Yamaha M-70 Thread

Hopefully you should be ready to go now. Sounds like you building a nice little arsenal of test gear.

Working on it. I was given a frequency generator last year for my 60th birthday, and now with the 2nd VOM on hand the only thing that I really want is an oscilloscope. I've started looking anyway. Of course after that I'll learn how to use it. ;)

Cheers,
James
 
I got impatient. I woke up an hour ago and felt good about this. So I got a cup of coffee, hooked up the 2 multimeters, plugged in the amp...and pushed the power switch.

The amp powered right up and came out of protection in about 5 seconds. :banana::banana::banana:

A bit of tweaking and I was able to get the photo coupler and -B power supply adjustments pretty close to their 1.2v and 70v settings.

DSC05188.jpg DSC05189.jpg

So this just makes my day. Time for breakfast.

Oh. I realized that I made another small mistake, only one of observation this time thank goodness. The bad light bulb is for the power switch, not the protection light. Rather convenient actually.

Cheers,
James
 
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The pre-drive board has now been pulled and worked. No major surgery here. Just re-capping, re-flowing, cleaning and new trim-pots.

The old elcos all measured OK but most of them were bulging on the bottom and had started leaking. The 47µF were starting to corrode where the leads met the bottom seal.

For C909, 910 (47µF/6.3v) I used Panasonic FC 47µF/16v.
C927, 928, 929, 930 (22µF/100v) got FC's also, 100v.

I've read often enough of old mylar caps giving problems that out of an abundance of caution I replaced C921~924 (0.047µF/50v) with Wima MKS2 63v.

The old trim-pots were removed and then the board got cleaned and dried before installing the new ones. Inspected with magnifying glass for solder bridges and that's it for this board. No other parts were replaced.

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Cheers,
James
 
And the main amp pcb has been recapped now.

Again, all of the old caps checked good and in this case none of them were leaking. Again, I also replaced the mylar film caps out of caution.

The 22µF/100v were replaced with Panasonic FC 100v.
The 100µF/6.3v were replaced with Panasonic FM 25v.
The 10µF/25v were replaced with Wima MKS2 50v film.
The 4.7µF/50v were replaced with MKS2 63v.
The 0.047µF/100v mylar were reolaced with MKS2 250v (overkill voltage but that's what I had on hand).
The 0.01µF/100v mylar were replaced with MKS2 400v (again, that's what I had on hand).

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Cheers,
James
 
Next to do.

Replace the 2x MV12 diodes with 2x 1N4148 (2 each, in series).

There are 2 sets of 39k ohm 1/4w resistors that have gotten hot enough to bake the pcb dark brown. I'm going to replace them with Vishay metal film CFM and raise them up off of the board.

There are also a couple of 56 ohm fusible resistors that are obviously not original (light tan body color instead of the light green of all of the other fusibles and the leads are bent differently than the originals). Now when we heard that the 1/4w fusible resistors were going EOL I bought everything that I could from Mouser and Digikey as fast as I could but I was late to the party and missed the last of the 56 ohm. So I'm going to have to use Vishay CFM series for these. They are flame proof but not fusible.

After that I still have to replace the trim-pots and re-flow the rest of the board.

I'm coming up hard on having to make a final decision concerning buying speaker posts and buying resistors to make a pair of test loads.

For the speaker posts I'll decide within this week, I have some likely candidates from HiFi Collective. ;) There will probably actually be rhodium involved.

For the load resistors I'm thinking a pair of 8ohm 250w. Darn things will set me back almost €40 each with VAT but I don't see any way around this. I'll have to set the ZDR and meter pots under load unless I'm seeing this incorrectly, and that means buying some hefty resistors . At least I already have a large finned heat sink to mount them on. And I'll look around and find a pair of fans to mount over them as well.
https://www.mouser.de/ProductDetail...bXrIkmrvidNw3CpujCJIJtOnk5nyAaHRAtMXS2D43ow==

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Cheers,
James
 
For the resistors I'd just buy the eBay equivalent and use those. You can get 100w resistors in enough flavors to wire a few together to get the required ohm/watt rating for a lot less than $40.
I've used my 100w rated resistors up to well over 150w for short periods of time and have had no problem. For cooling I just had them sitting in a puddle of distilled water in the bottom of a square baking dish (yes, not the correct way to do it but they survived). Just don't touch the resistors to anything you don't want melted/boiled because even at 10-20w those little things get hot.
 
For the resistors I'd just buy the eBay equivalent and use those. You can get 100w resistors in enough flavors to wire a few together to get the required ohm/watt rating for a lot less than $40.
I've used my 100w rated resistors up to well over 150w for short periods of time and have had no problem. For cooling I just had them sitting in a puddle of distilled water in the bottom of a square baking dish (yes, not the correct way to do it but they survived). Just don't touch the resistors to anything you don't want melted/boiled because even at 10-20w those little things get hot.

Well, back at work today after 2 weeks off. And I remembered that a while back my foreman offered me some 12 ohm 100w resistors that he was planning to toss in the recycle bin. Brand new. I didn't take him up on that at the time because my tiny little pea sized brain didn't know what to do with 12 ohm resistors when I needed 8 ohm. Sigh.

So today I tried to get my brain to do something other than act as a counterweight for my ass and looked around and found the resistors. Finally figured out that if I put 3 of these things in parallel I'll get 4 ohms. Then take another set of 3 and wire them in series to the first set and oh(!), I get my 8 ohms. And 600w of load? That should be enough for anything that I do in the rest of my lifetime.

And I have a massive heat sink to mount them on. I'm going to look around and see if I can find a fan or 2 to mount on this also. The only problem is that I only have one heat sink and enough resistors to make 1 load array, so I can only test/adjust 1 channel at a time. Will that be a problem? I wouldn't think so but better to ask before...

Photos later in the week.

Cheers,
James
 
I got the load bank built today. The heat sink had a fair amount of holes in it so I had to stagger the resistor positions somewhat but they fit without having to be mounted over any holes. I used 2.5mm solid core to wire them together and a ceramic terminal block for connections. No fan(s) yet but it's a pretty massive piece of heat sink, easily 10 times the weight of one of the amp's sinks. A quick check showed right at 8 ohms. I won't have it home until Friday so, photos on Friday.

And the variac arrived today. Looks pretty decent for the price. Photos, see above. :)

Cheers,
James
 
Looks like you are all ready to test/adjust. Glad you got out of the resistor thing cheaply and that variac is a nice looking piece of gear.

so I can only test/adjust 1 channel at a time. Will that be a problem? I wouldn't think so but better to ask before...

I don't see a problem with running one channel at a time on an SS amp (apparently not a good idea to run a tube amp unloaded) , to be sure maybe just run the 1khz test signal into the one channel being tested not the other.
 
The bench 30v power supply arrived. It's certainly not high end but should suffice for my needs.

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Cheers,
James
 
And I ordered RCA sockets and speaker binding posts from HiFi Collective. I've been reading about rhodium and it looks like there are some advantages to using it. Hi conductivity and high hardness (resistance to abrasion) as well as high corrosion resistance (noble metal).

For RCA's I ordered these...
https://www.hificollective.co.uk/phono-sockets/cmc-805-2.5-rhodium-plated-rca-sockets.html

For the binding posts I ordered these...
https://www.hificollective.co.uk/binding_posts/spk-gld-gold-plated-speaker-terminals.html

I wanted these silver/rhodium/platinum plated ones but they are out of stock...
https://www.hificollective.co.uk/binding_posts/spk-sil-silver-plated-speaker-terminals.html
If the get some in stock before I get far enough to be installing the posts I might still order some and save the gold plated ones for some other project.

Cheers,
James
 
Looks like you are all ready to test/adjust. Glad you got out of the resistor thing cheaply and that variac is a nice looking piece of gear.

I don't see a problem with running one channel at a time on an SS amp (apparently not a good idea to run a tube amp unloaded) , to be sure maybe just run the 1khz test signal into the one channel being tested not the other.

Good to know, thanks.

I was looking over the manual today and have come up with another question. Adjusting the ZDR pots it says to adjust to the lowest distortion. So I'm getting the feeling that I would need a distortion analyzer for that? (Which I don't see a way that I can afford?) Or can that be done with an oscilloscope? All of this is something that I've never done yet so I still have a lot to learn.

Cheers,
James
 
I used some M5 x 80mm carriage bolts to make some feet for the thing. There were plenty of holes already there so I had lots of positions to choose from.
Standing it up next to the amp gives a bit of perspective of it's size.

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Cheers,
James
 
I was looking over the manual today and have come up with another question. Adjusting the ZDR pots it says to adjust to the lowest distortion. So I'm getting the feeling that I would need a distortion analyzer for that? (Which I don't see a way that I can afford?) Or can that be done with an oscilloscope? All of this is something that I've never done yet so I still have a lot to learn.

Yes you are correct, you would normally use a distortion analyzer for this measurement, but l believe you can also use some scopes (although maybe not as accurate). I don't own either so won't be much help in this respect, maybe someone familiar with testing THD will chime in. Here are a couple of links l found on the subject.

http://audiokarma.org/forums/index.php?threads/signal-generator-and-scope.794416/#post-10988165

http://www.diyaudio.com/forums/equi...-oscilloscope-audio-noise-distortion-etc.html
 
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