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Old 5th June 2011, 05:41 AM   #11
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Default Pictures of the PSU

I googled the PSU board and found images, AND schematics. Unfortunately, I still don't see any info that I can make much sense of.

I did try to measure the load using one of the motors from the VCR. I can make it spin by connecting the 12v line. But when I put the meter in series on the supply line, the motor doesn't spin. Does that mean the PSU can't supply enough current to get through the meter and still power the motor?
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Old 5th June 2011, 06:35 AM   #12
Mooly is online now Mooly  United Kingdom
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First, if the motor spins when connected to the 12 volt supply and doesn't spin when the meter is in series with it then the meter has a problem... fuse, damaged in some way, current sensing resistor burnt out, or even the current is above what it can measure and the volt drop caused by the meter is too high reducing the voltage available to the motor and yet the meter has survived.

The PSU. First safety. The primary side of this is always at mains potential... that's anything in that dotted area.

Pin 3 of CN102 is the 12 volt supply. This voltage should only appear when pin 6 of CN102 goes "high". That would have been from the logic/system control of the player. The input to this regulator is the AL15 supply (all time 15v that is not switched).

The AL5-8 rail is the one used for feedback and regulation (this is the line thats monitored). For the supply to run correctly you may need to load this rail whether used or not.

I feel this is not a suitable PSU to use for a DIY amp for several reasons... not least safety.
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Old 5th June 2011, 11:55 AM   #13
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Default ...stupid question

are you using the correct terminals on the multimeter?

For ranges above 400mA or so, there is always a third terminal to be used for current measurements. Usually you have to move the red lead to this third terminal.

Otherwise, as suggested above, you have burnt the meter fuse and/or shunt resistor. Cheap meters often do not have a fuse on the high current range...

The easier and safer option, also mentioned above, is to simply measure the voltage across a known load and then calculated the current.
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Old 5th June 2011, 12:00 PM   #14
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Quote:
Originally Posted by Mooly View Post
I feel this is not a suitable PSU to use for a DIY amp for several reasons... not least safety.
Looking at the pictures and schematics....this is good advice....
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Old 6th June 2011, 05:49 AM   #15
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Default Success! What's the safety concern?

After Mooly telling me it was a bad idea, I was ready to throw in the towel. I thought I'd have a friend measure the VCR PSU's amperage and help me test my meter. Turns out he already OWNs the Sure 25w amp board I was planning on using in this project.

We hooked up HIS external power supply to his amplifier, and measured the amperage. The amplifier never drew more than 120 mAmps at 12v, even with the volumn up plenty loud. We were driving 2 8 ohm speakers.

We then hooked up the Sure amplifier to the VCR power supply on the 12v rail. We turned up the volume (on the iPod supplying the line in), and the VCR PSU would trip its internal protection circuit and shut down once the amplfier was drawing about 30 mAmps.

We switched to the 15v rail in the VCR power supply and all was well. We could turn up the volumn on the iPod (and hence the amplifier) to louder than we'd ever listen to it. No problems.

My problems measuring current were because I'd blown the fuse in the meter.

So now I have empirical proof the VCR PSU can drive the amp. Can you help me understand your safety concerns? One reason I wanted to use the VCR is that the power supply is safely encased in the original metal enclosure. I expect to locate the amplifier board in the VCR case where the tape mechanism used to reside.

I'd like to understand more about the VCR circuit board. It would be cool to pull power off a circuit controlled by the original power switch. But I suspect this is beyond my skills.
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Old 6th June 2011, 07:18 AM   #16
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Quote:
Originally Posted by Randallm View Post
After Mooly telling me it was a bad idea, I was ready to throw in the towel. I thought I'd have a friend measure the VCR PSU's amperage and help me test my meter. Turns out he already OWNs the Sure 25w amp board I was planning on using in this project.

We hooked up HIS external power supply to his amplifier, and measured the amperage. The amplifier never drew more than 120 mAmps at 12v, even with the volumn up plenty loud. We were driving 2 8 ohm speakers.
The Sure 25 watt amp.

Basic electronic theory... 12 volts DC at 120milliamps is W=I*V which is 12*0.12 = 1.44 watts drawn from the PSU.

25 watts requires just over 2 amps at 12 volts. An amp that were 100% efficient would need that.

So either something has gone astray with your measurements or you really are measuring 120 ma and are happy that the sound level that produces is loud enough. If so it is nowhere near what 25 watts would be. It is surprising how little power is needed with efficient speakers but that is a whole different topic...
for interest,

So how much power do you really need for domestic listening ?

Quote:
Originally Posted by Randallm View Post
We then hooked up the Sure amplifier to the VCR power supply on the 12v rail. We turned up the volume (on the iPod supplying the line in), and the VCR PSU would trip its internal protection circuit and shut down once the amplfier was drawing about 30 mAmps.
If you look at the circuit then the 12 volts (pin 3 of CN102) is a "switched" 12 volt line and unless you apply a logic "high" to CN102 pin 6 this 12 volt line is "off".

Quote:
Originally Posted by Randallm View Post
We switched to the 15v rail in the VCR power supply and all was well. We could turn up the volumn on the iPod (and hence the amplifier) to louder than we'd ever listen to it. No problems.
The 15 volt rail is on CN102 pin 9. This is unswitched and so that is why the amp worked.

Quote:
Originally Posted by Randallm View Post

So now I have empirical proof the VCR PSU can drive the amp. Can you help me understand your safety concerns? One reason I wanted to use the VCR is that the power supply is safely encased in the original metal enclosure. I expect to locate the amplifier board in the VCR case where the tape mechanism used to reside.
Its great that you are learning and figuring things out for yourself and working empirically is great sometimes too but, you have to remember that you are working with unknowns. If you handed me a 12 volt bulb and I didn't know what it was I could say its fine on 15 volts... I've had it running for an hour so it must be OK. A 1 amp fuse will pass 1.25 amps for quite some time before it blows... but you wouldn't design using that basis.

Safety... well tbh even discussing this type of PSU is actually against all forum rules... they are not suitable for the novice to work on. The primary side is live at all times. Touch any component on that side and anything remotely grounded and you will get a shock. When running there can be significant electrical "leakage" via the inductance of the chopper transformer windings. You can see this in action on many TV's VCR's DVD's etc that use this type of supply. Plug in switch on (no leads connected) and you can often draw an arc from the aerial socket to the aerial plug. Hold one in each hand amd you'll know about it. There's not much current there... but it hurts

Quote:
Originally Posted by Randallm View Post
I'd like to understand more about the VCR circuit board. It would be cool to pull power off a circuit controlled by the original power switch. But I suspect this is beyond my skills.
Not sure I understand that... the power switch (standby) will be microprocessor controlled and is used to turn off that switched 12 volt line. The PSU is still running at all times feeding the microprocessor.


So I say again that this particular PSU is not suitable for a DIY amp.
Keep learning and keep asking though
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