Sunday, June 13, 2021

Making Your Famicom into the Best Famicom it can be : A/V Mods Done Right

Modifying the RF-Only Famicom to output separate video and audio is nothing new, people have been doing it since the 1980s.  But many mods I have seen involved video circuits of dubious quality, drilling and cutting into aged plastic and difficult to reverse without replacement parts.  In this blog post I will go over what I believe are the best ways to modify your Famicom for AV output.

The Two Famicoms

Nintendo manufactured the original incarnation of the Famicom for approximately 10 years, from 1983-1993, longer than any other of its video game systems.  The design was completely stable from the exterior, only the label badges changed somewhat and year one systems came with square rubber button controllers and smooth bottom plastic, which were soon replaced with more reliable round plastic buttons and textured plastic bottoms.  Internally the motherboards underwent frequent revision, and most first 1983 motherboards were recalled due to a hardware defect.  

Generally speaking, there are two styles of motherboards.  The earlier models are marked with an HVC-CPU label, the later models are marked HVC-GPM.  The most common revisions are HVC-CPU-07 and HVC-GPM-02.  With the HVC-CPU boards, the Power/RF Boards are connected to the main PCB only by a 7-connector ribbon cable.  With the HVC-GPM board, the Power/RF Boards are connected by a four ribbon cable but also by four large tabs which attach to the RF shield.  

Nintendo Famicom HVC-CPU-07 Motherboard (courtesy of Evan Amos)

The older HVC-CPU motherboards have fewer ceramic capacitors and also lack the shield over the cartridge connector, so they may be a bit noisier than the later HVC-GPM boards.  The HVC-GPM systems are more difficult to work with due to the soldered RF shield making a very strong connection to the main PCB and difficult to remove without high heat and a lot of solder wick or a powerful motorized suction desoldering tool.  If you are looking for a HVC-CPU or HVC-GPM motherboard, HVC-GPMs always have a metal shield over the cartridge connector and may have the "FF" Famicom Family on the FAMILY COMPUTER label or have a VCCI logo on the underside of the system's enclosure.
Nintendo Famicom HVC-CPU-GPM-02 Motherboard (courtesy of ifixit)

The highest quality video signal of any official Nintendo Famicom or NES console can output is composite video.  The NES' PPU generates NTSC or PAL color video signal (chroma) internally as well as a monchrome signal (luma), but mixes the chroma and luma signals together before sending them out of the chip on a single pin.  Older consoles and home computers like the Atari 2600 or the Commodore 64 perform the mixing externally and are capable of s-video output without complex circuitry, but the NES is more limited in this regard.

The Backoffice AV Replacement Board Power VAMP

The new standard for Famicom AV modifications is the Backoffice AV Replacement Board Power VAMP, currently in v3.  This board replaces the original Power/RF board of your Famicom and has a 3.5mm TRS jack to carry composite video and audio output with a 3.5mm to RCA video/audio cable.  The Power VAMP, is designed so that it screws into the same holes as the original Power/RF boards did.  The cut out for the AV connector is set where the old RF hole was and no cutting of the Famicom's enclosure is required.  All that needs to be done to install the board is to remove the old board, which can be done with a heated soldering iron and gentle pressure for the HVC-CPU boards.  The HVC-CPU boards will also need the wires for the power switch to be removed from the old board and soldered into the Power VAMP.  There are two video guides given in the link above which show you how to install the mod.

Backoffice Power VAMP Board (v2 shown, courtesy of RetroRGB.com)

The Power VAMP also has other benefits.  It has a bridge rectifier (the four diodes) which will permit the use of either a positive or negative tip 9V DC power adapter.  This is rather useful if you cannot find a good Sega Genesis Model 1/Master System adapter, which are my go-tos for powering Famicoms, AV Famicoms, PC Engines and the Roland MT-32, CM and SC modules.  These use a 10v/850mA, 5.5mm/2.1mm Type A barrel connector, center negative, but are getting less common.  You should probably not use a NES AC adapter, which is only a transformer which steps down the volager from 120v to 9v.  While the Power VAMP has a bridge rectifier which can convert AC to DC, it does not have the large smoothing/filter capacitor which a true AC to DC capacitor would provide.  Finally, along with its "no cut mod" design, you can revert back to the stock RF output if you keep your old Power/RF Board.

Reducing Jailbars

Once you have your Power VAMP installed, you can kiss your need for a VCR or a TV which can accept channels 95 & 96 goodbye.  But you might find the resulting image to be a bit disappointing, especially in comparison to an NES front loader or an AV Famicom.  The problem is that the original signal picks up interference on its way from the PPU to the video amplifier to the RF modulator on the Power/RF board and the system has poor filtering in general.  This causes vertical jailbars to appear, most noticeably in areas of solid color, and upgrading to composite video will not affect their visibilty.  The newer HVC-GPM boards with their extra filtering and shielding were designed to meet Japanese radio frequency emission requirements, any improvement in video quality was an afterthought.

In order to get the best video quality from your Famicom, you must bypass the original video output circuit and replace it with one derived from the AV Famicom.  I have seen many designs, including one from the Twin Famicom, but the Twin Famicom has jailbars, so it is not the best solution to the problem.  The same thing can be said about the NES Top Loader, Nintendo did little more than the bare minimum needed to convert the HVC-GPM board into a 72-pin board.  The Power VAMP board's AV output is based on the Twin Famicom's, which uses the pre-amplifier transistor circuit on the Famicom mainboard.

CatHouse Games AV Mod Board

The schematic for the AV Famicom's circuit can be found here.  However, you do not need to build this circuit from discrete components if you do not want to.  This board sold by console5 and designed by CatHouse Games has all but two of the components required by the schematic, the 560pF capacitor is optional and the tantalum capacitor should be connected directly between PPU pins 20 (-) and 22 (+).  This board's stereo audio feature is not compatible with the Power VAMP board and that stereo audio mod gives weird balances and does not account for expansion audio.  That part of the circuit, including those 1uF/50v capacitors, can be ignored.  

Planning the Mod, Yellow is Video, Black is Ground, Red is +5v

The best place to anchor the CatHouse Games video amplifier board is on the lower right screw post of the Power/RF board on the HVC-CPU-0x boards.  To give the board more space, I trim the pointy bits underneath the CatHouse Games board with a flush cutter as much as possible.  If you are using the HVC-GPM-0x  If you use the CatHouse Games board, you must remove the transistor at Q3.  That begins the noisy video circuit.  You can use the B(ase) hole of Q3 to connect to pad marked PIN 21 IN on the CatHouse Games board or connect directly to PPU Pin 21.  There are many places to find +5v and GND for the input side of the CatHouse Games board on the Famicom's mainboard.

For the output side of the CatHouse games board, connect the GND to someplace on the Power VAMP board, pin 7 on the 7-pin ribbon cable pads or that lonely via next to the "R" in "MOTHER" are good places.  However, the Power VAMP v3 and the CatHouse Games both have video amplification circuitry and they cannot be stacked.  If you wish to use the CatHouse Games board with the Power VAMP v3, you must bypass the video amplification circutry on the Power VAMP v3 board.  To do this, lift the leg of the resistor marked R2 on the Power VAMP v3 board where the trace runs to the TRS jack, then connect the Y pad on the CatHouse Games board to the via you just cleared on the Power VAMP v3 board.  I used a 10uF/35V tantalum capacitor on the PPU between Pin 20 (+) and Pin 22 (-).  

The Completed Mod

If you wish to use the CatHouse games board without the Power VAMP v3, then you will need to keep the original Power/RF board and get creative with your output connector.  Cutting may be required or you may need to repurpose the RF output connector.  Also, to connect audio, you can use the pads for audio marked on the CatHouse Games board.  Connect a wire from cartridge connector pin 46 to one of the two CPU Px pads on CatHouse Games board.  Then straddle a 220v uF capacitor (axial works best here) where the positive lead of the capacitor is connected to the wire coming from pin 46 and the negative lead is going to the audio output.  This is reversed from the polarity shown on the board for the audio caps, but that application was using them in a different manner.

Things to Avoid

With the above components and methods, you should be able to AV your Famicom with no cutting or trimming of original Nintendo components.  Generic components like wires and transistors are easy enough to replace to reverse the modification back to a stock Nintendo console, but shells and PPUs are becoming harder to come by.  There is no more reason to cut PPU Pin 21 anymore and you should never, ever do it.

PPU Pin 21 is the video out pin, and it is obviously rather necessary for video output.  However, it is also necessary for an NESRGB mod, and if you snip the pin you will have to find a way of making that connection if you want to use that mod.  Sometimes people both snip the pin and bend the leg to make it easier to solder a wire to, but this puts stress on the pin and if it shears off right at the plastic, that PPU may be useless.  Removing the Q3 transistor takes care of the noise just as well as snipping the pin.

Covering the PPU in copper foil was a solution some people have proposed to reduce jailbars, but you have to cover from underneath the PPU, which pretty much requires removing the PPU.  With the AV Famicom circuit and lifting Pin 21, using copper foil has shown no visual improvement from reports of other users.

Comparisons

In order to make a fair comparison, I have made three videos.  I have started with my stock Famicom, which uses an HVC-CPU-07 revision board (which is very common), RF output.  Then I have progressed to the Famicom modified with the Power VAMP board and the stock video amplification circuit.  Finally I show the Famicom modified with the Power VAMP and the CatHouse Games board.  

Jailbars are best seen with games that use wide areas of solid color.  Super Mario Bros. 1-3, The Legend of Zelda, Slalom and Kirby's Adventure are good games to look for jailbars.  Games using black backgrounds or complex backgrounds are not so good.  Here are three screen captures for reference, 

Stock RF Output

Power VAMP v3 Output

NESDev Ideal AV Mod Output

Jailbars may be more noticeable on a CRT than an LCD or vice versa, depending on your display.  LCDs are generally unkind to the gritty signal that comes out of the NES/Famicom PPU.  If you really want to get the the best video quality that is compatible with high quality scalers and will give you the best quality video, you will need to modify your Famicom with an NESRGB board.  This mod will give you the option to upgrade down the road.  The NESRGB board has a Power Board replacement which replaces the Power/RF board and provides its own video outputs, so you will have to remove the Power VAMP and the CatHouse Games boards.  

As you can see from the videos and screen captures above, my mod was not 100% successful in eliminating the jailbars.  I have an older Famicom with a rev. E PPU and CPU, later revisions of the chips, rev. G and rev. H, may be inherently less noisy.  Modding a Famicom for AV output is still a worthy endeavor, although I am not sure I would have gone to the trouble of adding the NESDev Ideal AV Mod had I seen the difference between it and the Power VAMP v3 on my Famicom beforehand.  

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