Yamaha MX-1 Thread

[Bratwurst7s]

I have to admit that now after some break-in, my CX-1 now sounds so good with my B-2x that I'm curious about how it sounds with its intended mate, the MX-1 (but I am re$i$ting). The CX-1 really does give the C-2x a run for its money. Can't wait to hear about your "new" MX-1 once you get it hooked up!

Glad it got to you safely even if it was delayed. I bet it's going to make an excellent pairing with the CX-1!

MX-1 is definitely a keeper...

OK, so I'm 3 albums in now. Workingman's Dead, Animals "Before We Were So Rudely Interrupted" and John Fogerty Centerfield.

I was loving the combo of the CX-1 and M-70 and until today could easily have kept that setup. Now I can say for a fact that going back when I re-cap the MX-1 will be a disappointment.

There is a synergy working with the CX-1+MX-1+S7s. After 35 years of living with the Speakerlab 7s this is the first time that I can say that I have an amp that really does them right. The MX-1 just seems to be doing a much better job of controlling the 7's 4x big fat woofers and still giving wonderful sparkling mids and highs. They are working tighter and deeper than ever before. This is kicking ass.

The MX-1 is a keeper indeed.

Now I have to go back out front and spin another disk.
James

 

[avionic]

I think this maybe the same issue with my MX-1 and MX-2. Have to check it out when I get some free time.

http://liquidaudio.com.au/yamaha-mx-1-power-amplifier-repair/

Under the MX-1

 

[Bratwurst7s]

I placed an order with Mouser today with all of the caps that I need including some variants. And wouldn't you know it, I just realized that I forgot to order 10k trimpots and relays. Facepalm.

Cheers,
James

 

[808_state]

View attachment 1193253Under the MX-1

MX-2000 or 10000?

 

[avionic]

MX-2000

 

[808_state]

Nice!

 

[Bratwurst7s]

The mouser order shipped, roughly 14 hours after the order closed. Should be here by Thursday, Friday at the latest. If Mouser and Digi-Key ever close down I will cry.

Cheers,
James

 

[OMGCat!]

I think we'd all cry if they were to go away. My closet would become an electronics graveyard rather than a staging area for future projects.

 

[Bratwurst7s]

So FedEx didn't deliver on Thursday, or yesterday. Tracking shows that the order is sitting in a warehouse outside of the city. First time that FedEx has let me down.

Cheers,
James

 

[Bratwurst7s]

My parts have all long since arrived and in fact are now installed and the amp is back in place in my system. This is probably going to be my least documented or photographed project yet but I'll try to provide some useful info.

The solder connections throughout the entire amp look pristine, very shiny and nice. So I did not reflow everything.

For the front soft-start board I used a Panasonic FC, 470µF/35v. That's all that I did there.



Cheers,
James

 

[Bratwurst7s]

The 2 heat sinks with attached amp boards are pretty easy to remove. The ground wires are held with a gold plated screw and 2 wire hangers and there are 3 plugs to pull. Then one only has to remove the 4 screws holding the sinks in place and pull the assy out.



Cheers,
James

 

[Bratwurst7s]

So now is when others can start to benefit from my mistakes. At least they weren't big mistakes thank God.

I didn't think through what the SM shows concerning disassembly and started by pulling the V-amp board. I figured that after that I would try to get at the PSU and protection boards by removing the bottom plate. More on that in a bit.

The v-amp board has 4x 100µF/10v Elna Duorex caps, C28, 29, 42, 43. I replaced them with Panasonic FM 25v. Pretty straightforward there.

On the other end of the board are the 3x 1000µF/100v that I forgot to measure when I first opened the amp. The are mounted with 2x vertical (C19 & C39) and one horizontal. I did open it again later and measured the 2 upper caps and found them to be 22mm dia x 40mm long, "Elna For Audio". Here I made another mistake and failed to measure the 3rd one, C18, and assumed that it was the same. Fail. It turned out to be a Nichicon "Great Supply", 22 x 35mm.

I used UCC KYB 1000µF/100v in my M-70 but I only had 2 extras on hand. When I placed my order with mouser they were back ordered (July). I went ahead and ordered some and as a backup ordered some Nichicon PW and Panasonic ECA.

The KYB's are 18mmx x40mm and are rated at 105°c, 3.51A ripple current and 10000 hours. When I finally did get them they measured 1045µF and 1048µF, 0.03ohms ESR.
The PW's are also 18mm x 40mm, 105°c but are only rated at 1.52A ripple and 8000 hours. They all measured around 1040µF, 0.05ohms ESR.
The ECA's are 18mm x 35.5mm, 1.3A ripple and 2000 hours and only 85°c. These only measured 940µF to 950µF, 0.6ohms ESR.

As it turned out Mouser shipped the KYB's separately but in time to use them, which I did. Using the ECA for C18 would have been easier but I didn't like how it measured and it's stats.

In order to use the longer KYB in place of the Nichicon I re-mounted the right side of the board (looking from the rear of the amp) on the inside (ie, towards the front of the amp) of the mounting bracket instead of the original rear side location. This does flex the board a little bit, so someone else might not want to make that choice. I couldn't find any other caps in the same quality level as the PW or KYB in less than 40mm height and didn't like the available choices in shorter caps. Unfortunately I failed to get good photos of the re-mounting. I'll try to remember to go back in at a later date and do that.

The old caps measured 897µF, 892µF for the Elna'S (ESR = 0.07/0.06ohms) and 892µF for the Nichicon (0.03ohms ESR).



Cheers,
James

 

[Bratwurst7s]

So here's the part about not understanding the correct way to disassemble the amp to get at the boards. After re-installing the v-amp board I went to remove the bottom panel(s) and then realized why the SM shows the heatsink/amp-board assy's being removed first, followed by the PSU board.

The psu board, protection board and transformers are mounted on frames that are themselves mounted fwd-aft between the front and rear frames. And the transformers are screwed through the mounting frames into the inner one of the 2 bottom panels. So one can't remove the bottom panels without first removing the transformers and the pcb's.

So back to square one. You unscrew the ground wires and nearby small plug attached to the psu from the protection board, unclip the cable bridge from the psu to the protection board, remove the 4x screws from the corners of the psu board, and then remove the 8x screws holding the filtercap mounting frames tho the lower frames. Then one can raise the board from the rear and tip it up towards the front of the amp. A small 18mm (or 3/4") board placed under the pcb positioned it just right and stable.

Then one removes the v-amp board and lay's it where the right heat sink was, and then you can pull the protection board to work on it.

It's easier than it sounds.



There is a single 2.2µF/50v Elna Duorex on the psu board. I replaced it with a Wima MKS2 63v.

I also added some bypass caps to each the 4 large filter caps, Panasonic ECW polypropylene 250v, 2.2µF, 0.47µF, 0.22µF and 0.1µF.
https://www.mouser.de/Passive-Compo...yzsq8i&Keyword=119837583&FS=True&Ntk=P_MarCom

Cheers,
James

 

[Bratwurst7s]

The main filter caps have tabs that are about 8mm wide, with about 18mm center-to-center, 26mm outside edge-to-outside edge.



Cheers,
James

 

[Bratwurst7s]

The protection board has 2x 10µF/16v caps (C254, 256). There was just enough room to mount Wima MKS2 film caps and still clear the mounting bracket for the v-amp board.
C-253 is a Nichicon 0.22µF/100v. I used a Wima MKS2 100v there.
Lastly C255 is a 220µF/6.3v Elna Duorex, I used a Panasonic FM in 25v.

Unfortunately I seem to have only taken 1 photo of this board. But luckily it's pretty uncomplicated work wise.



Cheers,
James

 

[Bratwurst7s]

The main amp boards each have 3x 47µF/16v Elna Duorex and 2x 100µF50v Elna brown elco's.
They were replaced with Panasonic FM, 47µF/25v and 100µF/50v.

For the 10k trim-pots I used Bourns 3296Y-1-103LF. These are top-adjust and the correct part would be a side adjust. But the only side-adjust in that series (3296P) has the pins oriented backwards of what we need. No problem. There is plenty of room to adjust the Y's after the boards are installed. They just barely clear a nearby resistor, but they do clear.
https://www.mouser.de/ProductDetail/Bourns/3296Y-1-103LF?qs=sGAEpiMZZMvygUB3GLcD7sSg8po3Zpo3JZ5ftUfaJLI=



Cheers,
James

edit: I forgot to mention that I measured the old trim-pots with a true-rms meter after removing them and pre-set the new ones to within 0.1ohms of the old ones.

 

[Bratwurst7s]

So now we come to the most puzzling aspect of this amp, and what I have to consider a failure by Yamaha.

Those amp boards with the output transistors etc have the transistors mounted to a sub-rail heat sink. All well and good, it makes working on the amp very easy in this area. And those rails are screwed to the main heat sink, with the boards being stabilized with 3x screws and stand-offs.

The main heat sinks have 5mm of wall thickness. And there is a 2mm slot in the sub-rails right where the mounting screws penetrate. The mounting screws have 10mm of threaded section, so theoreticly there is 3mm of screw actually attaching the rails. But the last 2 threads of the screws are tapered and in reality there is exactly 1 thread holding these things together!

When I removed the screws from the left sink half of them were loose and the rest roughly hand tight. I could actually see an air gap in some areas where the sinks were supposed to be contacting. The right sink was a little better.

After separating the rails/sinks I could see where the paste had never been properly spread, and there was only a polka-dot pattern that didn't look like it would have been very effective at all.



Cheers,
James

 

[Bratwurst7s]

So to fix this I found some 16mm sheet metal screws. Even with end-thread taper I now have a good 4mm of grip, 4 threads. I thoroughly cleaned the sinks with acetone and applied a full but thin and even layer of thermal paste. The rails are now held down very tightly, I got a small amount of paste squeezed out along the entire length of the rails.



After putting it all back together it was time to set the idle current. After powering up the test points showed 12mV L&R. After warming up they actually dropped rather than rise, down to 11.6mV. Setting them to 14mV was a breeze with the 20-turn pots.



I did not replace the 3 relays yet. I still haven't tracked down replacements. And I obviously didn't replace the main filter caps. I did measure them (in circuit) and got about 32400µF for the 33000µF and 34500 for the 36000µF. So this is something that I also still have to research but they haven't degraded all that badly in 18 years.

Eventually I will replace the speaker binding posts also but the ones that I want are out of stock.

The amp is now being used again and it sounds... ...Delicious.

Cheers,
James

 

[avionic]

Outstanding photography..

 

[Mark B]

Very nice writeup on refurbing your MX-1.

My MX-1 needs some system time.