Taito Trimline Space Invaders sound issue

modessitt

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I've been messing with this today, and am getting nowhere. The sounds play, but there is a loud hum in the background.

I have already changed all the electrolytic caps on the sound board, as well as the 5 on the back board. When I touch the 104 mylar cap near the amp, the hum gets a lot louder. I replaced that 104 cap, as well as the 5 closest to it and near the same circuit. No change. I swapped out the amp with another I had. No change. I checked the resistors in that circuit and they are good (admittedly one should be 40k-ohm and is reading 47k-ohm, but I doubt that difference is my issue). I followed that line where I touched the cap to a LM3900N, but replacing that didn't help either.

What's my next step? I'm not finding a very good schematic for this, so I don't know whether getting an audio probe in there is going to help me much if I can't trace the inputs and outputs properly.

The game came to me with no sound (speaker unplugged - most likely due to the hum) and hooking it up gave me the hum. I replaced the power supply with a known good one, but that didn't help. Added a jumper from sound board ground to power supply ground - that didn't help.

Turning the volume up and down on the board at the main pot makes the hum a little louder or softer, but the regular sounds get loud or soft at a quicker rate. The hum is there even with the sound all the way down. Adjusting the seven individual pots (for each type of sound?) doesn't affect the hum at all.

Anyone got any bright ideas?
 
Bumpity. Anyone got a copy of the schematic that actually shows the amp? I've found a copy of the datasheet for the amp, and can try disconnecting both the Input and Inverted Input one at a time to verify that the problem stems from the sound generation circuitry and not the Vcc voltage...
 
Personally I'd put a scope on it and look at the pre-amp audio to see if there's a hum riding on the audio. If not, then you've got something up in the amp circuit. That should atleast help narrow it down.

You can also listen to the pre-amp audio with a set of headphones wired up as probes. The ones with the volume control on the wires are nice, because it can save your hearing. :D That will also tell you what's up with your audio.

All the hum I've ever had to bang on ended up being PSU noise coupled over via bad caps, or failing (high ESR) decoupling caps.

Good luck.
 
you could isolate the final stage as a source of problems - lift the leg on the 47k resistor that feeds pin 6 and feed some low level audio to it via a 0.1uF cap from a battery powered source like an ipod - if there is no hum even as you turn it up, then its prob in the preamp where the final vol control is

while the preamp output is isolated, you could feed that to a powered speaker and see if hum is generated by the preamp independently of the main amp

send me your email addy and i can send u a small booklet on hums and buzzes explained it may help, it explains basics

schematic here http://www.brentradio.com/images/Other/Docs/SpaceInvaders/TaitoSI_P1P2_Schematics.pdf
 
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you could isolate the final stage as a source of problems - lift the leg on the 47k resistor that feeds pin 6 and feed some low level audio to it via a 0.1uF cap from a battery powered source like an ipod - if there is no hum even as you turn it up, then its prob in the preamp where the final vol control is

while the preamp output is isolated, you could feed that to a powered speaker and see if hum is generated by the preamp independently of the main amp

send me your email addy and i can send u a small booklet on hums and buzzes explained it may help, it explains basics

schematic here http://www.brentradio.com/images/Other/Docs/SpaceInvaders/TaitoSI_P1P2_Schematics.pdf

That's what I meant by using an audio probe - using it to probe each of the unamplified audio lines to see if one specific line has the hum. That would let me figure out whether it is caused by the audio generation circuit, and which section.

The problem with that schematic (which I'm not convinced is exactly for my game as mine has 3 boards and isn't color) is that it doesn't show the mb3712 amplifier that I can find, so it's hard for me to track each pin connection and where the feeds for signals and voltage are coming from. Even the board layout diagram and parts list in the trimline manual doesn't show it. I can see that removing the input to pin 6 SHOULD make the hum stop if it is part of the input. If it stays, then you'd think there'd be some ripple on the Vcc line, right? What about the Inverted Input on pin 8?

What about the LM377's just before the speaker output as seen in that schematic? It seems there is a 10uf cap on the line between the different AMP lines and the LM377 circuit, and removing that should either eliminate or confirm those as the source of the hum, huh?
 
That's what I meant by using an audio probe - using it to probe each of the unamplified audio lines to see if one specific line has the hum. That would let me figure out whether it is caused by the audio generation circuit, and which section.

The problem with that schematic (which I'm not convinced is exactly for my game as mine has 3 boards and isn't color) is that it doesn't show the mb3712 amplifier that I can find, so it's hard for me to track each pin connection and where the feeds for signals and voltage are coming from. Even the board layout diagram and parts list in the trimline manual doesn't show it. I can see that removing the input to pin 6 SHOULD make the hum stop if it is part of the input. If it stays, then you'd think there'd be some ripple on the Vcc line, right? What about the Inverted Input on pin 8?

What about the LM377's just before the speaker output as seen in that schematic? It seems there is a 10uf cap on the line between the different AMP lines and the LM377 circuit, and removing that should either eliminate or confirm those as the source of the hum, huh?

page 5, above ic 34 (the final amp) just above pin 6 is the MB3712 marking!
vcc ripple - hope so! otherwise have a look at the links below for some more technical hints
lm377s - yep - start with the main amp and work backwards. turn down each individual sound pot in turn

more tech hints:
http://www.analog.com/en/content/rarely-asked-on the money as a possible cause. quote questions/raq_diodes_issue45/fca.html :
"The classical reason for an electronic circuit to hum was line frequency (or, more usually, double line frequency) ripple on the dc supply. This is addressed by enlarging the supply decoupling capacitors or, better, by using an electronic voltage stabilization circuit–which has the side-effect of attenuating low frequency (LF) ripple. Modern switching supplies, as we have seen, generate large amounts of HF noise, which must be imprisoned at the supply and not allowed to escape. They rarely have LF noise problems, so your supply is probably not the cause. Measure its LF noise with an oscilloscope or spectrum analyzer to confirm this.
Another cause of hum is line frequency currents in the signal line or ground (the dreaded "earth loop") due to line currents or voltages being too close to signal circuitry. Again, possible cures were discussed in the earlier "Imprison Noise" RAQ and work well at LF as well as HF. If you have successfully implemented HF noise reduction, you have probably addressed most causes of power supply related LF noise as well. Test this by operating your circuit from batteries (with simple linear IC regulators to stabilize voltages if necessary), putting a resistive load on your switching supply to simulate the op-amp's consumption. Check noise with the switching supply on and off."
http://www.analog.com/en/analog-to-...rarely-asked-questions/RAQ_groundhog/fca.html
http://www.analog.com/en/content/rarely-asked-questions/raq_lockDown_noise/fca.html
 
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