Yamaha MX-1 Thread

Thanks guys!

Oh. Something that I forgot to mention. The holes where the mounting screws to attach the amp board heat sink rail to the main heat sink measured at 2.5mm, which is the correct hole dia to tap them for M3 machine screws if someone wants to do that job really correctly. But I didn't want to be doing any thread cutting without completely dismounting the rail, hence using sheet-metal screws like Yamaha did.

Cheers,
James
 
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One last small observation. Before I went in and recapped this unit I noticed that it seemed to be running fairly hot after an hour or so of use with moderate volume levels.

Now that the amp board heat sink rails are more securely attached to the main heat sinks the amp only gets lightly warm even after several hours of spirited use.

So for anyone else with one of these amps it may pay to check how securely the amp boards are attached to the heat sinks.

Cheers,
James
 
Interesting Thread! Thank you so much for the information and follow up on this restoration. I just scored one MX-1 myself!!! Although mine was $750 US... but hey! that's just hard to beat with any other offer. Just one question, reading this I see you commented on most (if not all) parts you replaced by codes. That's very helpful. I wonder if those parts you replaced would be the only ones I should have replaced to get a complete overhaul, for what matters the most, done? If so, I'll get the schematics and probably recap it this coming winter.

I'll test mine, but if it's in full working order I won't bother restoring until I have some time.
 
Interesting Thread! Thank you so much for the information and follow up on this restoration. I just scored one MX-1 myself!!! Although mine was $750 US... but hey! that's just hard to beat with any other offer. Just one question, reading this I see you commented on most (if not all) parts you replaced by codes. That's very helpful. I wonder if those parts you replaced would be the only ones I should have replaced to get a complete overhaul, for what matters the most, done? If so, I'll get the schematics and probably recap it this coming winter.

I'll test mine, but if it's in full working order I won't bother restoring until I have some time.

Hi Bastian,

The only parts that I didn't replace for a complete servicing that should still be done are the relays and the main filter caps.

Concerning the relays, I still haven't researched what to use. Most likely Mouser has something, I just haven't looked at that issue yet.
And concerning the main filter caps, I haven't found something in the right capacity & voltage that fits.

You are going to like your MX-1. It's a nice clean sounding amp. :thumbsup:

Cheers,
James
 
Picked up a 240V (selectable 120-240) Yamaha MX-1 that had been DROPPED on the ground. Paid 200 for it untested. Fortunately the chassis is so damn solid the damage only bent the corner of the front face a little, doesn't bother me.
Plugged in and powered on - checked DC offset;
4.2mV RC
3.9mV LC

Opened and checked DC bias, 14.4mV both channels. 67W idle current draw.

Plays music.

Now the big problem - the chassis has 90V A.C. reference to ground (vibrates fingers when touching). The teardown begins.
 
Now the big problem - the chassis has 90V A.C. reference to ground (vibrates fingers when touching). The teardown begins.
If your running this on USA 120vac -- Reverse the AC plug. Not sure about " down under " wiring standard. If your and Aussy.:biggrin:
 
Thanks for reply Avionic

I'm 240V here and cannot flip the plug but I can swap L-N inside the amp or plug (I'm ticketed electrician). I could just earth the chassis with a new 3c mains lead, but I'd rather keep the mains ground out of the audio circuit - as I tested the speaker negatives are connected to the chassis also. So I will try and find the source of the AC leakage. 90V AC is way to high to be inductance from those massive Tranny's - I would expect 5-10V max
 
Managed to get the transformers tested. Perfect results with insulation resistance testing @ 250V DC. Isolated them from the chassis w/nylon, earthed cases to mains grid and retested, chassis voltage down to 7V A.C. (expected).

Removed the voltage selector switch to rule that out also. New sleeves installed over all transformer and incoming mains cabling to rule out leakage there. Re-Installed main power board and connected, back up to 45V AC on the chassis. Hmm.. will test for distortion w and wo the chassis earthed to mains as thats maybe the only way to get rid of the ghost voltage? Is there a possible fault on a cap to chassis, or is this normal behavior that might go away when a speaker is connected. More testing to do tomorrow..
 
Finally got everything back together - still 90V chassis to true ground/earth. This is enough voltage to make your eyeballs vibrate, let alone your finger hairs tingle. Rest assured this is not a mains side voltage leakage but a leakage somewhere after the secondary side of the transformers. I suspect the main power supply caps but not sure if I can get a replacement. Ended up grounding the chassis with the 3 core mains lead and problem solved, and no seemingly obvious change to the amplifiers operation. Would grounding the mains supply side affect the operation seeing as its a floating chassis by design? i.e. the negative speaker cable and the power supply both connect to the chassis?
 
Hi guys. I'm an electronics noob. One thing we have in common though is that I have an MX-1. I was adviced to change the relays in the amp. DEC DE2TU. The issue was that the left channel volume is a bit low than the right channel. Also sometimes the left channel cuts out. But on turning the volume up it comes back to normal.
I see people selling Omron relays for the mx-1. Can someone help me by recommending an omron relay as an alternative to this DEC De2TU relay. Thanks in advance.

I intend to purchase the alternative relay locally.
 
Hi guys. I'm an electronics noob. One thing we have in common though is that I have an MX-1. I was adviced to change the relays in the amp. DEC DE2TU. The issue was that the left channel volume is a bit low than the right channel. Also sometimes the left channel cuts out. But on turning the volume up it comes back to normal.
I see people selling Omron relays for the mx-1. Can someone help me by recommending an omron relay as an alternative to this DEC De2TU relay. Thanks in advance.

I intend to purchase the alternative relay locally.

That is one thing that I still haven't done, find definitive replacements for the relays. Mine have been working and I guess that I've just been a bit complacent about it.

Cheers,
James
 
Well, I have now reached the point where I do have to replace the relays in my unit. It has started dropping out if the volume is turned up past a certain point.

First up is the 12vdc DH1U 15A relay in the start circuit. After a lot of searching I found a workable replacement, a Finder relay, part number 40.61.9.012.0000 12VDC 16A.

This relay can be found at Conrad and Farnell.
https://www.conrad.com/p/finder-406190120000-pcb-relay-12-v-dc-16-a-1-change-over-1-pcs-502921
https://de.farnell.com/finder/40-61-9-012-0000/relay-spdt-250vac-16a/dp/2365735

DSC05616.JPG

Cheers,
James
 
The Finder relay is SPDT but the DEC original part is SPST. Not a problem. And also, the Finder has paired 250v input and output leads wheras the old part only has a single asymmetrical lead for in/out. Also not a problem because the Finder legs are in parallel and all of the un-needed legs can be simply snipped off.

The coil leads are directly under the coil. The next pair of leads are for the normally-closed position and will be cut off. The next pair is for power input, the 2 legs are in parallel and 1 will be cut off. The last pair are the normally-open output and again are in parallel and 1 will be cut off.

DSC05617.JPG DSC05618.JPG DSC05619.JPG

Cheers,
James
 
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Once the un-needed legs are cut off the relay is a drop-in fit in the board with no other modding needed.

DSC05605.JPG DSC05608.JPG DSC05609.JPG

Cheers,
James
 
The contacts on the old relay were burned. These are not great photos but are the best that I can do at the moment.

DSC05611.JPG DSC05621.jpg DSC05622.jpg

Cheers,
James
 
I bought a USB microscope to try to get some better photos is things like relay contacts etc, but unfortunately it has fairly poor resolution (the last 2 photos in the last post). Here is another photo taken with my trusty 20 year old Sony 4mp digicam:

DSC05623.JPG

Cheers,
James
 
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