yamaha m 40 amp

moez

New Member
Hello everyone I bought a yamaha m 40 amp on ebay three months ago its has been working great up until last week then it just shut down on its own and will not power on , I opened it up and noticed the fuse was blown so i replaced it and tried to turn it with no luck but i noticed burning smell and smoke coming from a resistor. So i downloaded the service manual from hifi engine and looked up what resistor it was and replaced it with no luck again it lit up the resistor as soon as i turned it on . Is there is anyone who knows about this amp and could help me the resistor is R171 on the schematic
 
Welcome to AK. Looks and sounds like you have shorted output transistors in the left channel.Most likely alot of other stuff is cooked as well. M series amps like to frie alot of parts when the outputs short out. About all I can suggest is get a blown up copy of the schematic and start checking parts starting from the outputs and work your way forward. Plotting the debris field as you go. Concentrate on transistors, resistors , diodes and electrolytic caps.
 
Thank you for the help it looks like all the left side transistors are gone.I am going to replace both right and left transistors and the resistor and try it again ,don,t have much experience with electronics just the stuff i read online.
 
Thank you for the help it looks like all the left side transistors are gone.I am going to replace both right and left transistors and the resistor and try it again ,don,t have much experience with electronics just the stuff i read online.

If you have a bunch of bad transistors.Likely you have a handful of bad fusible resistors as well. Post a photo of the component side of the problem area.
If this unit has any dried glue around the the bases of the larger caps that needs to be addressed as well.
 
Forgot to mention after closer inspectionfound three more resistors burned out. 001.JPG
 
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Forgot to mention after closer inspectionfound three more resistors burned out. View attachment 570295

Did you also check the big white ceramic wirewound emitter resistors. They look like theres a pair towards the front and a pair towards the rear, under the heatsink fins. They should be around .22Ω
 
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I will check the resistors, ordered the transistor from ebay so i have lots of time to go thru everything ,would you notice if those large resistors are bad visually?also could i replace the 1.2k resistor with a 1.5k that's all i could find locally?.
 
Looks like two of them ceramic resistors are bad the two towards the back , thanks for the tip .
 
I will check the resistors, ordered the transistor from ebay so i have lots of time to go thru everything ,would you notice if those large resistors are bad visually?also could i replace the 1.2k resistor with a 1.5k that's all i could find locally?.

Where is local. EBAY would be the last place on my list to buy any transistors.:no:
Did you check each of the TO-92 transistors and the diodes in the bad channel? When these amps short out they usually take out alot of components.
 
local is niagara falls ontario canada we have a store called [the source] which used to be radio shack but they pulled out of canada , the have a very limited assortment of diy supplies and no transistors. Would it be possible for you to copy my last pic i posted and mark what i should check that would be greatly appreciated thanks moe
 
local is niagara falls ontario canada we have a store called [the source] which used to be radio shack but they pulled out of canada , the have a very limited assortment of diy supplies and no transistors. Would it be possible for you to copy my last pic i posted and mark what i should check that would be greatly appreciated thanks moe

Not with this company PC. Have to wait until I get off work (2:30AM CST)

Mouser , Digikey and BD Enterprise is your friend.:thmbsp:
 
Need to pull and check the 14 (TO-92 transistors) on the bad side of the board. Green arrows point to two of the TO-92's. Check all of the glass diodes on the same side of the board ( I usually just replace them!). Need to verify that all the fusible resistors (greenish ones standing upright) - pink arrow points to one of them, are ok. Do not apply any power until your damn sure there are no more bad parts still on the board or you will burn up more parts -guaranteed!!! In a nanosecond:yes:

Thats why repairing these are so time consuming.
 

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This is how I keep track of whats good and whats bad. It also gives me and idea of where the damaging current went and what other parts to verify.

Red is checked bad. Green is checked OK. This is a schematic of and M-80. You will notice that the circuit is some what similar topology to the M-40.
 

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That will be a big help for me thanks again. I have no way of checking caps and coil, only have a multimeter should i be worried about those to?
 
This is how I keep track of whats good and whats bad. It also gives me and idea of where the damaging current went and what other parts to verify.

Red is checked bad. Green is checked OK. This is a schematic of and M-80. You will notice that the circuit is some what similar topology to the M-40.

That is a great idea. Thanks for posting that.

Cheers,
James
 
That will be a big help for me thanks again. I have no way of checking caps and coil, only have a multimeter should i be worried about those to?

Coils (inductors) are not and issue with your amplifier.(unless your protect relay is borked) Ceramic,mylar and mica caps are usually not a problem unless they are physically damaged. I replace all of the electrolytics reguardless of being good or bad. If relays are readily available they also get replaced with new. I don't piss around with trying to clean them.
 
That is a great idea. Thanks for posting that.

Cheers,
James

I print the schematic on ledger size paper 11 X 17. Made dozens of copy's of the schematic and also the main board component layout. Mark'em up with notes or whatever. It just works best for me.
 
Back to the drawing board ,thought i went thru everything like you said so powered it up and nothing but a little smoke and a burning smell. also the power light only came on when i pushed the class A button, the protection light came on right away. I have no idea what damage I've done now.
 
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