Introducing the Mega Amp - The universal Genesis audio circuit

[villahed94]

I re-did the solder on the resistor. Still no sound for the right side though...

I'll take another look at it tomorrow, but it's getting late right now. I can't seem to get it working. I think I'll just have to get a replacement.

Check with a multimeter if the 160K resistor is actually measuring around 160K . If it doesn't and you can find a 160KOhm resistor at a local electronics shop, replace that resistor.

[Helder]

On the side you checked for sound could you check the opposite side to see if there is sound? If there is then it's definitely a bad resistor and I can send you one to replace it with.

[ComradeOj]

Yesterday, I tried attaching a wire to the bottom side on the 160kohm (the side farthest from the op amp) and I could hear the sound, but it was very quiet.

I tested the resistor with my multimeter and it read correctly, 157kohm to be exact so I don't think the resistor is bad.
I also tested various points against the resistor to see if it continuity was good.

http://i.imgur.com/addVECI.jpg

The image is color coded to show which side on the resistor were tested against what. In all four cases my multimeter read 157kohm.

[Ace]

Since you can't even get the right channel working in Mono, it might be possible for there to be a bad VRef going into the op-amp. Have you measured the voltage on pin 5 of the TL074? This is VRef for the right channel on Helder's circuit. If you're seeing something other than 2.99V thereabouts, then VRef isn't reaching the op-amp. If it measures fine, then the only possible explanation to your problems is a dead op-amp, which will require a replacement of the TL074.

EDIT: And that would seem to be the case. Looking back at your measured voltages here: http://i.imgur.com/St2rvnl.jpg

Your voltages are WAY too high on the right audio output. The 2.97V you measured at the op-amp is the left audio output and that same output going into the left audio input of the Mono mixing circuit. The 4.4V is the voltage from the right audio output which is far higher than it should be. The output should measure 2.97V thereabouts on both outputs. Try reflowing pin 5 of the op-amp. If reflowing doesn't help, you can try soldering a wire between pins 3 and 5 of the chip as it's possible there's a bad trace which is leaving the non-inverted input of that particular op-amp floating.

[ComradeOj]

Since you can't even get the right channel working in Mono, it might be possible for there to be a bad VRef going into the op-amp. Have you measured the voltage on pin 5 of the TL074? This is VRef for the right channel on Helder's circuit. If you're seeing something other than 2.99V thereabouts, then VRef isn't reaching the op-amp. If it measures fine, then the only possible explanation to your problems is a dead op-amp, which will require a replacement of the TL074.

EDIT: And that would seem to be the case. Looking back at your measured voltages here: http://i.imgur.com/St2rvnl.jpg

Your voltages are WAY too high on the right audio output. The 2.97V you measured at the op-amp is the left audio output and that same output going into the left audio input of the Mono mixing circuit. The 4.4V is the voltage from the right audio output which is far higher than it should be. The output should measure 2.97V thereabouts on both outputs. Try reflowing pin 5 of the op-amp. If reflowing doesn't help, you can try soldering a wire between pins 3 and 5 of the chip as it's possible there's a bad trace which is leaving the non-inverted input of that particular op-amp floating.

I tried re-flowing pin 5, but I still couldn't hear anything from the right. Attaching a wire between 5 and 3 didn't help either.
I feel like I've tried everything at this point.

[villahed94]

I tried re-flowing pin 5, but I still couldn't hear anything from the right. Attaching a wire between 5 and 3 didn't help either.
I feel like I've tried everything at this point.

Hmm. Measure the resistance between pins 6 and 7 at the opamp. It should be around 160K. If that doesn't work then it's a dead opamp.

[Helder]

Send me a PM so we can discuss a swap.

[AlmostOriginal]

I still haven't done this mod and i feel ashamed.

[Kumagai]

villahed94, se te llenó el inbox de mp's y no te pude mandar más mensajes por allá xD, así que te escribo por acá, cuando puedas respondes el último mp que te mandé

Por casualidad tienes Whatsapp o frecuentas otro foro en particular? Para seguir en comunicación por alguna de esas vías, dado que no se por qué este foro se carga lentísimo, mandar un post o un mp por acá es todo un suplicio...!! Esperando tu respuesta!

[Helder]

Villahed you need to prune some messages in you PM box or maybe have the site increase the size of the inbox a bit.

[villahed94]

Well, I have space for one more PM (still got some PMs from 2010)

[Ramsis]

So I've bought a Mega Amp PCB from Helder and installed it in my Mega Drive 2 (VA0 board). Right from the start, I had a very similar issue like ComradeOj -- there was pretty much no sound from the right channel, whilst the left channel was much too loud, and distorted. I tried reflowing all OP-amp pins, which didn't change anything. While waiting for the replacement Helder kindly offered, I played around some more with the board, giving all parts a reflow multiple times. By now it only produces sound (on both channels) that is barely audible at all.

The replacement finally arrived, and I replaced the old install with it (including most wires). It sounded ... well ... strange. Fed up with everything, I completely uninstalled the Mega Amp, undoing all related changes (i.e., putting back in all resistors and capacitors) in order to check if the ASIC within my MD2 was still working properly. Fortunately, it was, and still is.

Next, I re-installed the replacement board to record an audio clip that I could put up here.

Voilà: Sonic 1 with replacement Mega Amp

As you can hear, the music is totally messed up. Just listen to that Marble Zone theme (starting around 01:10) ...

Here are pictures of my latest install (the wiring is all correct):

1 (2,2 MB)
2 (2,37 MB)

So ... anybody know what's going on here?

Thanks!

[Ace]

For some reason, it sounds like PSG is either shorted with the YM3438 left input or the mixing is off. Do you have a multimeter on hand? Measure the two 100Kohm resistors for PSG mixing and the 51Kohm resistor on the YM-Left input (the silkscreen says 39Kohms, but ASIC YM3438s need 51Kohms).

Also, you don't need to bridge the two solder pads for the SN76489, they're already linked by a trace on the bottom of the board. You use one or the other since they're the same signal, so when you go ahead and reinstall the circuit, don't put that wire in place.

[BlockABoots]

I just come here to post about my issue and a few other appear to be having the same problem.

Though my device isnt the MegaAmp its the CCAM version, but im have intermittent audio from the right channel as well it will be working fine and then it will just stop, i though it was my scart cable but got a new cable and this new cable has the same issue, have tried different sockets on my Bandridge scart switcher but same results all my other systems work fine.

Any ideas what the issue could be?

[Ace]

That sounds like a dodgy solder joint or weak trace. If it's the latter, that's going to be a pain as you will have to bypass the entire trace and take your signal directly from the source. You will also need to cut the trace in question.