Space Firebird Audio Probelms

[conan]

I have a 3 board Space Firebird with some issues. The current problems are moving star field that slowly jumps up the screen (regardless of orinetation) and then accelerates into a scroll on player death. Audio that only plays explosion sound with other game noises so quite its impossible to hear them without volume on full and the the explosion noise is deafening.
I have been through the sound board and pulled every chip, tested what can be tested (they all pass an IC logic) and replaced with sockets. Fitted new LM324N's and replaced the DAC 0800LCN. After replacing the DAC the game sounds came back and could be adjusted with main volume and I thought I had fixed it until the audio started to become distorted and fade out leaving only the explosion sound again, this happenened after about 1 minute. The star field still jumps but much less. Has anyone got any advice as I'm now down to replacing the caps but really dont know what to look for. Thanks

 

[Ronnie Dent]

Is this in a cocktail or on the bench? I know there is a secondary sound amp on the monitor chassis with volume control, as you probably know the starfield is generated by the sound board, there are a few types of sound board I have seen on these sets, I'm no expert by the way

 

[Lurch666]

First check the voltages going to the sound PCB.

Then check all the electrolytic caps for bulging or otherwise damaged.
If replacing them makes no difference then it could be another cap gone bad but I also notice there a lot of transistors and they can fail too.

Do you have any test equipment? An audio probe would help here.

 

[conan]

Is this in a cocktail or on the bench? I know there is a secondary sound amp on the monitor chassis with volume control, as you probably know the starfield is generated by the sound board, there are a few types of sound board I have seen on these sets, I'm no expert by the way

Thsi is a cocktail board that im planning on putting in a scratch build upright. Its on the bench right now. I think its a bootleg board but its marked TST-01-ESS.

 

[Ronnie Dent]

Maybe post up some pics of the boards?

 

[Lurch666]

I have a bootleg firebird so if you want anything checking let me know.

 

[conan]

First check the voltages going to the sound PCB.

Then check all the electrolytic caps for bulging or otherwise damaged.
If replacing them makes no difference then it could be another cap gone bad but I also notice there a lot of transistors and they can fail too.

Do you have any test equipment? An audio probe would help here.

Unfortunately i dont have an audio probe and im not sure how to use one either. Ill look at the voltage and see whats up with that. The caps all look good to my eyes but i have ordered replacements just in case. Here are a couple of pics of what it looks like. As you can see I fitted sockets to every chip and I have tested all the SN's. Two legs broke off the 8035 on extraction and Im suspicious they were already fractured as I was very carefull, unfortunately I accidentally plugged it back in in reverse and I think it may now be damaged as I get no game audio at all after soldering on two new leg ends. I have another on order.
As i understand it the 8035 excecustes the code on the MB8516 Eprom. Could that be corrupt as I dont get any bullet fire noises or enemy ship explosion noises at all even when it was working breifly.
One of the LM324N's has a 102J capacitor soldered to two of its legs for some reason, was the a repair or a pcb defect they worked around? I added it back when i swapped out the chip but it didnt seem to make a difference when i tested it before soldering it back in.

 

[conan]

The MB3712 amp isnt laid down flat and bolted to the PCB due to the board being fitted with a MB3713 which as i understand works in reverse so looks to have been fitted standing up. It was like this when I bought it and I have ordered a new MB3712 to replace it.

 

[Lurch666]

Looking at the schematics I noticed there is a mute circuit which I presume cuts the sounds during demo mode.
There's a cap and a transistor. If you have an oscilloscope it might be worth checking to see if it's functioning correctly.

 

[conan]

Well I fixed the starfield issue by replacing the 8035 that was bad. Audio still crackles out after a few minutes and now i have lost all colours other than yellow, white and brown/grey. Something must have happened on the video board. The CRT is fine as i tested it with a pettern gen.

 

[Lurch666]

The fact you have white means you are getting all the signals off the PCB.
There's a prom at 3M that sorts colour out so that would be where I would start.

 

[conan]

Thanks. Ill take a look there.

 

[conan]

Is that on the video board I cant find a 3M...

 

[Lurch666]

Damn my eyesight. It's 3N.
Sorry dude.

 

[conan]

I see what you mean sorry M3 on the board edges. I dont have a chip there just a bunch of resistors and caps. There is an MB7051 at N3 (looks like N on my board). is that the one its got a sticvker on it that says SDM-V-3N

 

[conan]

Damn my eyesight. It's 3N.
Sorry dude.

just replied hahah. Its that MB7051 then.

 

[conan]

On closer inspection i dont have white. I have blue but only in the background with those blue blobs that fly out of screen and yellow and a mixture of browns. No blue in any enemy sprites but a little blue in the ship. No idea how its managed to break itself one minute it was working the next....