Yamaha DSP-AX750se........Dead, No Power - diyAudio
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Old 16th July 2012, 07:55 PM   #1
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Default Yamaha DSP-AX750se........Dead, No Power

Hi Everyone,

Was wondering if someone could help me as I'm either missing something obvious or having a stupid moment.

Have just got a Yamaha DSP-AX750se surround amplifier that I'd like to repair, I bought it faulty thinking it would be something obvious.

Plugged in and switching it on, nothing happens, the unit is completely dead.
Took the lid off, and looked for signs of a component blowing. But all looks fine.

Took some measurements AC in is fine, but there is no AC out to the transformer. Believe the fault to be in the Sub Trans area. Investigated after obtaining the service manual and a few of the voltages are missing.

Have the service manual, if someone would be willing to point me in the right direction. Or am I in the wrong area all together.

Never worked on a 8.1 amplifier before so all help would be amazing

Pictures and service manual on request
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Old 16th July 2012, 10:33 PM   #2
jaycee is offline jaycee  United Kingdom
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You'll probably find there is a main transformer, and a standby supply which is probably 5V and always on. If the standby voltage is missing, nothing will work
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Old 16th July 2012, 11:03 PM   #3
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Time to get out your meter and start measuring voltages!
Start from power supply.
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Old 17th July 2012, 02:37 AM   #4
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Perhaps the former owner had a big house party and ran it way too loud for way too long into multiple speakers and the main transformer has had its thermal fuse blown. You have no idea how many amps I have seen come in on a Monday in this condition. Check for continuity on the primary winding, if it is open the transformer is toast.
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Old 18th July 2012, 08:20 PM   #5
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Sorry for the late reply been busy with work projects.

Measured the primary for continuity all is ok, no voltage is getting to it though.

Had a look at the circuit diagram and did some meter readings.

Two test points are labelled on the diagram for the sub trans area T401, I get the 66v input but no 8v output so I'm guessing the stepdown transformer for the stand-by supply has gone down.

Attached a copy of the sub trans area. All help welcome. Even if its just to say im doing something wrong or in the wrong area
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File Type: jpg Ax750se1.jpg (106.0 KB, 152 views)
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Old 18th July 2012, 08:40 PM   #6
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Quote:
Originally Posted by mark31385 View Post
Sorry for the late reply been busy with work projects.

Measured the primary for continuity all is ok, no voltage is getting to it though.

Had a look at the circuit diagram and did some meter readings.

Two test points are labelled on the diagram for the sub trans area T401, I get the 66v input but no 8v output so I'm guessing the stepdown transformer for the stand-by supply has gone down.

Attached a copy of the sub trans area. All help welcome. Even if its just to say im doing something wrong or in the wrong area
That 66v on the primaries ...Where is it coming from? are you sure it is suppose to be 66v and not 110v?
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Old 18th July 2012, 08:49 PM   #7
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Sorry on the primaries of the standby transformer im getting 56v just re measured it and on the secondries 0v. Would I be better taking an actual picture of the circuit board and highlight where I am metering?
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Old 27th September 2012, 04:18 PM   #8
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Hi,

I have just found this post after experiencing similar problems with my DSP-AX750SE.
Initially the unit failed to power up, but after unplugging for 10 mins it would power up and work fine. Now it fails to power up at all, but a faint flash can be seen if you observe the blue (pure direct) LED closely.

I have checked for continuity of the primary and secondary windings on the main transformer and both are ok.

I have also checked for 240V supply to the primary windings and here is where i'm confusing myself.
If I disconnect the primary winding plug from the board, I have 240V between the two pins (on the board). When I plug the transformer back into the board, I have 0V between these two pins, I have even checked each pin to Earth and still nothing.

I was hoping someone on here may of had a similar problem and can point me in the right direction.

Many thanks,
Tristo
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Old 27th September 2012, 04:48 PM   #9
jitter is offline jitter  Netherlands
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Service manual here, no less than 128 pages, this is one complicated beast (PSU on page 86).

Did you check the mains fuse? Do you measure continuity from power cord plug to the pins the transformer primary connects to (transformer unplugged)?

Last edited by jitter; 27th September 2012 at 04:53 PM.
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Old 27th September 2012, 07:37 PM   #10
jaycee is offline jaycee  United Kingdom
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It looks like the small transformer isn't run straight from the line, but via some sort of pulse width modulation like an SMPS on its primary. Seems total over kill to me where a regular line-powered transformer would have worked!

Hm, having carefully read, it seems you should only have 56V on the primary in Japan, where the voltage is 100V. There should be 158V there in Europe on 220-240V. My guess is that the circuit around IC402 is trying to detect line voltage/frequency and getting it wrong. Too low a primary voltage means too low secondary voltage = nothing works.

Look at page 75 for a diagram of the sub transformer PCB, page 81 top right for the schematic. You'll see a parts matrix below it. Check the parts on that list in the "G" column. My gut says check C405, R412 and C406.

C405 should be a 22n X2-rated capacitor, such as this
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