Yamaha Computer Speakers With LA4700 - diyAudio
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Old 6th June 2014, 10:00 PM   #1
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Default Yamaha Computer Speakers With LA4700

Hello

Could someone help me troubleshoot the amp of Yamaha speakers?
This is from a friend that bout them about ten years ago and the sound was very sweet with them. Nice bass for computer speakers.

The problem is that only very low volume sound comes from speakers although i open the volume or turn it off nothing except low volume sound.

I replaced the LA4700 Chip but no difference. Problem remains

i will post some photos below for anyone willing to help of the pcb's and the voltage of LA4700
Hope this helps.

Pins Volts
1 6.5
2 0
3 6.5
4 16.5
5 13.9
6 0.6
7 0.6
8 0
9 0
10 0
11 0
12 0.6
13 0.6
14 0.6
15 0.6
16 6.5
17 0
18 6.5
Attached Images
File Type: jpg 2014-06-06 00.40.24.jpg (958.2 KB, 102 views)
File Type: jpg 2014-06-06 20.35.36.jpg (927.0 KB, 92 views)
File Type: jpg 2014-06-06 20.51.44.jpg (771.9 KB, 87 views)
File Type: jpg 2014-06-06 20.46.18.jpg (832.3 KB, 82 views)
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Old 6th June 2014, 11:33 PM   #2
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did you check the volume pot too ?
de-solder it, and see if it works properly.
You can mesure it with a DMM
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Old 7th June 2014, 02:03 PM   #3
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I did check the pot without desoldering it.
It seems like it is working but it is loose when turned.

Later today i will de-solder it and reply if it is ok

Thanks for helping.
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Old 8th June 2014, 01:15 AM   #4
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Also note pin 15 should have the same voltage as pin 4, most likely both are fed from the same trace. Maybe just an error on your part, but, no voltage here means one channel is not working.
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Old 9th June 2014, 07:53 PM   #5
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Finaly i found the time and did remove the pot from pcb.
The pot is Alps GT 503A.

I attached the DMM to middle pin and left pin but the measurements is 3K although i change post position.
So guess it is faulty pot since it is very loose to.

Problem is i can not find exact replacement.
From datasheet i found it is 50K
Can i connect simple stereo pot just for test purpose?
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Old 11th June 2014, 02:08 AM   #6
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The potentiometer part is single so a mono pot will work for testing. It appears it also contains a switch of some sort in with it so you will have to jumper or add a switch for the other 2 pins as well.
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Old 18th June 2014, 09:32 PM   #7
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Finally i found the time and with a little experiment and like friend above said i used a single pot and the speakers are working again !Pictures in below link shows everything i did to repair-modifications for this speakers

Speakers Pictures

Thanks to all forum friends for their effort.
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