AX-750 Thread

The phono section on function board C:

C144, 149 (10µF/25v)(signal path): Replaced with Nichicon ES Muse 10µF/35v bi-polar.
C163, 166 (22µF/16v)(signal path): Nichicon ES 22µF/25v.
C158 (100µF/10v): Panasonic FC 100µF/25v.
C164, 165 (330µF/16v): UCC KYB 330µF/25v.
C157, 159 (2200µF/6.3v): Nichicon FG 2200µF/6.3v.

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The Q120 voltage regulator transistor got a heat sink attached. Since the Tuner/Tape Buffer (input) board stands right behind that transistor with very little clearance I had to get a little creative there. The heat sink is a section cut from an old PC northbridge heat sink. I figured that any kind of heat sink had to be better than nothing.

Cheers,
James

edit: I forgot to mention 2 things.
1) The entire board was re-flowed.
2) Throughout the entire amp when a coupling cap was used in the signal path I replaced them with a Nichicon ES Muse bi-polar cap (with a couple of exceptions where I used a KZ Muse). In spite of them being bi-polar I dimly remember reading somewhere that there is an orientation to these caps and it's best to install with the longer lead in the former positive hole/position, so that is what I do. If only I could remember the reasoning behind the practice.
 
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The large board in the 3rd photo of post #18 is the Tuner/Tape (input) Buffer board. It has 8 elcos and a surprise. There are 4 10µF/16v Elna's as input coupling caps (C109, 112, 113, 116), and another 4 of the same as output (of the board) coupling caps (C132, 133, 136, 137), all in the signal path. The surprise is that when I looked the amp and schematic over before ordering caps I didn't notice that the output side caps have a 100 ohm resistor soldered to one leg in series. They aren't shown in the schematic.

C109, 112, 113, 116, 132, 133, 136, 137 (10µF/16v): Nichicon 10µF/25v ES Muse bi-polar.

I re-used the resistors because I didn't have any 100 ohm resistors on hand and they all measured 101 ohms with my parts checker. But if I do another of these amps I'll order some nice Vishay/Dale CMF 1% metal film resistors.

Re-using the resistors meant that I only had short leads to work with and I couldn't install the resistor/cap combo so compactly near the board as they were. So they caps stand somewhat high. As I understand that is not a good practice but I hope that the exposed leads aren't too high.

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Cheers,
James
 
Again, the entire board was re-flowed.

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Cheers,
James

edit: Another note. Recently I read here in the forums that it's good practice not to bend the leads where they come out of the caps more than 15° or so, to unsure that the seal isn't broken where the lead exits the cap body. So I've been bending a step into the leads to widen them for hole positioning rather than just shoving them down into the holes.
 
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Another note about the tape/tuner buffer board. To remove it one must first remove the volume amp card, input selector card, the cd buffer card and might as well pull the pure direct switch/card as well. Then there are 4 screws through the back case panel that hold the card in place by way of the input jack blocks. After that it possible to remove the tape/tuner buffer card. One can tip it up from the front to get it out but while I did that one time I decided that that is a bad practice as it puts side pressure on the small plug on the µCom/Phono board when pulling the board up that way.

I really think that it's best to remove the back panel first, especially as one will have to do that to re-install the card. There is no chance of re-installing the card by pushing the jacks through the back panel and tipping the card down without putting enormous nasty side force on everything. I took one look and pulled the back panel before trying to re-install the card. In fact, if I do another of these amps I'll remove the front and back panels right away and just gut the amp. Then tip it over on it's side and flip the psu and main amp boards out to the side to work on them.

Cheers,
James
 
The volume amp board alone has 10 elcos. Replacing them was very straightforward, no hidden problems here. The 2 ganged volume pots were deoxed and all connections were re-flowed. 4 of the caps are directly in the signal path and were replaced with Nichicon ES Muse b-polar.

C317, 318 (1000µF/16v Elna Duorex II): UCC KYB 1000µF/25v.
C305, 306 (470µF/6.3v): 470µF/10v Panasonic FC.
C311, 312 (33µF/16v): 33µF/35v FC.
C301, 302, 315, 316 (10µF/16v): 10µF/25v Nichicon ES Muse.

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Cheers,
James
 
The CD buffer card has 8 elcos. As with the other cards I re-flowed all connections and used Nichicon ES caps in the signal path.

C103, 106, 129, 130 (10µF/16v): Nichicon ES Muse 10µF/25v bi-polar.
C191, 192 (33µF/16v): Panasonic FC 33µF/35v.
C187, 188 (1000µF16v): Nichicon FG 1000µF/16v.

DSC03882.jpg DSC03886.jpg DSC03889.jpg

Cheers,
James
 
OK. The tone control board. 12 elcos, 4 of them in the signal path.
C401, 402 (100µF/25v): Nichicon FG 100µF/25v.
C409, 410 (0.47µF/50v)(signal path): I still have some Panasonic ECQ film caps and used them but they are NLA. Recommend Wima MKS2 0.47µF/63v.
C415, 416 (10µF/16v)(signal path): Nichicon ES Muse bi-polar 10µF/25v.
C417, 418 (10µF/16v): Nichicon FG 10µF/50v (had a lot of these in 50v, would normally order 25v).
C429, 430 (0.22µF/50v): Wima MKS2 0.22µF/63v.
C431, 432 (1µF/50v): MKS2 1µF/63v.

Potis and switches of course got some deoxit and all connections were re-flowed.

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Cheers,
James
 
Now I'm a little upset with myself because I just realized that I neglected to take any photos of the input selector switch board. The input selector is the Achilles Heel of all of the Yamaha amps with the motorized input switch. Their contacts get dirty and then things start to drop out. With my AX-1090 so far I've had good luck with spraying the switch with deoxit, but there is a better way. Yamaha made replacement switches. Here is another place where I made a mistake and bought the wrong one for this amp, but in the end got lucky and didn't need it. You get to profit from my mistakes.

This amp and the AX-1050, 1070 & 1090 have a 7 position switch. The AX-870 and 890 have 6 position switches. The DSP-A1000 and some other AV amps have 9 position switches. Supposedly there are some amps with 5 position switches but I can only guess that they would/might be amps in the 400 series, I haven't researched that yet, can't confirm. Anyway, I screwed up, didn't do my homework correctly and bought a 9 position switch. Argh.

The switch numbers go like this:
VT148000 = 9 position,
VT148200 = I have read both 5 position and 11 position for this switch. I can't confirm which is accurate!
VT148300 = 6 position,
VT148400 = 7 position.

I have found photos of the boxes of the 300 and 400 switches so I can confirm those P/Ns but haven't been able to find confirmation about the 200.
The VT148000, 200 and 400 are still available from different sources. Here in Germany HTV Service GmbH has them...
http://www.htv-service-gmbh.de/Webshop/product_info.php?products_id=2623

Anyway, when I looked the board over carefully it looked as the switch had been removed and re-soldered. Non-original looking solder connections and small chips in the paint around them. When I pulled the switch I was very happy to see that someone had replaced it with a gold contact one. The color of the gold plating is pretty unmistakable. I just took some pics of the unused VT148000 for you all to at least see how a new one looks.

DSC03954.jpg DSC03955.jpg DSC03956.jpg DSC03957.jpg DSC03958.jpg

One thing about this pc board. Check the 1st photo in post #19. The board has a daughter board soldered to it at a right angle with a slew of jumpers. That daughter board has 3 plugs that have to be inserted into the plugs on the end of the tape/tuner buffer board. One absolutely has to be careful doing this and give that tape/tuner board plenty of support! There's a danger of flexing the board to far and breaking it and/or traces on it.

After I replaced the switch of course all of the other connections were re-flowed.

Cheers,
James
 
Hey James,
I've noticed you using UCC caps in more of your builds lately. Have you noticed their GPD line?
I stumbled across them on mouser recently, and have been tempted to try some in my next build - they have very low ESR for liquid electrolytics (~20-40 mΩ claimed) and high ripple current in small packages.

I've also been considering trying Panasonic ECW for polypropylene film caps since the ECQ are NLA like you said. I've been wondering if they are a better alternative to the MKS2 (PET) where they'll fit - which is definitely not everywhere since they are larger.

Those inline resistors are interesting too! Must have taken some time.
 
I'm sorry Zaibatsu, I had it in my head that I had answered you and yet when I looked, not. Thanks for the links. I hadn't seen them but I'll check them out.

Yes, reusing those resistors and getting them hooked up to the caps with the short leads was a pita. Lucky for me I work in a hospital and have access to the metal recycling bins. Discarded long nose surgical clamps are very useful for such work. :) But I do need to expand my bench stock of spare resistors.

The only other things that were needed to do to finish up the amp were re-flowing the connections on the pure-direct board. After that everything was re-assembled and it was turned on and let sit overnight. Then a re-check of idle current before closing it up. It was tested with a pair of throw away speakers and then with a pair of Canton Karat 200's & Denon DCD-1290 cdp. Sweet sounding combo, although the amp deserves larger speakers. I'll be doing an extended listening test on the coming weekend and will hook it up to my Speakerlabs and will take a few photos.

Cheers,
James
 
Great work, James :thumbsup:

Now I'm a little upset with myself because I just realized that I neglected to take any photos of the input selector switch board. The input selector is the Achilles Heel of all of the Yamaha amps with the motorized input switch. Their contacts get dirty and then things start to drop out. With my AX-1090 so far I've had good luck with spraying the switch with deoxit, but there is a better way. Yamaha made replacement switches. Here is another place where I made a mistake and bought the wrong one for this amp, but in the end got lucky and didn't need it. You get to profit from my mistakes.

This amp and the AX-1050, 1070 & 1090 have a 7 position switch. The AX-870 and 890 have 6 position switches. The DSP-A1000 and some other AV amps have 9 position switches. Supposedly there are some amps with 5 position switches but I can only guess that they would/might be amps in the 400 series, I haven't researched that yet, can't confirm. Anyway, I screwed up, didn't do my homework correctly and bought a 9 position switch. Argh.

The switch numbers go like this:
VT148000 = 9 position,
VT148200 = I have read both 5 position and 11 position for this switch. I can't confirm which is accurate!
VT148300 = 6 position,
VT148400 = 7 position.

I have found photos of the boxes of the 300 and 400 switches so I can confirm those P/Ns but haven't been able to find confirmation about the 200.
The VT148000, 200 and 400 are still available from different sources. Here in Germany HTV Service GmbH has them...
http://www.htv-service-gmbh.de/Webshop/product_info.php?products_id=2623

Anyway, when I looked the board over carefully it looked as the switch had been removed and re-soldered. Non-original looking solder connections and small chips in the paint around them. When I pulled the switch I was very happy to see that someone had replaced it with a gold contact one. The color of the gold plating is pretty unmistakable. I just took some pics of the unused VT148000 for you all to at least see how a new one looks.

View attachment 875138 View attachment 875139 View attachment 875140 View attachment 875141 View attachment 875142

One thing about this pc board. Check the 1st photo in post #19. The board has a daughter board soldered to it at a right angle with a slew of jumpers. That daughter board has 3 plugs that have to be inserted into the plugs on the end of the tape/tuner buffer board. One absolutely has to be careful doing this and give that tape/tuner board plenty of support! There's a danger of flexing the board to far and breaking it and/or traces on it.

After I replaced the switch of course all of the other connections were re-flowed.

Cheers,
James

That switch makes me wonder, it looks quite similar to the input selector switch of my C-4, but then motorized :naughty: I wonder how the pin-outs are related to the C-4's, maybe with some modding :idea:
 
Great work, James :thumbsup:
That switch makes me wonder, it looks quite similar to the input selector switch of my C-4, but then motorized :naughty: I wonder how the pin-outs are related to the C-4's, maybe with some modding :idea:

Thanks!

Well, for my use remote control of source selection is an un-needed luxury. But remote control of the volume is imho an essential luxury.

Cheers,
James
 
Today I needed an amp to test out some cd players so I gave the 750 it's 1st serious listening test while doing so. The speakers were/are freshly e-capped Canton Karat 200's and the cdp(s) were Denon DCD-1290's. I needed to do an A/B test to check out the effects of a mod. Dire Straits 1st in both players.

Nice! Really, a typical Yamaha experience, the amp did it's job super cleanly and stayed out of the way. Of course Canton speakers generally do the same thing, sound great and let the music do the talking. Imho Yamaha and Canton is a match made in heaven. They sound great together, and with it's 110 watts the 750 powers the 70w Karat 200's effortlessly, delivering superb clean sound. The listening area was small and cluttered, I'll have to do this again in a larger room, and will hook it up to my Speakerlabs then also.

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Cheers,
James
 
Looks great James, good to hear it's running well.
I've found it takes a few solid hours of use before the sound tightens up and you can really judge it (time seems to vary depending on component choice), so hopefully it gets even better once there's some miles on it. Just my experience though, I respect the mixed opinions on the topic! :D

Which caps did you choose for the speakers?
 
Looks great James, good to hear it's running well.
I've found it takes a few solid hours of use before the sound tightens up and you can really judge it (time seems to vary depending on component choice), so hopefully it gets even better once there's some miles on it. Just my experience though, I respect the mixed opinions on the topic! :D

I know that there is a school of thought that says that we aren't supposed to believe that, but I agree with you.

Cheers,
James
 
Hey James,

This seems to be the authoritative thread on the AX-750! :p

Got a question you might know the answer to! I have one of the European AX-750 models, and it's clearly labeled 275 watts on the back (which is for 4 ohms), but then it says it can only push 6 ohms in a) the user guide and b) on the back of the unit.

For what was a pretty powerful unit at the time, I find it strange that it may or may not do 4 ohms.

Any insights?

Also - what was the end of this story then?

Robert
 
Hi Robert,

The 275w noted on the back panel is the maximum load that the amp draws at the mains and has nothing to do with output power to the speakers. :)

The manuals sure state a mishmash of figures concerning output power.
20hz - 20khz 0.015% THD 8 ohm ... 110wpc

But note this: DIN Standard Output Power Per Channel (European Model) = 1khz 0.1% THD 4 ohm ...185w. (Peak power, not to be confused with continuous power)

In any case the amp has no problem driving 4 ohm loads. In fact the one in this thread has only driven 4 ohm speakers while I had it and is now owned by a friend and he is using my old Canton Karat 60's (4 ohm) with it. (that's the happy end to the story :))

Cheers,
James
 
Hi Robert,

The 275w noted on the back panel is the maximum load that the amp draws at the mains and has nothing to do with output power to the speakers. :)

The manuals sure state a mishmash of figures concerning output power.
20hz - 20khz 0.015% THD 8 ohm ... 110wpc

But note this: DIN Standard Output Power Per Channel (European Model) = 1khz 0.1% THD 4 ohm ...185w. (Peak power, not to be confused with continuous power)

In any case the amp has no problem driving 4 ohm loads. In fact the one in this thread has only driven 4 ohm speakers while I had it and is now owned by a friend and he is using my old Canton Karat 60's (4 ohm) with it. (that's the happy end to the story :))

Cheers,
James

Thanks for this James. I am now doing some things to the app and finally back to this thread, which is invaluable. Awesome! The Internet rocks!
 
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