Famicom AV board check

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soop
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Joined: Thu Jul 19, 2018 2:32 am

Famicom AV board check

Post by soop » Tue Feb 11, 2020 7:21 am

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Hey guys, I'm reaching out to you all out of frustration. I've been working on this for some time, and I got these boards made up after reading up on this thread: viewtopic.php?t=10554.

Short story, it doesn't work, and I literally have no idea what's going on. The board is supposed to replace the entire video and power board, so when the whole unit didn't work, I thought I'd replace the video/audio and keep the existing power then replace the power while keeping a known working (simple) AV mod. That didn't work either. Can someone take a look and tell me if I've fucked up somewhere? Bear in mind the usual video link on the board isn't wired up normally, I'm taking video straight from pin 21 of the PPU here.

*edit* oh and the diagram of the A1015 on the board is the wrong way around, because mine are ECB

krzysiobal
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Re: Famicom AV board check

Post by krzysiobal » Tue Feb 11, 2020 8:36 am

I don't know who designed you that circuit, but it is completely wrong. Neither of the transistors is connected correctly.
Draw proper schematic and proper PCB design according to valid transitor pins sequence, so we can talk further.

soop
Posts: 11
Joined: Thu Jul 19, 2018 2:32 am

Re: Famicom AV board check

Post by soop » Tue Feb 11, 2020 9:25 am

krzysiobal wrote:
Tue Feb 11, 2020 8:36 am
I don't know who designed you that circuit, but it is completely wrong. Neither of the transistors is connected correctly.
Draw proper schematic and proper PCB design according to valid transitor pins sequence, so we can talk further.
I mean I designed it, but I may well have messed up the transistors. Does anything else look wrong?

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Ben Boldt
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Location: Minnesota, USA

Re: Famicom AV board check

Post by Ben Boldt » Tue Feb 11, 2020 7:16 pm

The arrow is going the wrong way on your 2SA1015 transistor -- it makes it the symbol for NPN, so it isn't clear what you intended to do. The 2SA1015 is actually a PNP transistor.

Image

Pay extra-special attention to the pinout of the transistors. Your layout does not correspond to your schematic, there is mixing up of the pinout between the two.

What AC adapter are you using? Does it put out 9V DC or 9V AC? Is the pin in the center positive or negative? Test it with a voltmeter to be sure.

I am not really sure about your video circuit. Where did you get that circuit from? Here is a known-good one:

Code: Select all

                     (GND)
PPU.20 ----------------+---------------------------------+--------+
 |                     |                                 |        |  Ring
 |                  | / (c)  2SA937               560pF ---       +----O } Composite
 |         PPU.21 --|< PNP                      Ceramic ---              }   Video
 |                  | ^ (e)      220uF                   |        +----O }
--- 4.7-47uF           |     Electrolytic     110        |        |   Pin
--- Tantalum           +----------|(---------/\/\/-------+--------+
 |+                    |        +
 |   (+5V)     300     |
PPU.22 -------/\/\/----+
For something like this with so few components, it is a good idea to get a prototype working on a breadboard first, then you can easily modify and get it working before moving on to an actual printed board.

lidnariq
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Re: Famicom AV board check

Post by lidnariq » Tue Feb 11, 2020 7:24 pm

Ben Boldt wrote:
Tue Feb 11, 2020 7:16 pm
Where did you get that circuit from?
It's supposed to be a copy of the reverse-engineered path from some Famicom or NES, including the portion that's inside the RF box. It's the link in the OP's starting post. (Of course, the OP put all the transistors in the wrong orientations, so ...)

Your schematic is usually followed by another transistor as an extra buffer.

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Ben Boldt
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Re: Famicom AV board check

Post by Ben Boldt » Tue Feb 11, 2020 7:50 pm

lidnariq wrote:
Tue Feb 11, 2020 7:24 pm
Your schematic is usually followed by another transistor as an extra buffer.
I guess I haven't really thought about that before. The extra buffer is typically inside the RF box right? What is the benefit of adding the extra buffer? My AV modded famicoms only use the 1 transistor, I am very interested to know if I should change it and possibly retest stuff. Unfortunately I have absolutely NO IDEA where I put the famicom I was using for the experiments in that thread. I think I will only find it if I move at this point. I have a dozen famicoms sitting here, just missing #13...

lidnariq
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Location: Seattle

Re: Famicom AV board check

Post by lidnariq » Tue Feb 11, 2020 8:06 pm

Ben Boldt wrote:
Tue Feb 11, 2020 7:50 pm
The extra buffer is typically inside the RF box right?
Yes.
What is the benefit of adding the extra buffer?
Reduces the effective output impedance of the 2C02.

The voltage buffers that are being used here—both are common-collector amplifiers—effectively divide the output impedance of the previous stage by the gain of the transistor. Since the 2C02's output impedance varies with the instantaneous voltage, one would ideally use these current amplifiers to minimize the phase distortion that the 2C02 natively has, and then re-add impedance matching at the end.

soop
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Joined: Thu Jul 19, 2018 2:32 am

Re: Famicom AV board check

Post by soop » Wed Feb 12, 2020 5:36 am

Ben Boldt wrote:
Tue Feb 11, 2020 7:16 pm
The arrow is going the wrong way on your 2SA1015 transistor -- it makes it the symbol for NPN, so it isn't clear what you intended to do. The 2SA1015 is actually a PNP transistor.

Image

Pay extra-special attention to the pinout of the transistors. Your layout does not correspond to your schematic, there is mixing up of the pinout between the two.

What AC adapter are you using? Does it put out 9V DC or 9V AC? Is the pin in the center positive or negative? Test it with a voltmeter to be sure.

I am not really sure about your video circuit. Where did you get that circuit from? Here is a known-good one:

Code: Select all

                     (GND)
PPU.20 ----------------+---------------------------------+--------+
 |                     |                                 |        |  Ring
 |                  | / (c)  2SA937               560pF ---       +----O } Composite
 |         PPU.21 --|< PNP                      Ceramic ---              }   Video
 |                  | ^ (e)      220uF                   |        +----O }
--- 4.7-47uF           |     Electrolytic     110        |        |   Pin
--- Tantalum           +----------|(---------/\/\/-------+--------+
 |+                    |        +
 |   (+5V)     300     |
PPU.22 -------/\/\/----+
For something like this with so few components, it is a good idea to get a prototype working on a breadboard first, then you can easily modify and get it working before moving on to an actual printed board.
Thanks Ben! I did use a breadboard, but I think when I was designing the board some difficulties with Eagle, and perhaps improperly measuring the transistors have led to the current situation. On the other hand, I think I burned out a 7850 somehow, because I just replaced it and now have power and sound!

I'll look into your advice the next chance I get :)

*Edit* woop woop, got it working! Thanks everyone! Looks like the design needs a tweak regarding the transistor pins, but everything else works great!

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