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.

After:

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.
Happy repairs.

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.

P.S-  If you enjoyed reading this, click here to subscribe to my blog (free subscription). That way, you’ll never miss a post. You can also forward this website link to your friends and colleagues-thanks!

You can also check his previous repair article below:

https://jestineyong.com/main-smps-transformer-overheated/

 

16 Comments

1. 

   Parasuraman Subramanian

   July 5, 2019 at 9:17 pm

   A highly professional, neat and clean job! Well done! I really appreciate the very informative details! Very useful indeed! Many thanks! May God bless you!

   Likes(2)Dislikes(0)

   Reply
2. 

   Justice (City of Durban --South Africa)

   July 5, 2019 at 9:50 pm

   Wow Riaan what an interesting work and article this is. I will re read again , please keep up posting your work.

   Likes(1)Dislikes(0)

   Reply
3. 

   Eric Rice

   July 5, 2019 at 10:42 pm

   Thanks, good repair article. Would love to have the pcb drawing.

   Likes(4)Dislikes(0)

   Reply
4. 

   Albert van Bemmelen

   July 5, 2019 at 10:54 pm

   You did a great job adjusting a LM13700 to replace the obsolete CA3080 chip. But why not just buy the LM3080 instead?, they are very available on Aliexpress for less than a dollar a piece (five for $4.94) and even eBay still sells the old CA3080 or if you want the identical LM3080 opamp.
   You no doubt had a lot of work on replacing all 5 potentio's and connecting the new power AC power jack. The customer was lucky he brought the device to you for this professional repair after other repair firms simply refused fixing all defects.

   Likes(8)Dislikes(0)

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5. 

   Yogesh Panchal

   July 6, 2019 at 12:00 am

   Riaan, Very professional approach repair attempt.
   Thanks for sharing.

   Likes(1)Dislikes(0)

   Reply
6. 

   Cameron (Auckland, NZ)

   July 6, 2019 at 7:13 am

   A very nice repair indeed... It's great to see techs still exist that will go the extra mile/kilometer for their customers! A pity the repair industry tends to pay so little when everyone competes on price-cutting instead of quality.

   Keep up the great work, and repair articles. Thankyou.

   Likes(3)Dislikes(0)

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7. 

   Vincenzo

   July 6, 2019 at 8:57 am

   You, my friend are a pro audio God! Wish you lived in Adelaide South Australia and not South Africa, but my wishes are irrelevant to your outstanding performance repairing devices which otherwise would of been thrown out . Well done . \/

   Likes(4)Dislikes(0)

   Reply
8. 

   Sathish

   July 6, 2019 at 3:15 pm

   Can you please share the schematic

   Likes(5)Dislikes(0)

   Reply
9. 

   Myles McDonald

   July 6, 2019 at 11:37 pm

   What a great article to read. This is the artistry that I would like to learn.

   Likes(2)Dislikes(0)

   Reply
10. 

    Isaac

    July 7, 2019 at 12:44 am

    This is a good job well done Riaan

    Likes(3)Dislikes(0)

    Reply
11. 

    Donald

    July 30, 2019 at 2:14 pm

    I love reading your educative posts, we are learning a lot

    Likes(2)Dislikes(0)

    Reply
12. 

    Steve Griffin

    November 15, 2019 at 10:25 am

    Great work Riaan! Also your Servicing Work you carried out! That is truly a love of your work.
    I am wondering if you get much experience on JBL Pro Equipment, namely the MPX Stereo Amplifiers? I bought a MPX-600 supposedly ok but on opening up to check - all main fuses were blown! It is really heavy as the power transf. is huge! Just thought I would ask..
    Kind Regards,
    Steve G
    [Australia]

    Likes(1)Dislikes(0)

    Reply
13. 

    Steven J Griffiths

    January 25, 2021 at 10:49 am

    Great article and good job. I would like to know the details of the master volume [pot and a supplier (I am in US) I have a Kustom Groovebass 1200 with a damaged master pot. - Thank you.

    Steve Griffiths

    Likes(0)Dislikes(0)

    Reply
    • 

      CURTIS

      May 22, 2021 at 5:18 pm

      I have a kustom groove bass ( 2 actually ) with no sound from the 2 main speakons but sound with eq etc through a PA direct, leading me to think its the master pot as everything turns on. There's even the click from the relay when first clicked on. Did you find a solution yet?

      Likes(0)Dislikes(0)

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      • 

        Brandon J

        May 24, 2022 at 1:30 am

        I would love to know if you found a solution as well. I have the same issue with the master pot but I have no idea which pot I would need and where to find it.

        Likes(0)Dislikes(0)

        Reply
    • 

      Brian

      March 19, 2024 at 2:40 am

      Did you ever find any information on this. I'm trying to find a solution myself.

      Likes(0)Dislikes(0)

      Reply

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