(Topic ID: 73697)

Gottlieb System 80: Replace Obsolete LM379S Audio Amp with Modern Equivalent?


By Pinwiz1985

5 years ago



Topic Stats

  • 29 posts
  • 9 Pinsiders participating
  • Latest reply 9 days ago by Flippers_com
  • Topic is favorited by 6 Pinsiders

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There have been 3 images uploaded to this topic. (View topic image gallery).

LM379S.jpg
Heat sink for amp.jpg
NOS LM379S pin 1 mark.jpg

#1 5 years ago

Does anyone have a link or pdf on how to convert the circuitry from an obsolete hard to find LM379S to a modern equivalent chip? Its for the A6 sound board located at U23.

Rebuilding this board for a friend and the heat sink was missing... So the chip was on its way out.

The cool part of this project is I;ve been given schematics to add a subwoofer/external speaker mod for System 80 games. Sounds badass but want to get the original speakers going to get everything working.

#2 5 years ago

I'm planning to post this to the Wiki tomorrow.
Up too late now, but I have permission from the original author to post it.

I know that it works as I built one today.
--
Chris Hibler - CARGPB #31
http://www.Team-EM.com
http://webpages.charter.net/chibler/Pinball/index.htm
http://www.PinWiki.com - The new place for pinball repair info

#4 5 years ago

Thanks Chris! Yeah I got sound coming from the center pins of R15 and R16 but the audio amp is shot here. Marco Specialties actually shows the LM379S in stock, but brandon called me from there today and said they are having trouble finding them...no surprise there.

I've been in contact with Fred Swemmer and he sent me some stuff to add an RCA "unbalanced" connector for sending the sound to an external speaker/amplifier like he does on his boards. Unbalanced audio interfaces are susceptible to ground hum noise. An isolation transformer or balun in between the external amplifier and sound board decoupling both would solve this issue. The LM379S has a midrange frequency boost to accent the speech portion. The LM1875T has a more flat frequency response. This is probably why the LM379S is so desirable and hard to find.

I'm designing a "balanced" output circuit design for myself based off Fred's original design. I find balanced interfaces are more friendly for example when multiple pinballs are connected to the same speaker/amplifier system through a mixer section. I have a full PA system with dual 1x18 subs and 1x15H tops running about 2.5k continuous watts I intend to test with soon.

#5 5 years ago

Chris, one more question. its not made clear in your wiki post, but do you still need to modify the sound board power supply with the correct zener diode and resistor to provide 16 volts instead of the stock 30 volts?

I see you get the 12 volts from Pin 1 but I do not see any trace cut isolating the 30 volts if the sound board wasn't modified.

#6 5 years ago
Quoted from Pinwiz1985:

do you still need to modify the sound board power supply with the correct zener diode and resistor to provide 16 volts instead of the stock 30 volts?

Nope.
That's one upside to this mod.
The Gottlieb spec'd mod requires a change to the PS.
This mod uses the 30V as is.

After installing the LM1875, I can't tell the difference in the "sounds" portion of the audio.
I haven't gotten the voice calls working yet...the board has more problems than just the failed LM379S. These things are real head scratchers.
--
Chris Hibler - CARGPB #31
http://www.Team-EM.com
http://webpages.charter.net/chibler/Pinball/index.htm
http://www.PinWiki.com - The Place to go for Pinball Repair Info

#7 5 years ago

Perfect. I figured that it wasn't necessary given the supply voltage for the LM1875 can be 16-60 VDC according to spec. I just scored some NOS LM379S amps from another source. Marco Specialties said they could not find them.

I replaced the dried electrolytic caps on the sound board, but the amp burned out before I could test with the game running. The game starts fine without the sound board connected, but before doing work to the board the game would not start, the credit display goes blank, a delay, then returns to attract mode. If the game was left to warm up for 5 minutes and a game was started then it worked just fine...

If it wasn't a dried capacitor that caused this issue I'm thinking it may be a failing Z-chip on either the sound or MPU board. Time to break out the logic probe. Thankfully my SC-01-A speech chip is still good. These are even HARDER to find than the LM379S.

The LM379S has a slight frequency "bump" around the 300-400hz range and another around 800hz range for speech according to my o-scope. So the amp will sound a bit more "honky" than the newer amp which is more natural flat EQ sounding. Most wont be able to tell the difference but some will if the speakers are original.

1 week later
#8 5 years ago

Chris,

Question, I haven't installed the board back in Black Hole yet. I was able to obtain a new LM379S amp. Ive been using my computer's power supply to give the sound board +12, -12, and +5 volts. The +30VDC ive been providing with a few batteries in series. I hear the amp power up, and there is voice attract mode at center pin of R15/R16 and the self test proves the board is functioning, just no output from amp.

If I power the amp at pin connector 10 with +12 volts instead of 30VDC the amp powers up and works, but only for like 30 seconds before the output gets garbled and weak.

Am I correct in assuming that the LM379S input requires a bias voltage off the same power supply with a common ground reference to work properly???

#9 5 years ago

You're blazing new territory my friend. I do not know the answer to that question.
--
Chris Hibler - CARGPB #31
http://www.Team-EM.com
http://webpages.charter.net/chibler/Pinball/index.htm
http://www.PinWiki.com - The new place for pinball repair info

#10 5 years ago

Crap. Thank you anyway for the pinwiki post. If I hadnt found the 379 id be putting in the newer amp. When I get it in the machine ill report back.

#11 5 years ago

I'm sure you noticed that the LM379 pin 1 is down, which is the opposite of every other IC on the board.
--
Chris Hibler - CARGPB #31
http://www.Team-EM.com
http://webpages.charter.net/chibler/Pinball/index.htm
http://www.PinWiki.com - The new place for pinball repair info

#12 5 years ago

Correct. It was marked on the heat sink and masked on the board as well. Bottom right hand corner if looking at the board with the amp located in the top right hand corner.

#13 5 years ago

That's good. I wish you could see the LM379 heat sink pin 1 designation without having to remove the big finned heat sink.
--
Chris Hibler - CARGPB #31
http://www.Team-EM.com
http://webpages.charter.net/chibler/Pinball/index.htm
http://www.PinWiki.com - The Place to go for Pinball Repair Info

#14 5 years ago

It never came with it. I bought the TO-3 finned heat sink separately. The chip itself has a metal heat sink mounting bracket attached to it. On that there is a black dot designating where pin 1 is. As blownfuse says on here system 80 is not just a job its an adventure. By the time im done rebuilding this black hole, pretty much every single board ive rebuilt or serviced in some fashion.

#15 5 years ago

Pics for those not sure what's being talked about.

Steve

NOS LM379S pin 1 mark.jpg

Heat sink for amp.jpg

#16 5 years ago

Thanks for the pics guys. An update, got to work on the machine today, and Murphy's Law hit me. The sound board worked great after the repair outside of the machine. My F2 fuse blew and discovered my 10volt bridge rectifier shorted out! GAH! I know it wasn't because of the sound board as the fuse isn't for the sound board, all the bridge rectifiers are original, but i did change the filter caps a while back.

Guess i'm ordering some 35amp 400V bridge rectifiers before I can move forward. F1 and F8 were still good (associated with sound board), and prior me to me reinstalling the board I tested good voltages for the sound board. F2 is for the 5 volts so I can't do anything without it.

Moral of the story, don't just replace whats broken...just shotgun the whole f#$%ing thing the first time!

Repair all at once, cry all at once!

#17 5 years ago


Friggin' Murphy!
The RatShack will have that bridge.
--
Chris Hibler - CARGPB #31
http://www.Team-EM.com
http://webpages.charter.net/chibler/Pinball/index.htm
http://www.PinWiki.com - The new place for pinball repair info

#18 5 years ago

Don'tcha just hate that guy, he sure gets around.

Steve

Quoted from ChrisHibler:

Friggin' Murphy!

#19 5 years ago
Quoted from ChrisHibler:

Friggin' Murphy!
The RatShack will have that bridge.

That is a negative, RatShack does not have a 35 amp 400v. They have a 25 amp, but id have to order off the web, its not in store. I'll just order it from Todd at Bigdaddyenterprises. Hes always been a cool guy to work with.

...and yes, friggin Murphy! The only parts of the power supply chain I DIDN'T change and of course they fail. everything else I rebuilt (repinned connectors, power supply board, filter caps, etc)

2 months later
#20 5 years ago

Hey guys.

Okay, I was able to replace the 4 BR's in the bottom box. The entire power supply chain is modern and repaired. Machine boots up great and has all voltages present.

Now the bad news. I blew up the S&S board! I don't know how I did it but I did! The voltages are correct at the S&S power supply. I had the LM379S installed (being aware of pin 1 location) and installed the board. Soon as I did that within 5 seconds I lost the 5 volt and 12 volt indicators on the power supply. Nothing popped or burned, but I unplugged the sound board and the game powered up fine with all the voltages present.

Put the S&S board on my computer bench power supply, and the output at R16 its repeatedly saying "FAILS, FAILS, FAILS." The LM379S amp and the LM1875T amp do not output at all when installed.

I don't want to start blindly throwing parts at it. What does the error message mean? Did I blow both EPROMS? The 6502 MPU? The 6532 RIOT chip? HELP!

#21 5 years ago

Bump for this fine man

#23 5 years ago

Bump! Any techy Gottlieb guru's out there? Not looking to shotgun this board if I don't have to. I have Clay's guides btw, however the error message is not saying whats failed on the board. I'll even make a video of this behavior if it helps.

2 months later
#24 5 years ago

Did you end up getting this monster of yours to work?

#25 5 years ago

I haven't investigated it further, it is on my to do list. My friend bought a used working board to get the game going. I'm thinking it has to be either the riot chip or one of the DIP14 chips. Eventually when I get back to it I'll update everyone on here.

1 year later
#26 4 years ago

You can still find LM379's. Not easily but they are still out there.

LM379S.jpg

3 years later
#28 10 months ago

im only getting 28.3 volts to my A6 sound board and having problems, I've already replaced the bridge rectifier on the power supply side of the pinball, is that enough for the A6, I'm having fading volume problems, I've replaced the SC01-A chip already and suspect the amp on this board. Seeing that my R15 and R16 are fine, can I not just connect up a cheapo 12V ebay amp to the pins and connect it to the speaker bypassing the whole mess?

9 months later
#29 9 days ago

The original LM379S will work even at 12VDC - not terribly well, but it does work. So your problem is elsewhere...
John :-#)#

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