Joust 2 speaker hum

boogiemanspud

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I just picked up a Joust 2. It works well, but has a nasty sounding hum coming from the speaker. Anyone have any thoughts on what this could be and how to fix it?
 
75-80% of the time it is the amplifier. The rest of the time, it is AC hum getting through the capacitors.

Replace the filter capacitors and if it persists, then replace the amplifier.
ken
 
75-80% of the time it is the amplifier. The rest of the time, it is AC hum getting through the capacitors.

Replace the filter capacitors and if it persists, then replace the amplifier.
ken

I have been looking online and can't find any schematics or pics of the boards. Can anyone help me locate this capacitor? I can take some pics of the board if it would help.
 
The capacitors should have the values marked on them.

Unfortunately, there are no schematics for Joust 2. The closest anybody can do requires interpretation from the Mystic Marathon, Turkey Shoot and Inferno schematics.

ken
 
This is BAD! I hadn't even messed with the sound board yet and now I have next to no volume from the speaker (still loud though). I tried turning the volume pot inside the cab. I can hear the game but it is QUIET! Almost like a whisper sound level.

So, does this still sound like the same problem? (I would guess so.)

I recently figured out what the amplifier looked like.

This board has 4 amplifier chips. They are mc1458P, and also say 636 AB on them. Is there an easy way to tell which is bad, or would it just be wiser to replace them all at once?

I still don't know what filter capacitor needs replaced. Should I just replace all capacitors? They are cheap but that seems like a lot of de-soldering and soldering!

I wrote down all the caps on it, and the amplifier, in case anyone needs this info:

Caps with 2 prongs coming out the ends: (radial leads?)
2 - 16V 1000uF
2 - 16V 470uF
2 - 35V 100uF

Caps with Inline prongs (prongs coming out each end): (axial leads?)
6 - 030TK 25V 10uT
1 - 50V 1uF
1 - (says on it: 4n7 J63 U3 KS)
1 - (says on it: 1n2 J160 U6 KS)
1 - (says on it: 180 p J630 u3 KS)

4 - Amplifier Chips (636 AB - MC1458P)
 
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The audio amps should be the ones mounted on the heat sinks. They should be TDA2002 audio amps. The MC1458s are the mixing and preamps. While they might go bad, it is usuall the TDA2002s.

The audio card for Joust2 looks similar in a lot of ways to the one for SmashTV. That may help you as SmashTV has a very good set of schematics and parts diagrams.

The 1000mfd caps look to be on the output from the audio amps. The 470mfd caps appear to be on the feedback input of the audio amps. I haven't found the 100mfd caps on the schematic yet. The axial caps are probably of no consequence. I would replace the radial caps (1000, 470 & 100 mfd) and order up some TDA2002s. You can get them from Jameco.

ken
 
Thanks for the clarification. I wondered what those were on the heatsinks. I looked in radio shack but the ones they had that looked similar were transistors or something (cant recall). Plus not the same model/type.

I will give the heatsinks a good cleaning while I am at it. The cab is amazingly clean inside but there is some grime on them (waxy/dusty stuff).

EDIT: I assume these need thermal compound between the amps and heatsink, is this correct?
 
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There should be some white stuff between the chips and the heat sinks. That is supposed to be there. It is a thermal grease to help transfer heat from the chips to the heat sinks. You can get more from most electronics stores (used to be able to get it from Radio Shacks, but I haven't tried that in a while).

ken
 
There should be some white stuff between the chips and the heat sinks. That is supposed to be there. It is a thermal grease to help transfer heat from the chips to the heat sinks. You can get more from most electronics stores (used to be able to get it from Radio Shacks, but I haven't tried that in a while).

ken

I don't know if RS carries thermal compound anymore. Any computer store that does component sales of heatsinks will, or newegg has an entire section here: http://www.newegg.com/Store/SubCategory.aspx?SubCategory=85&name=Thermal-Compound-Grease . This stuff isn't exactly cheap and may be overkill but it will definitely get the job done.
 
O.K. I got the radial caps ordered, the Amps, and some thermal compound. I'll give them a shot and see how it works! Thanks for the help guys!
 
I attached a pic of a diagram. Maybe it can help someone with a similar issue.

EDIT: Also attached a pic of the soundboard with the components already de-soldered.

ANOTHER EDIT: That first diagram got a bit fuzzy somehow on upload. All + sides of the caps are on the right sides of the diagram, except red caps which have the + on top.
 

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Well I replaced the caps and amps and still have a problem. I do now have volume, so that part is fixed.

I attached a couple pics. The line I am pointing at on the soundboard goes to the MPU board where I am pointing in the other picture.

Anyway, if I unplug the wiring that I am pointing at on the soundcard or at the MPU, the buzz goes away, but of course it loses some of the sound effects (I was listening to it in attract mode).

Anyone have any ideas? Could the wiring between the MPU and soundcard be bad? Is there any way to test this easily? If it's not the wiring, could it be the amp on the MPU, or the caps right by where the wiring plugs into the header on the MPU?

Thanks again! :)

Edit: OK, for the ongoing saga... So I decided to try and run some jumpers from the MPU board to the soundboard. This seemed to get rid of most of the hum. I had plugged/unplugged the cable several times and I can't tell for sure, but I think it helped the humming. I suppose it could be corrosion on either the header pins or inside the female plugs. I will give them a good cleaning today and see if it helps anything. I am really thinking though, this has to either be corrosion on the headers/plugs or else the wires are getting feedback of some sort from the other wires, causing the hum. I will try cleaning contacts and if that doesn't work, I will have to seperate the wires from the other wires and see if that helps.


O.K. I think I got this figured out. I cleaned the headers and female connectors again, still had the hum. So, I took the wire out of the harness and ran it away from other wires. Hum stopped. So, if I am following the right logic here, the wire from MPU to soundboard is getting interference from the other wires.

So, any ideas what to do with this? Could I wrap the wires in foil tape or would that make it worse? I am thinking that would act as a RF shield and cancel out the interference.


Another EDIT: Yikes! Got to looking and this has no ground plug, somebody in it's past snipped it or pulled it out. That may have had something to do with it, but... not sure as the above stopped the hum.
 

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I am gonna chalk this one up as fixed. I rerouted the shielded cord and it stopped the hum. Really weird problem, but at least rerouting fixed the hum.
 
The shielded cord is the cable to the volume control as near as I can tell from the pictures. These are known to be susceptible to all sorts of interference. Verifying that the shield is connected to the ground is the first step. If the hum persists, grounding the case of the potentiometer (make sure it does not have continuity with any of the three wires first) can help and if all else fails moving it away from all other wires can work as well.

ken
 
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