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Old 24th May 2012, 07:02 PM   #1
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Default 10.2VAC for 9VAC units?

I have a 10.2VAC 900ma power supply from a modem no longer in use. Could it be used for some Alesis 9VAC 850ma modules? Thanks
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Old 25th May 2012, 01:08 PM   #2
AndrewT is offline AndrewT  Scotland
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Measure the no load voltage of the PSU.
It is likely to be between 11Vac and 15Vac.

I expect if you have >12Vac (open circuit) then a regulated 9Vdc should be very possible.

But, I misread you Alesis spec. Is that 9Vac or 9Vdc?
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Old 25th May 2012, 01:15 PM   #3
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Default AC adaptor compatibility

I have some sound modules and processors which take 9VAC which are usually rated `1 amp or 850ma. A no longer used modem PSU was rated 10.2VAC 900ma output so I thought it might be fine as a backup for my five Kurzweil MicroPianos and many of my Alesis drum modules. I wanted to know if the extra voltage would damage the units. I have been told that the amperage draw is important and not to under power in amps. Thanks, John Poole wyncoteacademy.org faculty
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Old 25th May 2012, 01:21 PM   #4
Pano is offline Pano  United States
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Ditto. Measure the stock power supply with no load to see how close it is to its nominal 9 VAC. Then measure the modem supply to see where it is. They may not be far apart.
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Old 25th May 2012, 01:29 PM   #5
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Default AC adaptor compatibility

Will do. I was always baffled why the readout using a $20 meter read way higher on most of my PSUs. I'm careful to match the DC voltage requested on my equipment but notice that some 9VDC units read almost 12. So I'm curious to see what the readout will be on the AC to AC PSUs. How much variance is safe? John Poole
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Old 25th May 2012, 01:41 PM   #6
Pano is offline Pano  United States
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Hard to say, but 1.2V is probably safe. Small power supplies usually read rather high with no load, under load is a different story - they can drop quite a bit. That would be harder to measure, of course.
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Old 25th May 2012, 05:26 PM   #7
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I replaced a dead Kurzweil 9V supply with a 12V in an emergency. The owner kept it and has been using it since.

The actual issue is not really voltage. The AC power line can vary from 110 to 132 volts. That means the 9 VAC can vary from 8.25 to 10.13 volts. So your 10.2 VAC could go as high as 11.22 VAC or 14.66 VDC after rectification. Now there are a few semiconductors that could actually fail at that voltage, but it is doubtful that any mass sold MI product would use them.

The problem is that if there is an internal regulator that the higher the voltage the greater it has to drop it, thus more heat build up.

So if the internal voltage is regulated to 9 VDC at 600 ma. (Which is about all you can get from 9VAC 850 ma.) With the 9V supply the design heat dissipation is 1.5W With the other supply the worst case is 2.5W. Now if there is internal voltage regulation, it most likely has an overheat shutdown circuit.

So your supply should work. If the Alesis parts get hot then stop using it. If they shut down and restart later don't use it.

If you can see the regulators on the cards are 78xx or 79xx they will be fine.

Of course you can check you AC line voltage. If it is 117 or below don't even worry about it.
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Old 26th May 2012, 01:31 PM   #8
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Default AC to AC PSUs

I have only a layman's understanding of electricity and circuits.
I used a cheapo meter and all my Alesis PSUs read exactly 9VAC. The disused modem supply rated at 10.2 came out at 10VAC. I assume professional meters can pick up digitally an exact readout. I always scout out curbside throwaways. I got lucky and found a 14VDC supply which I can use for a Boss drum machine. Thanks for all of your input. John Poole
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Old 26th May 2012, 01:45 PM   #9
AndrewT is offline AndrewT  Scotland
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Quote:
Originally Posted by dinnerpianist View Post
...........I used a cheapo meter and all my Alesis PSUs read exactly 9VAC. ..........
This is suspicious.
A transformer output voltage depends on both the mains input voltage and the load. If the load is open circuit, then I would expect the open circuit voltage to vary markedly throughout the day and week.
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Old 26th May 2012, 02:25 PM   #10
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Really cheap meters load the circuit and read .6 volts low!
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