Yamaha M-70 problem

[kyle bermingham]

My Yamaha M-70 (2-channel outboard amp) is acting up. The first one I bought seven years ago off ebay still works great. However, my second ebay M-70 purchase started acting up a few weeks after purchase.

Specifically, the protection light started going on, even when not playing. I was hearing popping sounds from my rears, which it powers, even when the pre- was turned off (I leave the amps on--every time they turn on I have to reset the breaker). I noticed the protection light was on, so I turned the amp off.

Today I checked a few things. First off, I noticed when I touch the connections running to the pre-amp, it made a loud popping noise. I turned it off again, checked the speaker wire polarity, and changed the RCAs. After resetting the breaker, it played for a bit, with the protection light on--then just as I was going to turn it off, the speakers stopped making sound. It doesn't seem to work at all now.

I am just about certain I will need to spend money and have someone fix it, if it is fixable. However, if anyone knows an easy fix, I would be most happy. Short of that, it would be nice to know the likely issue, so I can research how much it will be. Lastly, I am in the Tampa Bay area--if anyone knows a good electronics shop with knowledge of the old Yamaha M- series, I would be ecstatic.

 

[John Stuart]

Hello, I'm not sure, if it is helpful, but here is the manual, just in case - https://www.usermanuals.tech/d/yamaha-m-70-service-manual

 

[M Code]

Amp needs to go in for service..
That is a great product when working properly..

Just my $0.02...

 

[TLS Guy]

You will need a really good service tech, if you can still find one. I guess those may be good amps when going. I have reviewed the service manual and circuits in detail. As is so typical of Far Eastern products it is thrown together. The power supply has a huge number of taps going to multiple power supply boards, and the amp boards contain thousands of components. There are lots of delicate adjustments from multiple surface mounted pots which is always a bad sign, as these wander and set of lots of problems.

If I can I like to look at circuits before purchase. Unfortunately this is often not possible. Now I have seen that manual, it is in my book as a unit I would NEVER want to own under any circumstances.

This is the poster child for inelegant design if ever I saw it.

The circuit is too big to load here.

Now here is the circuit for my beloved Quad 909 250 watt per channel amps.

Note the low part count and absence of internal adjustments.

That is an example of elegant design.

The component count of that miserable M70 exceeds that Quad by at least a 1000%

I fear getting that M70 up and running is going to be a very expensive proposition, unless you are very lucky.