Battery backup for a VCR......

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Janitor Tzap
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Battery backup for a VCR......

Post by Janitor Tzap »

Has anyone here made a Battery backup for a VCR?

Why do I want to put one on a VCR?

I'm currently transfering VHS tapes to the Hard Drive vie a Video Capture Card.
The VCR is plugged in the same power strip as the computer.
Thus, when the power strip is turned off, the VCR looses it's AC as well as its settings.
Yes, I could run the VCR to another outlet.
It's just a pain to do that.


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philba
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Post by philba »

It's less of a pain to open up the VCR and figure out how to hook in a battery?
Or buy a cheap UPS and use that?

I'd think finding another outlet (or daisy chaining 2 power strips) would be a lot easier.
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Post by Janitor Tzap »

philba wrote:It's less of a pain to open up the VCR and figure out how to hook in a battery?
Or buy a cheap UPS and use that?

I'd think finding another outlet (or daisy chaining 2 power strips) would be a lot easier.
Well,
I've been wanting to do this on a few VCR's that I have in the house.
When we get a brown out or loose power for about a minute.
I then have to go around to each one and re-do the settings.:(


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haklesup
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Post by haklesup »

In a VCR we're talking mainly about the clock settings as there's not much else. So the search for the right chip to keep powered ends with the clock chip. (Oh, that assumption may be incorrect)

Assuming you figure out what chip is the clock you should then try to find a datasheet. If the clock has a display inhibit pin (blanks the display) you are in good shape. All you need to do now is devise a way to power only the clock chip (thus isolating it from the power supply so the other chips don't also power up) and assert the inhibit display pin at the same time. If you can't shut down the display, any small battery would last only a short time (seconds to minutes) same if you cant isolate from the other chips.

On the other hand, you might get lucky and find an unused battery backup pin on the clock chip and you can just put a battery on it. I wouldn't attempt this without a schematic or hours of reverse engineering the power connections to the chips at least.

Bottom line, if the chips with memory weren't designed for battery BU in the first place then you might as well put in a UPS. Still think an extension cord is a pain?
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jollyrgr
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Post by jollyrgr »

This could prove fun....

Likely there is a 12V and 5V supply (or similar) inside the unit. I'm not sure about what level the 12V supply might be but the 5V logic supply is likely there.

Check the PS board. Follow the line cord in, then to possibly a transformer (linear supply), then to a circuit board. I'd expect a couple of bridge rectifier circuits and possibly discrete diodes making up a bridge circuit. There may be two or more output voltages. In newer VCRs the supply will be a switching supply and you won't see a bulky transformer. These supplies you need to be VERY careful with as the "primary" DC voltage is taken right off the line cord (160 to 320 VDC). Anyhow find the 5V supply. Again, dending on the design, there may be two or more boards. One of these will be the logic board and may be just behind the front faceplate with the clock display. Add a battery across this 5V supply line. Use a forward biased diode in series with the battery or you will be recharging the battery off the power supply when the VCR is receiving power. You may want to add a diode in series with the primary 5V supply of the VCR so that the battery does not "back power" the power supply and drain quickly.

I'm surprised the clock does not recall its settings for at least a few minutes. If the VCR DOES remember it settings for half an hour or more then the cure is even easier. Locate the 0.5F to 1F memory capacitor near the clock chip on logic board. Put a battey (with protection diode) across this capacitor.
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Chris Smith
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Post by Chris Smith »

Companies like "Polaris" used to sell 12 volt models.

Also All electronic, and dozens of other companies used to sell the 12 volt model.

Other that that Hooking into the main PS and over riding the DC signal is easy.
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philba
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Post by philba »

If you're going to do it, I'd look at the electronics section. There may be a clock chip though if the model is "recent" (as VCRs go) it will just part of the microcontroller rather than a separate chip.

I suspect you will have to power the entire controller. If there is any spare space, I'd opt for as large a cap as you can fit in between the Vcc line and gnd. You can get 5V supercaps that aren't very large. I'd use either a schottky diode or (dare I say it?) a PMOSFET in series between the cap and the PS for blocking. If you are able to measure the current draw, you can calculate out how long you a given cap will last you. The key word here is "small".

To really do it "right", you'd want to switch just the power for the micro and associated circuitry. That's going to take a bit more sleuthing (tracing the PCB out). Fortunately, it's probably single-sided so should be easy.

You could, of course, use a battery instead of a cap but then you'd probably want to keep it outside the case so you can change it when needed. Unless you build in a charging circuit, it WILL need replacing.

It does seem like a lot of work to make 80's technology work the way it should have in the first place.

Good luck.
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Post by Janitor Tzap »

Thanks guys for all the suggestions.

Hmm.......
I guess I'm going too have to hunt down the schematics for the VCR's.

I know the one I can get from the local library with the "Sams PhotoFacts".

The other I may need to see my friend who is still into TV/VCR/Radio/Stereo Repair.
And see if he has it.

Other then that.....
It doesn't sound all that tough to do.

Thanks again.


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