Yamaha M-65 wont power on

Rob B

2010-12-26 6:08 pm
Hey all, I have an old m-65 thats been serving me well for a few years until last night. First time I used it for subwoofer duty, I had it running a single 4 ohm driver at around 200 watts on the meter for about 3 hours (pretty close to full tilt really, but it could do more). It was getting a little warm but not as hot as i have seen it get. I figured driving just one speaker on one channel it should be okay to handle it until I found a sub amp... I was wrong.

Anyhow, at some point it completely shut down, no smoke, no pop, no visibly damaged parts, didn't even go into protection first. Now it wont even power on, light up, anything. As if it has no power supply.

My question is, does this amp have a fuse anywhere in it or is this thing cooked? First time I blew an analog amp and didn't see any parts visibly fail... normally an overheated class a/b will go up in smoke in my experience.

I have been using this amp daily and consistently hard use on the weekends as my main amp for the past two years, so its not like i blew the dust off this thing and then put to work, I know the amps limits (or thought so lol).

Any input is appreciated!
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Measure the primary of the transformer for continuity - i bet you find it open, meaning the internal thermal fuse has failed. Normally this means you are SOL as the thermal fuse is buried in the windings, but you may find that the connections to the fuse are brought out onto terminals. If so, you could bypass it.

Rob B

2010-12-26 6:08 pm
I cant do much without a diagram but i tracked the circuit down from the plug to the transformer and it has power to the transformer, so I'm assuming the transformers dead. i noticed though there is a separate fuse and circuit for the 240 section. Is there a possibility the transformer will work if i switch it to 240?

I don't know much about amps so sorry if that sounds stupid.
It's best not to assume anything......

I cant do much without a diagram but i tracked the circuit down from the plug to the transformer and it has power to the transformer, so I'm assuming the transformers dead.

Keep going......

Rob B: Since you have a multimeter and you have already confirmed that the transformer is getting juice, just measure the voltages of the transformer secondaries with it powered up. That's the easiest way to determine if your transformer has gone tits-up.

With the meter on AC volts you can't fry anything unless your probe slips and you short something out.

and, as already mentioned, check for continuity of the primary windings with your ohmmeter.......
the transformer has 6 wires, I don't know which are the primaries. The owners manual calls it a dual something or other special transformer. I doubt I'l find a replacement part once i find the problem anyways, no?

Sounds like you've already convinced yourself it's the transformer.......if you find AC voltages on the transformer secondaries then it's something else.

You have eliminated the obvious stuff like the mains fuse, correct? Did you remove it (or them) and check for continuity across the fuse with your meter?

Only six wires? That should be very straightforward. Does the amp have a 120/240 switch/dial on the back? Many that were sold at military bases have that (the int'l model). These have a tapped primary winding for 100/120/240V. With your meter on AC volts, it has a very high resistance, so you will not damage anything by checking the voltages on various transformer taps/wires.

With the amp powered up take the negative meter lead and place it on a tap, then take the positive meter lead and probe the remaining 5 taps carefully since some are at mains voltage. Write down the results. Move the negative meter lead to another tap and repeat.

Let us know what you find.


2005-01-01 3:33 pm
Amplifier transformers sometimes have a thermal cutoff that trips if the transformer get hot enough, and as you said you drove the amp hard when it quit, its possible the transformer overheated and blew this thermal cutout.

If your lucky, the thermal cutout can be bypassed by bridging two of the taps.

Rob B

2010-12-26 6:08 pm
Okay, i checked the fuses first thing obviously, both good. It goes from the cord to a board that tees off to two fuses. from the fuses these two circuits go to a 110/120/220/240 switch and then there are a few wires going to the transformer. I only tracked the 110/120 path to the transformer and found voltage all the way down.

the output of the transformer will be dc voltage no?

You will have to excuse my ignorance, I'm a professional mechanic and really only know automotive electrical.
The transformer output will also be AC. It doesn't turn into DC until it gets through the rectifier and the smoothing power supply caps and resistors.

.......so the wires out of the transformer on the secondary side are still AC. Check all combinations of the secondary wires with your meter on AC volts and let us know what you get.