Dear All
I recently got a Yamaha M-4 power amp ("perfect condition", ebay blahblah). It was damaged in transit, due to poor packaging and an obvious misassembly. There is an internal support for the main pcb, which was assembled incorrectly and the heavy transformer etc. and poor packaging played together and that pcb broke. I repaired the damaged leads, which supply the speaker relays with power. I also replaced the feeble speaker terminals with standard terminals for bananas.
I tried to get the DC offset down and finally arrived at about 3mV per channel, but it would fluctuate about +/- 5mV - just within specs (10mV according to the SM). I did not replace anything else.
After that operation the amp played fine and with a very nice sound for a couple of weeks, though I am still undecided whether it is better than the Kenwood Basic M2-A it replaced.
Then a couple of days ago, while listening, there occurred popping noises from the M-4. Also the level began to fluctuate, i.e. the music level went down considerably and the bass even more. After a couple of seconds the level went back to normal and so on. Of course I took it out of the chain and put the Kenwood back into place, which plays nicely since. So it is clear, that my pre is not the culprit (a Kenwood C2, which I rebuilt more or less completely (mute relais, some transistors, all electrolytics with Pana FM, all ceramics with styroflexes, desoldering and cleaning the source selector, replacing balance pot with resistors).
The questions I have are circling around where to start with the diagnose of the M-4 problem. To the more experienced buffs, does it more look like a simple voltage problem (due to the miserable trimmer pots) or is it more related with bad caps or transistors?
I have a full set of tools at hand (DMM, capacity and esr meters, oscilloscope, though I just learn how to use it), soldering/desoldering iron etc., but I have no electronics education and need to slowly understand more of the intricate things...
Thanks a lot for any suggestions or perhaps experience with that amp.
Ben
I recently got a Yamaha M-4 power amp ("perfect condition", ebay blahblah). It was damaged in transit, due to poor packaging and an obvious misassembly. There is an internal support for the main pcb, which was assembled incorrectly and the heavy transformer etc. and poor packaging played together and that pcb broke. I repaired the damaged leads, which supply the speaker relays with power. I also replaced the feeble speaker terminals with standard terminals for bananas.
I tried to get the DC offset down and finally arrived at about 3mV per channel, but it would fluctuate about +/- 5mV - just within specs (10mV according to the SM). I did not replace anything else.
After that operation the amp played fine and with a very nice sound for a couple of weeks, though I am still undecided whether it is better than the Kenwood Basic M2-A it replaced.
Then a couple of days ago, while listening, there occurred popping noises from the M-4. Also the level began to fluctuate, i.e. the music level went down considerably and the bass even more. After a couple of seconds the level went back to normal and so on. Of course I took it out of the chain and put the Kenwood back into place, which plays nicely since. So it is clear, that my pre is not the culprit (a Kenwood C2, which I rebuilt more or less completely (mute relais, some transistors, all electrolytics with Pana FM, all ceramics with styroflexes, desoldering and cleaning the source selector, replacing balance pot with resistors).
The questions I have are circling around where to start with the diagnose of the M-4 problem. To the more experienced buffs, does it more look like a simple voltage problem (due to the miserable trimmer pots) or is it more related with bad caps or transistors?
I have a full set of tools at hand (DMM, capacity and esr meters, oscilloscope, though I just learn how to use it), soldering/desoldering iron etc., but I have no electronics education and need to slowly understand more of the intricate things...
Thanks a lot for any suggestions or perhaps experience with that amp.
Ben
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IIRC the M-4 has input level controls on the rear panel.
If so, use a contact cleaner/lubricant.....or rotate them a million or so times.
Other reasons are crook solder joints....and Yamaha's are prone to this, in particular the hot/very hot running components.....try tapping boards with screwdriver handle.
Another reason is input diff pair transistors...look for black transistor leads....freeze spray.
Noisy/leaky input transistors can cause intermittent rumble/popping noise and/or shift in DC offset possibly causing triggering of output protection relays.
More help, just ask.
Dan.
If so, use a contact cleaner/lubricant.....or rotate them a million or so times.
Other reasons are crook solder joints....and Yamaha's are prone to this, in particular the hot/very hot running components.....try tapping boards with screwdriver handle.
Another reason is input diff pair transistors...look for black transistor leads....freeze spray.
Noisy/leaky input transistors can cause intermittent rumble/popping noise and/or shift in DC offset possibly causing triggering of output protection relays.
More help, just ask.
Dan.
HI Dan
Thank you so far. I'll try your suggestions. By the way: the transformer coils are made of copper foil and look very ruffled, which means, not neatly wound, but looking quite loose and as whether the coil is disintegrating. All the images of older M-4s I have seen so far on the net show the same untidy transformer coils. Is that the Yamaha standard?
Ben
Thank you so far. I'll try your suggestions. By the way: the transformer coils are made of copper foil and look very ruffled, which means, not neatly wound, but looking quite loose and as whether the coil is disintegrating. All the images of older M-4s I have seen so far on the net show the same untidy transformer coils. Is that the Yamaha standard?
Ben
Hm, I need to have a look in my other amps, to see, whether all manufacturers used such, eh "things".
By the way, my Harman Kardon 655 Vxi has the exact same problem as the M-4, so I assume a similar fault. The 655 Vxi's pre-amp section has also been repaired three years ago by a very reliable shop, which leaves the power amp section as the presumable culprit.
Ben
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