Yamaha GA-15 Guitar Amp - diyAudio
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Old 16th March 2014, 10:44 AM   #1
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Default Yamaha GA-15 Guitar Amp

Hallo, I'm looking for service manual for my Yamaha GA-15 guitar amp. He looks dead. Can anyone help me?
Bye...

Last edited by Dinosauro; 16th March 2014 at 10:47 AM.
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Old 17th March 2014, 01:29 AM   #2
Enzo is offline Enzo  United States
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I don't have the manual, but for a basic practice amp like this, if it is "dead", meaning no sign of life at all, you may get by without one. Pull the plug from the wall outlet, but turn the amp power switch on. Now measure resistance with your meter between the two main prongs of the power plug. You are measuring the power transformer primary through the power cord, fuse and switch this way. If you get an open, then check resistance right at the transformer primary wires. I don't know the resistance, but expect something like 50-200 ohms maybe. Point is we don;t want to see an open. If there is resistance instead of open, it is likely to be correct.

With plug in wall outlet, is there main voltage across that primary? And find the secondary wires, is there AC on them as well? We are checking the health of the power transformer.

If you get secondary voltages - oh I guess 25-40vAC with center tap - then it should be easy to find the main power supply. Those secondary wires will connect on the board right to either a rectifier bridge or to some discrete diodes. And the resulting DC will be filtered by a couple larger filter caps. Got DC on those?
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Old 17th March 2014, 11:54 AM   #3
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The amp is not really dead, but is in a coma. Fuse is OK, LED lights up and you hear a slight hum, but no signal is amplified at the entrance to the exit. Neither the speaker nor the headphones output. The signal is interrupted somewhere or the transistors are faulty. That's why I am looking for circuit diagram. thanks
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Old 17th March 2014, 01:21 PM   #4
teemuk is offline teemuk  Finland
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Contact Belcat Co., Ltd. and see if they can help. They made those amplifiers for Yamaha.
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Old 21st March 2014, 10:02 AM   #5
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The schematic looks fairly simple: Yustech GT-Guitars and Basses Solution.: Yamaha GA15
Try to follow the signal with a scope (input jack, first opamp, tone stack etc.) Look for bad jacks, switches etc.
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Old 21st March 2014, 11:09 AM   #6
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severely overdriven TDA type outputs blow i stock a couple dozen for replacements i'd start there.
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Old 22nd March 2014, 01:12 PM   #7
JMFahey is online now JMFahey  Argentina
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Quote:
Originally Posted by fastfolkert View Post
The schematic looks fairly simple: Yustech GT-Guitars and Basses Solution.: Yamaha GA15
Try to follow the signal with a scope (input jack, first opamp, tone stack etc.) Look for bad jacks, switches etc.
The blog answers
Quote:
his blog is open to invited readers only
http://yustech.blogspot.com/

It doesn't look like you have been invited to read this blog. If you think this is a mistake, you might want to contact the blog author and request an invitation.
Since you seem to be able to read it, can't you just download the schematic to your computer and then upload it here?
Thanks a lot
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Old 25th March 2014, 08:08 AM   #8
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No one can enter in this blog. Can you upload the schematic on our forum, please?
Thanks
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Old 25th March 2014, 07:20 PM   #9
yustech is offline yustech  Malaysia
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Here you go.I'll leave it open until friday.
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yustech.blogspot.com
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Old 25th March 2014, 08:27 PM   #10
JMFahey is online now JMFahey  Argentina
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Dear Yustech.
Thanks for opening your Blog, but found no schematic there, only pictures.
Am I missing something?
Thanks.
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