RC-M70 12VDC jack

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Fatdog

Fatdog

Well-Known Member
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I have had a couple of these for quite a while. They work like a charm for me.

https://www.walmart.com/ip/n-a-Univesal-AC-DC-Adaptor-1000Ma-Max-DV-1000-dv1000/28445270?athcpid=28445270&athpgid=athenaItemPage&athcgid=null&athznid=PWVUB&athieid=v0&athstid=CS020&athguid=4ec3d671-1b0-169f9677707614&athena=true


663f6def-e33c-4259-9b52-a3ed307f6e2d_1.3591e51ed523cb71374b7127cdd7df73.jpeg
 
caution

caution

Member (SA)
You guessed right, Dan. I checked some different adapters on my M70. The inner diameter needs to be 3mm. Anything less will not fit over the pin. The outer diameter of my plug is 6.25mm, but 6.5mm will work just fine. 5 amps is plenty but you could get away with less.

It uses 15V with batteries, and the jack is marked 15VDC, so get that if you can. 12V isn't at good but will work. The owner's manual calls that input an "External DC input jack (DC 12 V)" so I'm not sure they would suggest 12V if it was damaging.
 
Transistorized

Transistorized

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The optimal voltage is 15V (1.5V X 10D batteries). A brand new D Alkaline will read 1.6x Volts while an exhausted D battery will be around 1.1V. (1.1 X 10 = 11 Volts). When you factor that some of us are powering these devices with rechargeable batteries that are only 1.2V X 10 you get 12 Volts...so yes. 12V will power the box. It will show that the batteries are going down on the meter....but it will work.

*Edit* I am not going to modify this section below. It is wrong. Read it and laugh but please do not use it :lol: Caution has the correct formula below for determining mA and Amp draw for DC. You are NOT to use line voltage when determining DC amperage. I hate giving wrong advice but am willing to leave and admit my mistakes. With that here is my blunder in gray :lol:

The next thing to consider is the max amp draw the box will consume. The above adapter is rated at 1000mA (or 1.0A). Since the box is rated at 40Watts using a 120V home outlet, you divided the wattage by the outlet voltage (not battery voltage) and you get 0.333 amps which is roughly 330mA so 1000mA is fine on amperage. You can go as high as you want with mA as the circuit will only pull the amount it needs but I would not go below 400mA.

Also make sure you get the center tap polarity correct. Some devices are not protected with a diode to prevent reverse polarity damage.
 
dan

dan

Member (SA)
are you sure? that particular adapter says its DC output current is 1 amp so its output would be 12v x 1a = 12 watts. its total input power is stated as 18 watts. my m70 is rated for 47 watts so i think you need at least a 12v 4a dc supply. to not stress the power supply, i would go as high as 5 or 6 amps. of course to use 47 watts, it would have to be on full blast with all the bells and whistles going. you might get away with a 1 or 2 amp adapter if you keep the volume bellow a certain level.
 
caution

caution

Member (SA)
The current draw on the AC side is inconsequential, you care about the DC current, so you have to start with 12V. 40 watts at 12V is 3.3 amps. The actual current draw under normal use is much less than that, but you'll have peaks of higher use.
Another way to determine wall wart wattage is that you only need to be as good or better than using batteries. The maximum sustained safe discharge current for D cells is about 2 maybe 3 amps, but the box wasn't designed to demand that much.
 
dan

dan

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the 47 watts must include the losses in the transformer mounted inside the M70 while powered on AC.
 
Transistorized

Transistorized

Member (SA)
I learned something again today :-) So when dealing with a converter you use the wattage of the device 40 (or in Dans case 47) and divide that by the output voltage of the adapter 12V to get 3.3 Amps. So he would need 3300mA or higher to be on the safe side.

Chalk another one up for future reference

Thanks Eric.
 
hopey

hopey

Member (SA)
You must follow the DC voltage on the Boombox. You can use higher voltage than labeled but never lower as this will damage your power capacitors as they will keep trying to raise the voltage and overheat. DC circuits are designed at twice the voltage.
 
caution

caution

Member (SA)
Sure you can. If that was true, then you'd damage your box when the batteries get low.
Never use a voltage higher than what the product specifies.
 
Superduper

Superduper

Moderator
Staff member
caution said:
The owner's manual calls that input an "External DC input jack (DC 12 V)" so I'm not sure they would suggest 12V if it was damaging.
Click to expand...
The RC-M70WH owner's manual says that jack is for a car adapter, and suitable for 12V to 16V input. Circuit wise, the M70 has an array of internal voltage regulators that ensures pretty much every internal circuit runs at it's designed voltage except for the output amplifier circuit, which is unregulated and runs at input voltage. So if you are running at 12V-16V, you will be fine, on ACTUAL DC. The reason I say actual is that a lot of these wall power supplies are unregulated and may or may not be outputting the voltage you think. In fact, I would even go so far as to suggest you test the output voltage of a new and unfamiliar power supply. It might be well over 20V. The reason is that most of these supplies were designed for a particular application/draw. So if it is rated for 12V 1800mA draw, the voltage might be 16V at no load, and which drops to 12V when draw is 1800mA. As long as the voltage with boombox at idle is below, around 18V or so, you are probably safe.

As for current, 1A doesn't sound like much and the boombox amplifier certainly can't perform at it's maximum output specifications with that small current, but it's probably fine for most light listening use. A larger (current) supply would be best for any kind of volume though, and NOT because low voltage is going to hurt any capacitors either. Speaking of which, like Caution says, you can go lower voltage, but don't go higher than what product specifies.
 
dan

dan

Member (SA)
Ended up modifying a nintendo Wii power adapter. That adapter I got from digikey was awkward to use.

20190516_203125.jpg
 
Transistorized

Transistorized

Member (SA)
The way I understand DC power demands is this.

Voltage is more critical and should generally be within a 3% tolerance of suggested voltage. They realize that there is a +/- percent of margin here because a properly working car DC system can swing from 12.6 all the way to 14.4. Even though the jack says 12V, 10 D batteries at 1.5V is 15V so we know this is still a safe range.

As superduper touched on, they probably stamped 12V on case by the jack because they knew that there was a high possibility someone would use an unregulated PS, that when idle, could output voltage out of the safety range. Had they stated 15 or 16V and someone used an unregulated PS rated for 15 or 16V, it could push the voltage over 16V and damage could occur.

Again, I personally keep the listed voltage within a 3% tolerance. The mAh/amperage output of the PS should meet the power consumption of unit using cautions DC math equation but can exceed it without any issues. Supplying too little mAh/amperage overloads PS circuitry and will cause poor radio performance and an overheating PS at high demand but you could have a regulated PS with a DC output of 12-15/16Volts capable of supplying 10,000mAh (10A) and they'd both be happy. The box would merely sip the 3300 measly milliamps at most...and the PS will run cool because it is only providing a fraction of its potential mAh output level.

Of course all this could be made easier (when the option is available) by plugging it in. All you'd need to focus on then would be AC voltage and leave the rest up to the engineers at JVC :-)
 
nikonfoo

nikonfoo

Requiem Æternam
Interesting
The only reason for using DC is that you are running it in a place where the voltage doesn't work with this M70 or any other box
for example a Japanese domestic 100v in a higher voltage country
but didn't see :cool: any talk of this
other wise just plug it in to AC
 
jerrymilton

jerrymilton

Member (SA)
Superduper said:
I got a question.... why bother with all of that, and not just plug it into AC?
Click to expand...
I came across this thread becuz I own a Victor M70GX 100V box and live in the US, so there is my answer.
 
hopey

hopey

Member (SA)
caution said:
Sure you can. If that was true, then you'd damage your box when the batteries get low.
Never use a voltage higher than what the product specifies.
Click to expand...
I was refering to the capacitors in the power pack. Also when Running on batteries they do get damaged by going flat. Batteries are a form of capacitor.
 
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