Buzz from speaker on hacked Mac Color Classic

[vldmrrr]

I've installed mini-ITX motherboard into Color Classic. All works well except the speaker produce quite loud buzzing sound that changes with what is shown on the screen. The loudness of buzz increases with the contrast of the image on screen: it is loudest on full-screen linux terminal filled with white-on-black text.

The buzz was extremely loud at first until I connected the motherboard ground to chassis metal cage with a thick wire - that made it much quieter but still annoying. I am not sure what else to try to reduce the noise further so asking for advice here.

The sound input on mac harness is connected to front panel audio header of motherboard. The volume regulator input of mac is hardwired to 5V.

 

[VERAULT]

Line out vs speaker out issue?

Or ground issue. is the analog board sharing same ground plane as mini itx board?

 

[vldmrrr]

Line out vs speaker out issue?

I assume the front panel audio header on PC motherboard has line out level and also assume that analog board expects line out signal (because it has regulated volume level). So you suggest to try splicing in speaker amplifier? In this case maybe it would make more sense route its output straight to the speaker. I will think about this. Thanks

Or ground issue. is the analog board sharing same ground plane as mini itx board?

Yes there are multiple wires routed between ground pins on motherboard and analog harness. But these are on video signal, sound, brightness, power on signals inputs. The motherboard does not get the power from mac, it uses separate (dell) laptop style power supply. So power supply pins from harness are not routed to motherboard. I guess I should try routing those. Thanks again

 

[NeXT]

Sounds like a ground loop is picking up noise from the analog board.
Also, have you recapped the analog board completely? You might also just have bad filtering.

 

[vldmrrr]

Sounds like a ground loop is picking up noise from the analog board.

Ok, I was thinking how to troubleshoot this and as a first step I disconnected the ground wire pin at motherboard side. The buzz was gone, but the sound itself was gone as well. So the analog ground for sound is separate from common ground, and ground loop on analog ground can be excluded.

I also tried using rear line out connector on the motherboard with all the same results.

Also, have you recapped the analog board completely? You might also just have bad filtering.

I am not a believer of complete recapping and only replace capacitors proven to be problematic. This analog board worked fine with original mac motherboard a month ago, so I do not think any caps may be problem for the noise.

 

[GiGaBiTe]

I had a similar problem when I restuffed an old Compaq iPaq. I tried to use the internal mini-sub speaker and it would just buzz and have bus noise on the output.

How I fixed it was I made a separate amplifier using a TDA7052A and used a comically large toroidal choke that I pulled from the input filtering stage on a dead PSU on the power input to the amp. It solved all problems with buzzing and noisy audio. Without that choke on the input power, it would have noise, with it, no noise at all.

To mix the sound channels on the mono amp, I just used a 1 uF 50V cap per channel into the input of the amp.

I am not a believer of complete recapping and only replace capacitors proven to be problematic. This analog board worked fine with original mac motherboard a month ago, so I do not think any caps may be problem for the noise.

You need to get used to completely recapping stuff in Apple land. Macintosh machines have some of the worst problems with failing capacitors. The only cap on the analog board you can usually leave alone is the horizontal deflection capacitor. For whatever reason, those tend to not go bad. As for the rest of them, get them out of there. Especially if the analog board has RIFAs on it, only a matter of time until those blow up.

 

[VERAULT]

Ok, I was thinking how to troubleshoot this and as a first step I disconnected the ground wire pin at motherboard side. The buzz was gone, but the sound itself was gone as well. So the analog ground for sound is separate from common ground, and ground loop on analog ground can be excluded.

I also tried using rear line out connector on the motherboard with all the same results.



I am not a believer of complete recapping and only replace capacitors proven to be problematic. This analog board worked fine with original mac motherboard a month ago, so I do not think any caps may be problem for the noise.

Neither am I but macs are the exception for their complete shittiness in components used. All color mac classics need complete recaps. I know I have already completely redone 3 mac color classics. And I dont even know how many caps that is per machine as the analog board has so many.. a literal ton.

p.s. Dont forget the power diodes in the middle of the analog board. You cant miss them with the heat scorch marks to the pcb.

 

[vldmrrr]

I ended up directly connecting the speaker to "Internal speaker" header on motherboard at which I previously did not pay attention. The sound is clean, no noticeable noise.

So my problem is resolved, while the problem of buzzing of analog board sound is left unresolved.

 

[VERAULT]

Fairly certain sound problems are well eocumented as cap related on the color classic. I had at least one system with sound issues prior to recapping the analog board.

Its a 90s mac. Al lthe caps are absolute garbage.

Now an early macintosh ii with through hole caps? Leave it alone its fine...

 

[vldmrrr]

Just to recap - the system did not have any sound issues with original CC motherboard. The sound noise problem was only with PC motherboard line out connected to sound input of analog board on CC (actually to a replaced harness from 6400 series, as prior to PC board hack I did takky upgrade on this mac. And by the way, sound did not work at all with known good 6400 motherboard that does have sound in native chassis. So maybe I screwed up somewhere in between. Also there were no noise from takky config either - just silent.)