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Old 20th October 2007, 12:22 PM   #1
TroelsM is offline TroelsM  Denmark
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Default Yamaha AX700 Amplifier

Hi all.

I found an AX700 on the street.

It doesn't play. (Doh, - it might have been on the street for a reason).

Well the lid and bottom came off easy and the internals seems to be pretty high quality. Elna "audio-caps" and so on.

On the back there's a selector between 220V and 240V. Here in Denmark we have 230~ and the amp was in the 220V setting. Shouldn't be a problem.

- but apparently it is... The voltage on the main caps was +/-74V.. on 71V caps.. that seems a bit high, right? I changed to the 240V-setting. Voltage are now +/-64V.. OK, but not a very big margin...

Stranger even.. the output-transistors are 2SA1265 and SC3182.. these a rated at 140V..

I know that commercial stuff are always pushed to the limit, but seems to be a semi-high-class amp and still all the major parts are stressed to 90%-110%...

One output is at rail-voltage (go figure) and to of the output devices shows zero ohms CE. The others measure OK (I guess).

Opinions? Does anyone have a schematic and/or a service manual?

Is this worth fixing?

Regards TroelsM
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Old 20th October 2007, 12:26 PM   #2
TroelsM is offline TroelsM  Denmark
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Birds eye view.

A lot less chaos than one might expect from a jap´ amp.

Regards
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Old 20th October 2007, 06:27 PM   #3
Bigred is offline Bigred  Canada
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71 & 74v was definitely too high. I believe the filter caps are only rated for 71v. If you have the rails at 64v now this sounds about right. Considering you found this on the curb I think its definitely worth fixing. The outputs are readily available and are cheap.
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Old 20th October 2007, 07:00 PM   #4
anatech is offline anatech  Canada
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Hi TroelsM,
I agree with Bigred. Worth fixing for sure.

For outputs, use the On Semi MJW series. They will probably work out much better than the originals. The ampis not a problem model, so you should get many years of service from it.

-Chris
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Old 21st October 2007, 01:58 PM   #5
TroelsM is offline TroelsM  Denmark
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Hi

Thanks for the answers.

I'll give it a go something. If it's an easy fix it sounds like it's worth it.

Anatech: Could you be a bit more specific about the MJW-devices?

Assuming that the outputs are toast due to excessive rail-voltage, how likely is it that the rest of the amp is OK?

I'll have to measure, but I would appreciate all the pointers you can give me.

Regards TroelsM
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Old 22nd October 2007, 10:46 AM   #6
jaycee is online now jaycee  United Kingdom
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A small amount of overvoltage shouldn't have done much damage. You will want to look at capacitors connected to the main supply.. and see if there is a regulated voltage (such as 15v rails for any opamps).

Check all of the semi's in the power amp part. Those transistors look a bit under rated for 64v rails IMO.

I am guessing Anatech means MJW0281A and MJW0302A from Onsemi. They do look like quite nice parts. Only one question Anatech - what about the higher Cob on these parts? (nearly double the Toshibas)
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Old 25th October 2007, 04:43 AM   #7
anatech is offline anatech  Canada
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Hi jaycee,
The higher capacitance does not generally cause any trouble. It's a low impedance circuit and the drivers have more than enough current to deal with this. I can't even remember what the figures are.

If the MJW0302A and MJW0281A can be had, get them (discontinued). On Semi has other parts in that package that should work well. The driver transistors and other parts should be looked at too. Yamaha has a reputation for blowing up all the way to the diff pair. They are probably damaged even if they still work. Reverse breakdown is very hard on signal transistors. Throws off the beta and noise specs big time.

-Chris
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Old 25th October 2007, 06:35 AM   #8
es44 is offline es44  Denmark
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Hi TroelsM. Do you have an email addresss capable of holding about 3.5 Mb ?
In that case, i have the schematics, pcb-layout, partslist and so on.

Have a nice day.
Ebbe
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Old 25th October 2007, 08:02 AM   #9
TroelsM is offline TroelsM  Denmark
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Hej Ebbe.

I guess my G-mail could hold 3.5M?

I'll send the address in a PM.

Last night I got to work on the amp for an hour and got one channel playing.

One "funny" thing: Bass-control is dead. There seems to be loose connection somewhere and adjustment is almost impossible. It looks like a special pot. Could maybe be replaced with a normal pot. An org. spare-part would cost en arm and a leg?

Looking forward to seeing the schematics.

Regards TroelsM
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Old 14th July 2008, 03:33 AM   #10
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Hello!

A friend of meine gave me his non-working AX-700. The problem is, that there is only very low volume on the speakers though the volume control is turned to max. You can also hear scratching noise when turning the volume regulator.

I guess either the volume control potentiometer broken or there are issues with the transistors in the amplification paths.

Can someone please mail me the schematics and layouts and the part list?

you can contact me best via personal message or ICQ (137-134-880).

thanx a lot!

best regards from germany!
andy
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