wah, which changes the frequency response of the wah based on the signal input level
- Delay and reverb
In addition the amp uses Roland’s COSM (composite object sound modeling) to model eight types of bass amps:
- SUPER-FLAT – a wide frequency response amp
- FLIP TOP – models the Ampeg B15
- B MAN – models the Fender Bassman 100
- T.E. – models the Trace Elliot AH600SMX + 1048 (4 x 10”) + 1518 (1 x 15”)
- BASS 360 – models the Acoustic 360 bass amp
- SESSION – models the SWR SM-400 + Goliath (4 x 10”) + Big Ben (1 x 18”)
- CONCERT 810 – models the Ampeg SVT + 810E (8 x 10”)
- OCTAVE BASS – an original amp type that creates sounds one octave below the input sounds and layers them together to create a fatter bass sound.
EVALUATION AND REPAIR
Repairing the input jack on this amp is straightforward. The chassis cannot be fully removed until the speaker wires are disconnected, so the first step is to remove the front grill and speaker and disconnect the speaker terminal connectors. Then remove the top and side screws, and plastic bezel screws, so that the chassis can slide out. With the chassis removed there is easy access to the small circuit board that holds the input jack, as shown here:
We used our desoldering tool to demount the old, broken jack, and then used an on-hand replacement jack. The jack is the same JALCO type we used in an earlier repair of a Roland Micro-Cube amp, except that one of the terminals needs to be clipped off, as there is no hole in the PC board for it. The only source we’ve found for this jack is Roland, so order it from them http://www.rolandus.com/support/ or call (323) 890-3740.
After soldering in the new jack we tested the chassis with the speaker to make sure everything is working. To reassemble the amp you first need to feed the speaker wires back through an interior cabinet wall into the speaker cavity. Then slide the chassis in place and install the mounting screws. The speaker wires can then be reconnected and the speaker and grill screws replaced.