Videopac+ G7400 PSU

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qq1975b
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Videopac+ G7400 PSU

Post by qq1975b » Sun Jan 03, 2021 3:56 pm

Hi,

I hope someone with electronics knowledge can help me. I have been replacing all the caps from one of my Videopac G7400. After it was done I have measured the VC output and I have got 4.99V for the 5V channel and 14.75V for the 12V. Is it a big gap on the 12V channel?
I have read another 2 G7400 PSU I have where the caps have not been replaced yet and I have read 6.5V and 11.75V. A big gap in the 5V channel but the console works fine.

So I am really confused...What should I do? Could be something be wrong in all of them?

http://home.kpn.nl/rene_g7400/download/ ... supply.jpg

Admins, please, move this topic where you think it fits better, thank you.



Thank you.


G7400_power-supply.jpg
G7400_power-supply.jpg (124.22 KiB) Viewed 194 times

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exxos
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Re: Videopac+ G7400 PSU

Post by exxos » Sun Jan 03, 2021 4:10 pm

No regulation on the rails that's why.. Are you checking it under load ? You could tweak R168 ? But 5v will likely also drop.
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qq1975b
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Re: Videopac+ G7400 PSU

Post by qq1975b » Sun Jan 03, 2021 4:55 pm

Without load (disconnected from the motherboard) the one with 5V and 14,75V. The other two are attached to the motherboard but not cartridge inserted.

There is a trimmer but as you say, both drop in similar proportion. Just tweaked a little bit and I get 4,93V and 14,67V

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Re: Videopac+ G7400 PSU

Post by exxos » Sun Jan 03, 2021 5:01 pm

The only way which may work is to try increasing R158 ..
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qq1975b
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Re: Videopac+ G7400 PSU

Post by qq1975b » Sun Jan 03, 2021 6:34 pm

This is R158:
7ACC506E-8DAC-4B38-B921-DAC41F1C717F.jpeg
7ACC506E-8DAC-4B38-B921-DAC41F1C717F.jpeg (557.29 KiB) Viewed 176 times
It looks like a 1k 5% resistor. And the R228 looks like a 100E.

So, how do I know for which one I should replace it?

Edit: I tweaked the trimmer a little bit and I got 4.69 and 13.80.

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Re: Videopac+ G7400 PSU

Post by rubber_jonnie » Sun Jan 03, 2021 6:37 pm

I have a supposed 9v PSU for a zx81 that gives about 14V when off load, but is correct when on load. I'd suggest you need to find something to load it with and check what the output is like then before going and making adjustments.
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qq1975b
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Re: Videopac+ G7400 PSU

Post by qq1975b » Sun Jan 03, 2021 6:41 pm

Ok. I will do that.

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Re: Videopac+ G7400 PSU

Post by Maximilian » Sun Jan 03, 2021 7:14 pm

IMHO:
I dont know the Videopac machines but lets take a look.
Testing this without a load is meaningless, but to use your Videopac as a load is a gamble ofcourse.
Also I would not test the PSU without load as I have seen some (bad) PSU's go up in smoke because of that.

Finding a compatile load is going to be an issue, it looks like the 12V is only using 70ma and the 5V could be up to ~1a?
(2 big ass 5V fans and a teensy 12V fan?)

If you are within ~8% of 5V and 12V under load you will probably be ok.
For example:
Measured under load: The pal video encoder that uses 12V can handle a minimum of 10,8v(-10%) and a maximum of 13.2v (+10%)
(the closer to 12V you are the better.)

The other PSU with 6,5V on the 5V rail is probably dangerous and probably needs a recap at the minimum.

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Re: Videopac+ G7400 PSU

Post by Maximilian » Sun Jan 03, 2021 7:21 pm

Did you change the cap right next to the pot with a new one?
Looks a little bulged in the picture.

qq1975b
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Re: Videopac+ G7400 PSU

Post by qq1975b » Sun Jan 03, 2021 7:45 pm

Yes, I have changed it but yes, it seems not good. I will replace it.

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