System 18 background noise

[sk8er000]

Hello everyone,
Today I've received my Clutch Hitter Sega system 18 board (I'm preparing for the future multi ). I ran every test from the test menu and everything is working (RAM, ROM, VDP, IO, etc). My only issue is that when there is no BGM (when there is no sound or when there's only some audio effects) I hear a quite strong hiss noise. It seems like a shielding problem. On the board I didn't notice anything strange (except a green wire near the super capacitor but I don't think that could be related to that).

Anyone has some suggestion?

Thank you in advance

 

[sk8er000]

While I was doing the capacitors list I noticed another green wire an a broken ferrite but I don't think that those are the cause of my issue.

The very strange thing is that both green wires connect a point of the board to GND but those points are already connected to GND by the board itself!

I think that I'll try to recap the board soon hoping that can help to solve the problem

 

[stj]

linking ground or power points is often done if the original track was too thin for the current, or was passing an area that induced noise.
that and to bypass burned or cut tracks by lazy repair guys who couldnt be assed to get to the damaged track to directly bridge over it.

 

[sk8er000]

linking ground or power points is often done if the original track was too thin for the current, or was passing an area that induced noise.

I think that's the case since there are no damaged tracks on the board, I've tried to remove those "fixes" but still the same noise, so I put they back.

 

[stj]

hissing could be worn capacitors or a bad volume control.
probably other possible causes of course, but that would be my starting point.

 

[sk8er000]

I think that too since there is no visible damage to traces or pins and the audio is always reproduced right. There's only this background noise that changes according to which colours and how many of those are on the screen (for example when there is a lot of yellow or withe the noise increase).
The volume potentiometer doesn't make any scratchy noise when I increase or decrease the volume but I will change that too for completeness.

The only "strange" thing is that I never read about capacitors problems on the system 18 (I mean that is not a common problem like in the m92 for example)

 

[stj]

caps get old, start with any on the 12v rail to the amp.

as for the volume pot, usually the wiper(center pin) goes to the amp,
the right-side of the track goes to the audio source,
and the left-side goes to ground.

so as you turn the pot you give the amp more or less audio and as it becomes less the amp input gets slowly grounded - to prevent hum getting into the amp!!!
i have seen the left side have a bad connection internally a couple of times leading to this problem.

to test,
put the volume near the center, and meter the resistance between the outer pins - it should match the marking.(often 5k)

 

[sk8er000]

Thank you that's really useful, I will try as soon as possible!

 

[Blackfish]

Another area to check is the 12V supply from the Power Supply. If your power supply is getting old noise will also be introduced in the 12V supply itself. Some PCB seem to be more susceptible to this than others. After re-capping the PSU in my cabinets I noticed a substantial reduction in noise in both the Audio and Video signals. This noise would also often change with the video signals as the caps are used for filtering.

 

[sk8er000]

Another area to check is the 12V supply from the Power Supply. If your power supply is getting old noise will also be introduced in the 12V supply itself. Some PCB seem to be more susceptible to this than others.

That's true, I usually don't have problems with that PSU (it doesn't make any audio noise with various boards) but anyway with every system I use I have some random diagonal lines with every board. So I think I'll buy a new one soon since mine is from mid-90s.


I had some good capacitors lying around so I decided to recap as much as I could the board starting from capacitors that are connect to the 12v rail, but unfortunately the issue is still present.

I'll try to buy the missing ones as soon as possible..(I usually buy components from mouser but every time I need to reach at least 50$ for every order to get free shipping to Europe)

In the picture the new capacitors are the violet ones

 

[stj]

panasonics, what series are those?
i use FR for everything.

btw, if your in EU check the local places - they may be cheaper.
i'm talking about RS, Farnell, TME, Reichelt etc.

 

[sk8er000]

I used farnell once, they are pretty fast but the minimum shipping price is around 20€ that is more than how much cost a single recap, if I'm not wrong they offer free shipping but you have to reach an higher price than mouser

The capacitors I used are those one

 

[stj]

NHG series.
that's an unusual choice for a gameboard.
i usually use those for stuff like monitors because they go to pretty high voltages.

 

[sk8er000]

I did not chose this capacitors specifically for this game board, I had only that lying around, there are more suitable models of course but if one of the capacitors that I swapped was causing the issue they could solve it anyway (is that right? ). Anyway I still hope that the issue can be caused from one of the capacitors that I still have to swap or from the volume potentiometer

 

[stj]

they are ok caps for a gameboard - just dont use them to fix a psu.

 

[sk8er000]

Finally I recapped the whole motherboard and romboard, changed the volume potentiometer, the super capacitor and swapped the broken ferrite with a good one from the CN8 port.
Unfortunately the noise is still present (maybe it's a little bit weaker but I don't know if it's a placebo effect). A very noticeable thing is that the sound is way more louder now. I set the new pot at the level of the previous one (around 50%) and some samples were cracking on my stereo (not on the headphones) but lowering the volume solved the saturation.
I don't know if I can try some more fixes other than changing the PSU as suggested from @Blackfish. I'll buy another one soon

 

[stj]

well next suspect is op-amps
anything starting with TL0 or LM would be worth looking at.

 

[sk8er000]

Thank you for the suggestion. I think that there are only 2 TL064CN in the audio section.
Unfortunately I have no spare op-amp, I've bought a couple of them from Spain


Thank you very much @stj for helping me I really appreciate

 

[sk8er000]

In the meanwhile I desoldered the 2 op-amps and put 2 sockets to swap them easier.
The strange thing I noticed is that on the motherboard silk screen under the op-amps there's written TL084 but the op-amps are TL064CN

 

[stj]

i have seen that, maybe i should look at the datasheets to see the difference.