Midway Gunfight With Weak Audio

[Call Sign ACE]

I have a Midway Gunfight PCB that I have fixed, except for some weak audio. The sounds should be strong and full with somewhat of a lingering echo. The sounds are there, but lack the full sound as described.

I have already turned up the volume on the potentiometers, which helped at first, but that has not corrected the rest of the problem. Any ideas what might be wrong?

 

[ArcadeTechGW]

Assuming this is an EM Gun game....

...check the amp on the inside of the top deck, right side. I had bad connections on the one I repaired, and had to clean it up and adjust it.

 

[Lurch666]

I have one of these I have resurrected from the dead.
All I need to do now is sort the sound out.
It's a midway 8080 game like sea wolf and space invaders with a dedicated sound board.
The right hand side sound on mine is fine but the left hit sound is missing and the left shot sound is quiet.
When I get around to having another look at it I will post if I figure out the problem.
I'm going to try replacing the electrolytic caps for the left audio first.

 

[KenLayton]

Sound board likely needs new electrolytic capacitors since it's 40 years old.

 

[security0001]

i would start with the caps on that game and we stock them all if you need them.

 

[Interlock]

Assuming this is an EM Gun game....

Haha, nope. Classic Midway 8080 game! They made a game called Boot Hill which is also popular in the "golden age" community that is very similar

 

[Lurch666]

Well I've changed all the electrolytic caps in the left sound circuit with little difference.
Will investigate further when I get the chance.
Transistors next I think.

 

[creepykenny]

changing the caps in the audio section usually alters the delay/decay of the gunfight shot sounds, but of course ymmv
you need to look at the 3900 chips and schematics (i think,but of course i should prolly look at my set when making such a bold proclamation doh!) , and if you happen to have an audio probe it would be extremely helpful to you
schematics are pretty straightforward on these

 

[Lurch666]

Been a while but finally got back to this now I have an audio probe.
Found that the left gunshot is being generated properly but the sound is not making it out but the left hit noise is just a short noise.
I'm thinking maybe the fault in the left hit is also stopping the left shot from getting to the main amp.
I have ordered some 2N4125 transistors but am having problems finding the A-138 type.
Does anyone know of an equivalent?

 

[blinddog]

From what I've found it looks like nte102a should work. I don't remember if I have had to replace one. Seems like the issues usually ended up being one of the electrolytic, mylar caps or the lm354. Not saying they can't fail of course. Do they test okay with a multimeter?

 

[GA61XX]

Cool.. another Midway 8080 game set
Not got one but now i do Want

2N4125 = 2N3905

For what you are saying it just not got the echo as it should
a few months ago i had problems with a SID sound board, with the sound only being a short note & was missing the reverb, after changing the LM3900 , all the cap's i then went 1 by 1 through the resistors, yep, found that with Midway's manufacturing back in the day , the time / day' / weeks between the PCB being etched & components being added the copper had tarnished, hence the solder did not take..
i only worked this out from the many 8080 SI & SID sets i have worked on & that a lot of problems seam to be dry joints..
any way, it is not just to test the resistors, as will test good... just remove all components & replace with new after you have cleaned the tracks..

 

[Lurch666]

I managed to get 2N4125 but couldn't find A-138 so will try nte102a.

Will try removing the solder and replacing it with fresh just to see if that makes a difference.

I'm ok with digital troubleshooting but am kinda lost with analogue so apart from using the audio probe to see if it's just the main amp I don't really have the experience to diagnose this stuff.

I replaced the electrolytics because they have a history of failing and next it's the transistors since they sometimes fail.
After that all that's left is capacitors and resistors which in my experience tend not to fail but will replace them if everything else fails.

Thanks again dudes.

 

[GA61XX]

Transistor A-138
strange not to have it's full name ??
any details ?

just went & found your schematic for the sound PCB
Ok.. 2N4125 = PNP equivelent 2N4402
looks like the A-138 is NPN complementary ?
so i would look at trying NPN :- 2N4123 Hfe 150 , or 2N4124 Hfe 360 . also :- 2N3903 & 2N4400
looking at the gain of the 2N4125 being Hfe 150
seam to be in the low range..
a good transistor tester will give you an answer if you can pull a good one ..

 

[blinddog]

Looked into it a bit more and I doesn't look like nte102a is right, looks like it was another A138. The schematic shows it as a NPN and nte102a is a PNP. So it seems I was mistaken.

 

[Lurch666]

For what you are saying it just not got the echo as it should
a few months ago i had problems with a SID sound board, with the sound only being a short note & was missing the reverb, after changing the LM3900 , all the cap's i then went 1 by 1 through the resistors, yep, found that with Midway's manufacturing back in the day , the time / day' / weeks between the PCB being etched & components being added the copper had tarnished, hence the solder did not take..
i only worked this out from the many 8080 SI & SID sets i have worked on & that a lot of problems seam to be dry joints..
any way, it is not just to test the resistors, as will test good... just remove all components & replace with new after you have cleaned the tracks..

This post needs putting up in BIG lights for anyone trying to fix this sort of thing.

Just went and de-soldered/re-soldered the components (didn't remove them just sucked the solder then re-soldered) in the left hit sound section of the daughterboard and now the left hit works.
I'm impressed.
Now to do the same with the left amp to see if that comes back to life as well.

 

[GA61XX]

Cool.. BIG smile..
it was as i removed components i found that the copper had tarnish & solder had not take to the tracks....
very happy what i fund has helped another
i found it very strange, as all components would check good & all tracks would buzz out, but it seams current flow for sound just was not enough..
Answer strip out components, clean tracks, re-solder all back..
i have stripped out many IC's from these board set's only to find once i get the chip out to see the pad's "brown" not the silver as if it just had solder on it, they is Brown / Tarnished copper.... mostly it seams to be on the components side of the board..
it would seam that Midway left some boards up-to weeks/months before adding components & solder ..
the original of this finding is in a Space Invader thread ..

 

[Lurch666]

Found that the transistor just before the the main amp is what is failing on the left audio.
On the right and left I can just about hear the sounds using the audio probe but after the transistor on the right it's loud but on the left still quiet.
It's an A-138 so might just try a generic NPN and see if it makes any difference.

 

[Lurch666]

https://www.youtube.com/watch?v=1uVV4UoEYlA&feature=youtu.be

Looks like a BC547 NPN transistor did do the job.

In the video the scores are missing off the top of the screen but they are there it's just the LCD I've been using for testing has no adjustment.

It's a pity that I can never properly play this as you need the proper controls on an original cab to control the game so in my test setup up and down controls the left cowboy and left and right controls the right cowboy but I can't move the guns up or down or the cowboys left or right.

I bought this on ebay as it was listed as a space invaders and I didn't know back then that there were other games using the midway L shaped PCB setup but it's been fun to fix and I've learned lots.

Thanks again for all the help dudes.

 

[GA61XX]

The only thing to watch out for when using a generic NPN is the Hfe ( Gain )
is best to get close to the originals
hence why i have a small transistor tester, just carefully remove a know working & test
it will give you pin out & Hfe
from that you can then find a close generic NPN