yamaha m-50 repair (amp goes into protection mode)

[Zorac]

i recently purchased a yamaha m-50, the problem with it is that once it is up to tempeture if you turn the levels up to high the protection clicks on and doesnt come off till you turn the levels down. the led level indicators on the front also tend to fade at random (ie not with the music) is this generally an easy fix? what generally causes these problems? what can i expect for a repair bill?

im contemplating doing the repair myself, although ive never done something like that, im am comfortable soldering on boards an working with electronics...

thanks!

[Cal Weldon]

Is the protection circuit cutting in because of low impedance?

Not sure about the LED's.

[Zorac]

my running 8ohm satilite speakers off it (cerwin vega ht-s5) so that shouldnt be the problem, i believe the amp is 4ohm stable.

i did notice that it is sucking alot of power though (my power conditioner has an amp meter), its using more than my m-80, which is driving much large speakers at 4 ohms, and using more (double) than my m-45 which is powering another set of ht-s5s

[rhr9]

Hi folks ,

I recently got a M-50 amp to check . When I select "Speakers A" , the protection indicator is lighted and I have only the sound on one of both
speakers .
When I select "Speakers B" , all is ok , I get sound on both speakers .
So my question is : what is the difference between "Speakers A" and "B" ?
Is ther any trouble with the output relays ? I think all the rest of the circuits
are common .
I only found the Service Manual on the web , but not the amp. schematic !
So if anyone could show me the whole amp. shematic I could progress in my repairing .

Best wishes ,
@ + Philippe.

[sakis]

you have one Chanel shorted and also you connected your speakers in a wrong way

[rhr9]

Hi Sakis ,

In order to be accurate : I first connected 2 speakers on the "A" outputs .
On a second time , I connected the same speakers on the "B" outputs .
The speakers are allright and well connected .

The only difference between A and B is the relay that switches the outputs .
I think the power transistors are common right before the relays .

Best wishes ,
@ + Philippe.

[jitter]

I agree that the power stage is OK, otherwise the fault would occur on both channels.

My next suspect would be channel A relay. Very simple way to find out is to put channel B relay in channel A and see if that cures the fault.

[rhr9]

Hi jitter ,

Thank you for the reply , I will interchange the relays and see what happens !

Any solution to get the shematic of the protection circuit ?

Best wishes ,
@ + Philippe.

[Tejaus]

I have a M-80 that would do that too. Turned out to be the adhesive that Yamaha used to glue the two 1000uf 100v caps to the PCB is conductive. Once I removed that it worked fine. I'm not familiar with the M-50 but it might something to check.

[jitter]

Originally Posted by rhr9

Hi jitter ,

Thank you for the reply , I will interchange the relays and see what happens !

Any solution to get the shematic of the protection circuit ?

Best wishes ,
@ + Philippe.

After googling a bit I found out that you're not the only one looking for the schematics of the M-50. However, the schematics of the very similar M-70 are available and most answers seem to indicate the M-70 schematics are suitable for M-50 owners too.
Search for M70 on this website: http://www.eserviceinfo.com/.
The speaker protection circuit is on page 4 of the schematic.