Kustom Groove 1200 Bass Amp Repair
A customer dropped this bass amplifier off for repair. It had been at another repair shop. They weren’t able to repair it.
Upon assessment I found that the input strain relief was missing.
I wasn’t able to find a suitable size strain relief to fit the rather huge hole so I decided to fit an IEC connector socket. Looks better to me anyway.
I slowly applied power to the unit with my Varaic and Isolation transformer. Luckily it didn’t draw much current. I also checked for DC offset on the speaker output.
All these tests turned out to be good.
I then applied a 440Hz sine wave to the input of the amplifier and while monitoring the output with a scope. It soon became apparent that the clip limiter circuit wasn’t working.
This circuit limits the power amplifier stage output signal to prevent clipping.
I confirmed this by applying the same signal to the fx loop return socket.
Looking at the schematic I could see that the amplifier stage uses a CA3080 Transconductance amplifier IC. These IC’s are obsolete and are difficult to get. This must be why the other shop declined the repair.
A bit of research found that there are a couple of Transconductance amplifier IC’s available. One of these is the LM13700. This is a 14 pin device and has 4 single amplifiers in the same package.
I quickly drew up a double-sided pcb on Easy PCB to take the 14 pin device and some PCB header pins to take the 8 pin DIL space on the board. I used only one of the amplifiers inside the LM13700 and obviously the supply pins.
I also found that all 5 of the front panel potentiometers were faulty. I eventually decided to replace these as the original pot wafers were all cracked.
I had to cut the shafts slightly shorter as the ones I had were longer. The unit was reassembled and tested. Here one can see the max output signal into a 4ohm load. (1kHz). This confirms that the limiter is working. Shown: 53,78V RMS. This amounts roughly to 723 Watts.
I cleaned all of the controls, switches and sockets. Closed the unit up and informed the client that his unit was ready for collection.
If someone is interested in the PCB layout, they are welcome to contact me.
This article was prepared for you by Riaan Diedericks. He runs his own electronics repair shop in Pretoria, South Africa. He specializes in Pro Audio repairs. Please give a support by clicking on the social buttons below. Your feedback on the post is welcome. Please leave it in the comments.
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You can also check his previous repair article below: