Famicom AV/Power Board

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HyldenKing
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Famicom AV/Power Board

Post by HyldenKing »

Hello
I'm interested in making a Power/AV PCB for HVC 07 Famicom
I drew these schematics based on the HVC 05-06 diagram and Ben Boldt's AV mod posts

https://imgur.com/U2JDUvb.jpg

I'm a newbie to the electronics and can't say if this will work or not, so I have some questions:
1. Is everything correct?
2. Is it okay to add the power switch before the regulator?
3. Which caps would you change?
4. Is it better to solder a tantalum cap to PIN20-22 or add it to the board?
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krzysiobal
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Re: Famicom AV/Power Board

Post by krzysiobal »

This is the board I have made some time ago:
Image Image

It is both compatible with
* first revision famicom (with a big metal shielded dc/rf pcb, a'ka 5 pin), which has power switch on the 5V rail
* second revision (with a small dc/rf pcb a'ka 7pin), which has power swtch on 9V rail

(just connect JP1/JP2 differently according to the description)

It does not have the gretz's bridge, just a simple schottky diode placed on the DC input to protect against wrong polarisation.
Video amp is as in most of famiclones.

And the most important thing - there is a place to solder either
* mini jack A+V connector (this way you don't have to drill any holes in your shell)
or
* Audio RCA + Video RCA where originally two switches were placed (you need to enlarge those holes a little)
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aquasnake
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Re: Famicom AV/Power Board

Post by aquasnake »

It is recommended to set the power switch on the 5V power rail after 7805 output. And keep the video amplifier part powered forever, which is conducive to prevent the surge at the moment of switching and will not make the video output flash white


For LCD TV, the highlight at the moment of switching will not have any problem, just interference. For CRT TV, the white level of highlight output exceeds the standard, which may lead to the instantaneous high voltage excitation of kinescope filament
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HyldenKing
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Re: Famicom AV/Power Board

Post by HyldenKing »

Thank you for the response.

krzysiobal
1. Why the large cap is at the input and not the output?
2. Can I use the schottky diode bridge instead of the regulars? What's the difference?
3. I can't find the quality brand ceramics for the regulator's input/output, can I change it to the 470nf film caps?

I've made this on EasyEda , should I connect 5v pins on the right side?

aquasnake
That makes sense. I think I'll add the holes for the second switch
Last edited by HyldenKing on Sun Feb 13, 2022 8:28 am, edited 1 time in total.
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krzysiobal
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Re: Famicom AV/Power Board

Post by krzysiobal »

HyldenKing wrote: Sun Feb 13, 2022 7:45 am krzysiobal
1. Why the large cap is at the input and not the output?
2. Can I use the schottky diode bridge instead of the regulars? What's the difference?
3. I can't find the quality brand ceramics for the regulator's input/output, can I change it to the 470nf film caps?
1. Why large cap at the output, not at the input?
Cap acts as a charge reservoir for fast current transitions. If some chip needs lets say 100 mA for a fraction of second, this current need to be sourced from somewhere. 7805 is linear voltage regulator, so if you load 100mA at output, it will acquire 100mA from input, so this current will be first driven from the input capacitor. So putting larger cap at output than at input is useless.

So what about putting big cap at the output and at input? Better put many small caps near vcc of every chip, instead of one big.

And finally, cap is similar to voltage source for AC analysis. It means that cap during power-up acts as almost a short circuit for some time (the bigger the cap is, the longer it lasts), so it might trigger some overcurrent protection or even break the voltage regulator.

During power down, the energy stored in cap need to be discharged somewhere.. If you will put bigger cap at output, you will go with a situation that the input cap will be discharged before output one, so the 7805 will have higher voltage at its output that input, this is no good.

And some low drop voltage regulators (which 7805 is not) have expclicitly said not to put big cap on output as it will result in oscillations.

2. Schottky diode has lower voltage drop, giving you higher margin. 7805 to give proper 5V at output needs at least 7.5V at input. So if you have 9V DC power supply, then if it is a transformer one, the voltage at output will still fluctuate between VL and VH, depending on the current consumption (9V is a average voltage).

Puttin schottky diodes on the graetz bridge might not always be good idea as they have low reverse breakdown voltage (but for 9-12V this should not matter)
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HyldenKing
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Re: Famicom AV/Power Board

Post by HyldenKing »

Quite a lot of info there, thanks! :beer: :beer:
So I'll change the input cap to 1000uf or lower if it's possible and the output cap to 470uf and use 100nf decoupling caps.
stefanobaron
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Re: Famicom AV/Power Board

Post by stefanobaron »

krzysiobal wrote: Sat Feb 12, 2022 3:36 pm This is the board I have made some time ago:
Image Image

It is both compatible with
* first revision famicom (with a big metal shielded dc/rf pcb, a'ka 5 pin), which has power switch on the 5V rail
* second revision (with a small dc/rf pcb a'ka 7pin), which has power swtch on 9V rail

(just connect JP1/JP2 differently according to the description)

It does not have the gretz's bridge, just a simple schottky diode placed on the DC input to protect against wrong polarisation.
Video amp is as in most of famiclones.

And the most important thing - there is a place to solder either
* mini jack A+V connector (this way you don't have to drill any holes in your shell)
or
* Audio RCA + Video RCA where originally two switches were placed (you need to enlarge those holes a little)
Hello Krzysiobal!

I've download your awesome Eagle project and I'm trying to update it changing the SMD components by through-hole components.
It's the first time I use Eagle PCB and I'm having hard time to learn the PCB part :lol:

Maybe I'll make a manual PCB using the classic pen as it's a simple design.

Thanks again!
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