Hi guys, I just bought a Famicom AV on eBay and I have an issue I just cant figure out. I have a black screen and a buzzing sound. I've opened it, tested it on the multimeter, 7805 has 9.4 V ac in (that's what my power supply is providing). And 1.4V DC output... So definitely something wrong here. Now here's the complicated part. I've changed my 7805, And I still can't get 5V Output, But what I don't understand is that the input changed to 6.9V AC... My 7805 are L7805CV that from STI Microelectronics, That I have used on other repairs without problem... So first I can't figure out what is wrong with my console... Changed most Cap : The 2000uf, the 0.47 & the Reset button Cap. I could never output 5V. And I really can't understand why the input power is changing when I change the 7805. If I put the old one back I have 9.4V again, but still no 5V output. Any idea?
The 7805 is probably *trying* to output 5V but a component on the board is fried. It's fairly common for famicom CPU or PPU chips to die. A crude test to see if this is the case: Let it run for a minute, then touch the CPU and PPU. If you almost burn yourself on an IC, you found the problem.
Is this actually an "AV Famicom" (Japanese version) or a NES2 (US version)? They take different power supplies, and the AV Famicom needs DC input, not AC.
@TriMesh The Famicom AV (HVC-101) has a 7805 regulator and can take AC Power and DC up to 12V @rama thanks for the reply I thought that a working 7805 would output 5 V no matter what. So bacically, considering I have ruled out bad regulator and bad Caps, you're saying it's quite probably the CPU or PPU gone bad ?
The HVC-101 is NOT designed for AC input. It's designed to use the same AC adapter as the original HVC-001 Famicom, which is 10V DC @850mA center negative. It does have a reverse polarity protection diode which will also function as a half-wave rectifier, but this will subject the power supply circuits to abnormal stresses and will likely result in eventual failure. If you really have AC on the input of the 7805, then D1 has gone short (probably as a result of repetitive overload from the use of an AC supply). Note that in this case there is also an excellent chance that all the other electronics in the unit has also been destroyed. The NES-101 IS designed for AC input - it has a 4-terminal bridge rectifier on the PCB, but will also accept DC of either polarity.
@TriMesh Thanks for the detailed reply. I I'm quite puzzled because I have 2 other Famicom AV I've been using for ages with an original Pal Power Supply (outputting 9.4V, 1.3A AC) and nerver had any problem...
You can probably get away with it for quite a while, but I have seen a number of those consoles that were damaged by being used on AC. The basic problem is that since it's a half wave rectifier the cap has twice as long to discharge (20mS vs 10mS for full-wave with 50Hz mains), and hence when the diode starts to conduct the peak current is substantially larger - eventually, if can fail. If you're lucky, it fails open circuit and the console just stops working. If you're unlucky, it fails short and starts applying AC to the input of the regulator chip. What happens then depends on the internal design of the regulator chip - some of them will just refuse to work, but some have a reverse-connected parasitic diode across the internal pass transistor, and in this case, you end up applying about -9V to what should be a +5V line. Hence my comment about the rest of the electronics being quite possibly destroyed.
@TriMesh thanks very useful. WelI... I have ordered a DC power supply So one last question would a bad CPU or PPU could cause a 7805 to not output 5V
Yes - all those 3 terminal regulators will only supply a limited amount of current, and if you try to take too much (which a blown chip could well do) then it will drop the voltage to try and keep the total dissipation inside the limits (this is the "SOA protection" on the datasheet). But before you do anything else, make sure that the protection diode (I think it's D1) is still actually a diode and hasn't gone short, since a 7805 was never designed to tolerate voltages below the negative rail on it's input pin.
I frequently have had to replace cpu or ppu in these av famicoms. 8/10 tikes its tge issue from ones ive repaired.
Thanks guys, I will test my diode tonight. But what @TriMesh is describing seems right on spot. When I turn on the console, the 7805 gets really hot really fast (burning hot I mean).
Hi guys, hi @TriMesh So I'm having a hard time finding the diode. But from the look of it, it seems to be FC2 right ? So I have tested it with the multimeter and I get 0v from left to right et from right to left. So if it is a diode it seems it's down. Is FC1 a diode too ? I get a 0 result on both sides too.
Ok fixed So apparently not all revision of the Famicom AV have a Diode. Mine has 2 capacitors near the main cap instead. Anyway changed the capacitors, change the 7805, used a DC PSU and... all fine. I was so scared my CPU or PPU was blown. Anyway thanks for those who helped and cyu at the next issue
Thanks for the report and enjoy your console! I've a friend with the same issue on his famiclone and I'll try that same repair for him.
