Yamaha MX-1000U Protection always on..Help!

[adlpc]

Hello All,

I just found diyAudio forum! Great resource and great knowledgable, helpful, group here!

Have a MX-1000 that is always in protection mode. Haven't opened yet since I don't have a service manual, schematic, or even an owners manual.

Anyone have any of these or any suggestions?
Is this a usual or unusual problem for this amp?

Any and all help will be greatly appreciated!!
Thankyou in advance.

Read on if you wish.
My intro:
I'm an music lover that likes to tinker with equipment. Have designed and built a pair of two way true 360 radiating speakers, and also built my own amps from scratch (60W/ch simple amp on a chip type and a 350W/ch MOSFET monster KIT amp with a waaay oversized power supply that dims the house lights or blows breakers on power-up) so I do understand a little, but need hand holding when it comes to repair.

My Collection:
Turntables-
Denon DD Transcription with mod. Black Widow arm - forget CDs!
Garrard with 4 pivot Parallel tracking arm - good old engineering
I use different cartridges based on the recording - AudioTechnica, Denon, Grado, Shure

CDplayer-
Technics SL-PG300 - old & cheap with good clean sound

Reel to reel-
Sony TC-730 - old work horse - still good!

Cassette deck-
Yamaha KX-W382 - poor man's Nakamichi

Receivers-
Harmon Kardon AVR 210 - very nice
NAD 7600 - needs work, another monster!
Pioneer XS-1980 - needs work, Mother of all Receivers!
Pioneer VSX-9900S - good overall, outdated surround
Pioneer VSX-29TX - excellent when it works, DSP board problems
Yamaha RX-V995 - clean crisp and well behaved!

PreAmps-
Carver CT-17 - very clean sound

Integrated Amps-
Pioneer SA-9900 - preamp dead, works as a great power amp
Yamaha A-720 - great overall sound

Power amps-
A/D/S PH6 (have two!) - both a little noisy, but great at higher vol
Carver Premiere AV-705x - nice clean sound
Counterpoint SA-100 - sweet sound with surprising base
Hafler DH500 - good allround tank!
Sumo Andromeda - delicate highs and smooth
Technics SE-A5 - dynamic and crystal clear - Possibly my fav.
Yamaha MX-1000U - stuck in protection mode, haven't heard it yet
Yamaha M-50 - missing fuses, haven't heard it yet
Own 60W/ch - surprising solid sound
Own 350W/ch - very clean and SOLID sound
Yes, I am attending Power Amps Anonymous!

Note: I never believed amps actually sounded different, until I listened. Amps are far more speaker sensitive than I would have thought. Speakers are not as sensitve to amps as vica versa.

Speakers are generally my own full size floor standing design or Advent bookshelf with a subwoofer.

Yes, and I'm still married, 25 yrs now.

Thanks for the bandwidth!

 

[XEAGLEKEEPER]

adlpc
The first thing I would check is for shorted rails ie. output transistors. I dont have a schematic for a MX-1000 I do have one for a M-4 & M-45. MX series is much newer probably changed quite a bit. If no obvious shorted output transistors then possibly a power supply problem or even a problem with the protect circuit itself .Some would even say"Tap on the relay". Just if you should choose to tap on the relay dont have the amp hooked-up to your favorite set of speakers!Lookover the amp for other obvious causes such as swelled up or split-open capacitors , burnt or broken resistors etc.


Dave

 

[adlpc]

Hi Xeaglekeeper,

Thanks! One of the rails has very low voltage. I measures 90V and 50V on different ones, and one has only 2-3 V. Also found a a dark spot under a transistor attached to the middle of the right channel heatsink between the banks of the larger output transistors. Not sure if it was soldering heat or a damaged trany. Any recommendations on the best way to check a transistor?

I hope I don't need to disassemble the whole thing to get to some of the connections and soldered surfaces. The MX-1000 has two layers of boards with a connection in between that I'm not sure I want to tackle unless someone knows how they come apart easily.

Thanks again!

 

[halo0925]

MX-1000

Check the outputs with your VOM set to the HFE setting should get base to collector- emitter ranges of 450 to 650 or close, check for dead shorts or opens on all output dissconected from circuit.
Probably there will be more wrong then just the outputs, the circuit topology has not strayed much from the original M series.
Shorted outputs like to open the emmiter resistors, also check the Drive trannys and pre- drive they will go by by with the outputs.
The circuit can be a little hard to track, but on the lower PC board you will find a row of resistors and diodes for both channels right to left, check the diodes for foward conduction, by the way 4 of them will be zener diodes they also like to let loose when the outputs go.
Good luck

 

[XEAGLEKEEPER]

Well the most accurate way to check transistors is out of circuit or isolated from the circuit and to check for emitter to base,base to collector and emitter to collector shorts . A in circuit check for a shorted power rail would be to measure the continuity between the collectors on both of the channels and the central or star ground. All this will tell you is either + or - rail(s) on a given channel is shorted or not ie. through a output transistor but that gets us right back to transistor isolation. Really no easy way to do it. Other than paying someone else to do it. LOL

Halo sounds like you have " Been there" LOL

 

[XEAGLEKEEPER]

Pretty impressive specs on that amp.

 

[adlpc]

Thanks, both of you!

It sound more and more Like I have to take the whole thing apart and go through it resister by transistor and resolder everything.

I've heard a lot of good things about it, and those specs....! It's definitely worth doing.

Was given this one by a friend who got a new A/V system, and he knows I like to play with electronics. It's a little beat up, not too badly mind you, but does have a scratch in the large plexi covering the red meters and the front edges have some light scrapes. I really don't care how it looks, I care about the sound.

Guess I'll role up my sleeves and get to work

BIG THANKS for all the info and I will be keep you posted as time allows me to work on it, and will be asking more questions as I go through it.

Any specific info/service manual guidance? Any posted on a web site or anything like that? If not, I'll order a copy. I've learned never to dig without one!