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

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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!
 
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
 
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.
 
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.
 
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.
 
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often people make a common mistake connect one speaker to position A left and one speaker in position B also left ....so they "think" that both outputs work ...

beyond that protection circuits can be activated by excessive DC offset in the output but depending on the protection circuit ( seen before ) a faulty relay that causes spark in the speaker connection can also cause protection mode to activate ...

in this case a faulty relay ( also common failure )

after a very quick look at the schematic it seems that there is no clamp diodes in the output of the amplifier so that in a way completes the failure picture, so a faulty relay with loose contacts will create spark while operating that might cause a reverse current that protection will see and activate ..

still in a amplifier of this age the classic problems should be also present such is soldering issues and most of all small electrolytics

as about the service manual of the M 50/M 60 i have it if any one is looking for it please email me

kind regards
sakis
 
Yamaha M-80 in protection mode

Hi M-80 lovers, I recently acquired a M-80 power amplifier but is in protection mode with or without speakers (A,B or C) installed. So suspect it's not a relay problem. Visual inspection on the all PC boards shows no obvious issues ie; leakage, burnouts or bad solder joints. How should I proceed next? Thanks in advance.
 
Some of the most common issues on these old Yamahas include loads of bad solder joints and the Glue of Death. The latter, originally employed to fix larger components on the PCB before soldering, has a habit of absorbing moisture, discoloring, turning sour and attacking any components it covers; staying in protection is one possible symptom. So these should be checked.

Of course it could also be a plain ol' blown output stage issue. See whether all the output transistors pass a rough check with the diode test function when powered off, and attempt to measure output DC offset for both channels when powered on (I'd have to look at the schematic to tell where this is best done, have to hurry now).
 
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