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Old 4th December 2012, 08:41 PM   #1
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Default Yamaha M50 fixing or rebuild

Hi Everyone!! Wicked forum that is! Im new to it. Had a lot of reading done here so its time to post now
Got this amp few years back second hand. Nice sound and plenty of headroom for me bedroom. Then my ex spilled a tea on it blowing the fuse... well then it happened again and its not just a fuse now.
So here we go 5 years later i decided to do soming about it
downloaded the manual, done 'power supply troubleshooting' measurements all good.
When amp turned on heatsinks get warm but there is no amplification. Basically i can hear the music from speaker only tiny bit even with gain to the max.
Is it worth trying to fix it or is there any ways i can fit some preassembled module to match original PS??
Gonna upload some pics tomorrow.
waiting for questions and suggestions. Cheers!!
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Old 4th December 2012, 09:47 PM   #2
RS232 is offline RS232  United Kingdom
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If you are handy with a voltmeter check the power supply for the preamp.
Don't have the schematic but if the power amp is working the fault is probably in the preamp stage.

Ric
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Old 5th December 2012, 05:45 AM   #3
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Cheers Rick! I'd give a try to any measurements. The only thing is I'm not sure what readings I'm after.
I'll upload some pics and maybe schematics tonight for people to see what I'm dealing here with.
I'm not a great DIYer like most of guys here, so there may be some dumb Qs will be asked
It's a power amp and I not sure if there is a preamp section ( am I wrong?)
Looks like I'm in for fixing it instead of rebuilding hehe
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Old 5th December 2012, 08:31 PM   #4
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Here is some pics:[IMG]Click the image to open in full size.[/IMG]

[IMG]Click the image to open in full size.[/IMG]

[IMG]Click the image to open in full size.[/IMG]

[IMG]Click the image to open in full size.[/IMG]

[IMG]Click the image to open in full size.[/IMG]

Last edited by kytaez; 5th December 2012 at 08:59 PM.
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Old 5th December 2012, 09:02 PM   #5
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will try to post schematics its pdf file ..
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Old 5th December 2012, 09:14 PM   #6
amptech is offline amptech  Scotland
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Welcome to the show...

There's many things to check when working on faulty amplifiers....a handy piece of test gear is the lamp limiter that you connect to the primary side of the amp's 240v input.
This will let you,(1) know if there's a heavy short and save the fuse's from popping plus it will aid you to do some quick voltages check's...

if your amp has plug connections from the psu to the amp stage then disconnect them to test the psu part.
look at the the large filter caps they might say 47000uf at 50 or 60 vdc..say the ht will will be some were in the range of 40 to 47 volts if the cap is 50 working or 50/55 if 60 cap.
note to drain any voltage use a 10k resistor a fly lead connected to it and one to earth chassis and use the other lead to each side of the power caps..with the amp turned off....
Faulty amplifiers should not be connected to any speaker load in it's fault state as it will damage the speaker..a working amp will have very low mv dc say 10mv to as high as 100mvs..if there's higher dc over 1 volt or more do not connect a speaker load..

Things to check are cracked tracks/o/c components and faulty power transistors etc... start at the long tail in transistor these are the small devices then check driver stage and so on...

preamp voltages are from +/-12 to 15 volts dc..

power amp stage's are from +/-35 to 50 volts or more...
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Old 5th December 2012, 10:54 PM   #7
RS232 is offline RS232  United Kingdom
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Well Kytaez mentioned that the amp was working but the sound level is very low even with volume set to max, so at least he has a good starting point.
Can you post the schematic for this amp ?
Your pictures don't help and are very small.

Ric
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Old 5th December 2012, 11:30 PM   #8
RS232 is offline RS232  United Kingdom
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Ok, found the schematic...
Check the capacitor board all stages are supplied by that board.
The big caps supply the main amp and the smaller ones the rest of the amp (preamp/VU meter, etc)
Also check the fuses in that board.
The preamp board is the vertical one in the center of the main amp board and the supply most be around +/-70/0/70Vdc.

Good luck
Ric
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Old 5th December 2012, 11:41 PM   #9
amptech is offline amptech  Scotland
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The weak sound fault would be o/c resistors and s/c small transistors plus bias circuit due to the tea being droped in the amp.
The o/ps would have been damaged and need to be checked.

Post diagram so we can see things.
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Old 6th December 2012, 06:25 PM   #10
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Thanks for a quick responce!! getting use to forum now. Some bigger pictures this time [IMG]Click the image to open in full size. Click the image to open in full size. Click the image to open in full size. Click the image to open in full size. Click the image to open in full size.[/IMG]

For connecting the speaker to faulty amp.. its on its way to the bin anyway. as long as im not damaging the amp itself its fine..
In the servise manual there is troubleshooting for PS board. done that, readings is fine.
will try to see if the vertical board(preamp stage?) getting power...
Still dunno how to upload pdf (
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