MCR Kickman Audio Noise

[joeyoravec]

My Midway Kick makes a digital-sounding noise. Its not a 60hz hum, it's fluttering exactly like described in this old post: http://groups.google.com/group/rec...._frm/thread/c63c35e734b995f2/cbd022da1ceff2e5

Here's what I know:

• The noise is only present when the sound is unmuted (coin in, game playing). It's loudest with the volume pot at the middle volume setting; inaudible at the maximum or minimum settings.
• Replacing electrolytic caps, tantalum caps, analog switch, op amps, and the audio amplifier IC made no improvement. Replacing the coin-door pot for an on-board audio pot made no improvement.
• The audio power (unreg +12) is noisy but running audio off the main power (reg +12) made no improvement.
• Replacing electrolytics on the power supply made no improvement. Replacing the big transformer assembly caps made no improvement. Disconnecting the marquee light made no improvement.
• Service bulletin about CP34 has already been addressed (part is not even populated)
• Swapping the AY8910 between left/right made no improvement, but I don't have any known-good chips to replace them.
• I've never managed to identify the source of the noise. It happens right around the breakdown voltage for D102/D106, but I don't know if that instability is what's causing the flutter.

Do all MCRs with the same mute/volume circuit exhibit this problem, maybe this a design flaw? Any tips on solving this problem?

 

[ELutz]

MCR games seem to be "noisy" in general.....in varying degree, and some more than others. The biggest problem is tracking the problem down. A lot of stuff can introduce noise. This is a list of some of the things I've found to possibly introduce the humming/noise.....

Any of the three big caps associated with the transformer assembly.
Caps or amp chips on the small amp board
Caps on the power supply board
Caps in the audio circuit of the sound/input/output board
A weak ground connection associated with any of these boards, especially the small amp board.

Also, Midway released a service bulletin stating to cut jumper J4 on the power supply board......to reduce audio hum.

Edward

 

[joeyoravec]

Those are all great suggestions, although in this case it's not a 60hz hum it's a fluttering white noise. I might have expected white noise at maximum amplification but I can't explain why this noise is loudest at the middle of the range.

I hadn't seen the bulletin but did consider J4. These games have a separate unregulated filtered +12 power supply for the audio section. I'm not sure if it's because of high current consumption or for isolation, but that circuit delivers really choppy power. I tried running audio off the regulated +12. Even though it's not isolated the power is a lot cleaner.

I updated my original post... I forgot that I had replaced those parts too.

 

[joeyoravec]

Here's an audio sample of the noise... Starts with tiny 60hz hum, then I trip the coin switch (which unmutes the audio circuit) and you'll hear the overpowering noise. Not perfect but gives you an idea.

Had zip in order to upload to the forum. It's an m4a file recorded by my phone and it should open in itunes.

 

[mjenison]

I know this is a long shot, but there was a service bulletin a long time ago about some Kick boards having CP34 inserted backwards.

 

[DarrenF]

Have you tried disconnecting your marquee light?

Is your wiring from the sound board to the amp board shielded? The original design may have used the shield as one of the conduction paths; but ideally it should have your two audio signal lines inside the shield, and the shield should be grounded to earth at (only) one end of the cable.

 

[7146aaron]

Kickman

Ha.. Thats what all Kicks/Kickmans sound like.

 

[ELutz]

Ha.. Thats what all Kicks/Kickmans sound like.

I do find Kick/Kickman to be one of the "noisier" MCR games.

Edward

 

[joeyoravec]

I know this is a long shot, but there was a service bulletin a long time ago about some Kick boards having CP34 inserted backwards.

Just checked, CP34 is unpopulated on both boards. I couldn't find it on the schematic but it looks like a simple bypass cap.

Have you tried disconnecting your marquee light?

Good idea. Tried it, but it made no improvement.

Is your wiring from the sound board to the amp board shielded? The original design may have used the shield as one of the conduction paths; but ideally it should have your two audio signal lines inside the shield, and the shield should be grounded to earth at (only) one end of the cable.

The original has left/right audio wires plus shield for the audio ground. I'm concerned that this forms a ground loop (with vaudio return) so I'll have to try it today.

Ha.. Thats what all Kicks/Kickmans sound like.

I see on your VAPS entry that you have a few dedicated Kickmans. You're saying that your machines all make that noise when coined up, loudest with volume in the middle? If so, then it's safer to assume that I'm dealing with a design flaw.

 

[joeyoravec]

Is your wiring from the sound board to the amp board shielded?

Disconnected the shield at the amplifier end, no improvement.

 

[joeyoravec]

I pulled out my Fluke 9010A to play with this board. The AY8910 sound chips are at 0xB000 (F6 with the mute line) and 0xC000 (F7 without the mute line). To interact with them:

0xB000: Latch Register Address (write)
0xB001: Read Register (read)
0xB002: Write Register (write)

Pretty useful, now I can force the sound chips to mute/unmute or output a certain tone.

I configured IO Port B as output and unmuted the amplifier -- I heard the noise. Then I had the idea to remove both sound ICs and just ground pin6 manually; again I heard the noise. Since it happens even without the sound chips in circuit, I think it's safe to say the AY8910s are not the problem.

 

[7146aaron]

Noise

All 6 of the Kick/Kickmans I have make the noise that yours does. Hard to tell if yours is louder or not. But they all do make that exact noise. Never checked to see if it is worse with the volumn in the middle of the range.

 

[mjenison]

Just out of curiosity, what revision is your power supply board? Should end with a suffix like -A000, -B000, -C000, or -D000.

I had a Kickman that was fairly noisy but when I upgraded power supply revision from -A000 to -D000 it made a world of difference noise-wise.

 

[7146aaron]

90412

Just out of curiosity, what revision is your power supply board? Should end with a suffix like -A000, -B000, -C000, or -D000.

I had a Kickman that was fairly noisy but when I upgraded power supply revision from -A000 to -D000 it made a world of difference noise-wise.

All mine have 90412 Cs or Ds in them.

 

[joeyoravec]

Just out of curiosity, what revision is your power supply board? Should end with a suffix like -A000, -B000, -C000, or -D000.

It's a -C000.

 

[joeyoravec]

Ok more research... each AY8910 has a (goofy) variable-filter circuit on each of the 3 channels. The MCR2 troubleshooting guide describes how the 4-bit digital output controls a circuit that adjusts the filter cutoff frequency.

Assuming I did this properly, I set the AY8910 outputs for max-pass (4-bits = 0x0) and max-filtering (4-bits = 0xF) on one channel. At max-filtering you hear a staticy sort of noise. At max-pass that's gone but there are still components that I'm trying to solve. This is only one component of the noise problem but it's somewhere to start.

I've attached a screenshot of the oscilloscope at the output of the opamp (C10 pin 1) at the max-filtering. The small lines are dead-on at 50 kHz. The envelope is a sort of triangle at around 1.2kHz. Tonight's project will be to figure out if that's really what I'm hearing and why it's sneaking in.

 

[joeyoravec]

This repair has been on the back-burner for a while. Checked out a local kickman and it makes exactly the same noise. I'm convinced that the noise is a flaw in the original design.

I'm thinking that I might be able to fix the noise by cutting the LM3900 and volume control out of circuit, and running the audio into something modern that has a volume control. Can anybody recommend an audio amplifier good for arcade applications, meaning:

- Runs on +12v, drives a typical arcade speaker (15w?)
- Line level input
- Volume pot, best with a header to use a existing coindoor pot
- Has pin header connections; preferably not RCA and wall-wart plugs
- Bare PCB or mounting tabs to screw inside the game

I've seen a few but none have been perfect. Recommendations?

 

[DarrenF]

Can anybody recommend an audio amplifier good for arcade applications, meaning:

- Runs on +12v, drives a typical arcade speaker (15w?)
- Line level input
- Volume pot, best with a header to use a existing coindoor pot
- Has pin header connections; preferably not RCA and wall-wart plugs
- Bare PCB or mounting tabs to screw inside the game

I've seen a few but none have been perfect. Recommendations?

This runs on 12V, has plenty of power, line level inputs, PCB mounting... but the volume pot in on-board, and the headers aren't pins:
http://www.sparkfun.com/products/9612

 

[cleverlyj]

just to throw in my input, it seems to be an mcr thing. my satan's hollow and the tron i had both do the same thing. having a switcher definitely seems to make it worse, though.

 

[joeyoravec]

Looks like the sparkfun amplifier (here) is a possible solution. As described I eliminated the volume-control, ran the LM3900 audio output to the sparkfun amplifier. In this configuration the noise is gone, so I'm pretty sure that volume control circuit is the source of the problem.

Something is still wrong because my amplifier only runs with one speaker connected; with two it starts oscillating (peak, shutdown, startup, peak, ....). The game has a pair 4ohm speakers which should be fine. I suspect the +12v isn't supplying enough current for that amplifier but it needs further testing.