HI there, recently bought a Sharp Twin Famicom from ebay. I love it except it seems to have a low buzz when playing cartridges and when playing the disk system. I opened it up and the board looks great, I also cleaned the microphone slider on Controller 2 to make sure that there is no interference from there. Finally, I tested another A/V cable and another TV, it seems it isn't either of those things.
I'm thinking that the power adapter I was sold is causing the issue, it's an Enercell 7.5v 1000mah which I noticed is 250mah below what the Twin wants. Could this be causing my problem? If not, should I recap?
60Hz (crosstalk from PPU) or 120Hz (crosstalk from power supply)?
(You can record a sample using a computer or laptop and then use some wave editing software to determine which.)
Not 100% sure how to interpret this
http://imgur.com/dedkRRs,J3A9GgOEDIT: I grabbed this from an analysis of a 20 second recording of the Twin Sharp Famicom with no games in it
Yeah, 120 Hz and below is low enough frequency you're going to have difficulty seeing it in a spectrogram. Just look at the direct waveform, and count the number of milliseconds between zero-crossings (4 or 8) or repetitions (8 or 16).
Or downsample the whole signal by a factor of 16 to 64 and the low frequencies should pop right out in the spectrogram because each STFT window covers a longer time period.
Are you sure that's a recording of the buzz? I just hear brownian noise..
(In audacity, I enlarged the window by hitting Ctrl-F, Ctrl-Shift-F, and then increased the volume of the recording by selecting Effect/Normalize/Ok.
Counting zero crossings: zoom in horizontally, either by Ctrl-mousewheel or just the magnifying glass tool, until the range at the top of the screen (usually measured in seconds) varies by about .040 from left to right. If you see something repeat ~2.5 times, or cross through the x axis five times, that's 60 Hz. If it repeats ~5 times, or crosses through the X axis ten times, that's ~120Hz)
I do see a faint 15.7kHz whine from horizontal sync coupling in, which implies 60Hz could be present too ... but I can't personally hear it.
lidnariq wrote:
Are you sure that's a recording of the buzz? I just hear brownian noise..
(In audacity, I enlarged the window by hitting Ctrl-F, Ctrl-Shift-F, and then increased the volume of the recording by selecting Effect/Normalize/Ok.
Counting zero crossings: zoom in horizontally, either by Ctrl-mousewheel or just the magnifying glass tool, until the range at the top of the screen (usually measured in seconds) varies by about .040 from left to right. If you see something repeat ~2.5 times, or cross through the x axis five times, that's 60 Hz. If it repeats ~5 times, or crosses through the X axis ten times, that's ~120Hz)
I do see a faint 15.7kHz whine from horizontal sync coupling in, which implies 60Hz could be present too ... but I can't personally hear it.
Thanks! This helped a lot.
the signal crosses through x axis 5 times and repeats 2.5, so it's gotta be 60hz.
I recorded the buzzing again, it happens right after the FDS bootup theme.
http://www.mediafire.com/download/9hkqf ... i/buzz.wav
Yeah, that's PPU video output crosstalk.
I guess there's two ways that can happen ... the power supply can be too soft, allowing the PPU to couple into the audio via the voltage rail, which is fixed by recap'ing; or the audio traces go too close to the PPU traces and they're capacitively coupling. I don't actually know which is the case in the Twin Famicom. You might be able to touch your computer (or amplifier)'s input to CPU pins 1 and 2 (the two audio outputs) and see if you still hear the 60 Hz—if you do, it's more likely power supply coupling.
You can always troubleshoot using the PDF posted here:
http://shmups.system11.org/viewtopic.ph ... b#p1048939
Thank you both! This is invaluable information. I'll work on isolating the buzz
OK, sorry for the late reply. I have since been able to put a direct line to my CPU audio out and there is no buzz! So the signal must be getting mixed with video somewhere.. However, I took a look at the board and I don't see any capacitors touching or traces touching. Where do I go from here? I can post a bunch of pictures if that would help identify the problem
EDIT: Also, I purchased a Twin Famicom AN-505-BK to compare the audio circuits, seems this one also has a buzz
It's a bit fainter, but I'm not sure how many times it crosses the x axis. It's a bit 'fuzzier'. Here's a recording if someone could take a look for me:
http://www.mediafire.com/download/4sq4a ... bobuzz.wav
The next step is what ccovell said—follow along the audio path, touching your amplifier to the traces as you go, until the buzz appears.
Here's a zoomed in section from your most recent recording, note the following:
1- The length of one period, denoted at the bottom, is 0.017 seconds (or 1/60th of a second)
2- The waveform is briefly high, but mostly low
Attachment:
audacity-shows-ppu-crosstalk-60-hz.png [ 7.18 KiB | Viewed 3161 times ]
Both of those imply PPU video coupling into your audio.