Weird Pengo PCB issue / Sound problem

[Runik]

Hi there !

I have a strange Pengo issue : whenever the playfield is displayed and Pengo appears, the playfield disappears like when the level is over, and you go straight to the next level. It goes on and on, never displaying anything more than the playfield and Pengo before ending the round.
It behaves the same way during the attract mode, as the playfield disappears as soon as Pengo appears, and the scores are displayed ...
It's the unencrypted set 3 version, I checked all the roms against romident and they appear to be fine, so I'm a bit at loss there ...

Has anyone encountered a similar issue ?

I have a working bootleg version, if that can be of any help ...

Thanks !

 

[Lurch666]

dipswitch 2 switch 6 is a rack test.Is it turned on?

 

[Runik]

Oh my ... Looks like you're right !
I went through the manual many times, and this dip is labeled Normal / Continuous play, never thought it would have this behavior ... But that's logical when you think of it
I just need to fix the missing sound and it will go back to its owner ...
Thanks a lot

 

[Runik]

Ok, looks like my sound problem is harder than I first thought ...
Pin 1 of the MB3712 generates the waveform as I can see it through my oscope, voltage varies between 6V and 8V. Going though the U27 cap doesn't change it, looks like the voltage is too high (6V when the pot is at the minimum)
I have a working bootleg which is almost identical, and I have the same waveform generated from MB3712 pin 1, but the difference lies after the U27 cap : the voltage goes from 6/8V to 0.5/1.5V), thus allowing the sound to be heard perfectly ...

What am I missing there ? All the caps are good (I changed U27 a few times to be sure, and the others were tested good too), the pot is good, the wavefrom is there but apparently too high, what could cause this ?

Thanks

 

[AndyMcDandyCDN]

Ok, looks like my sound problem is harder than I first thought ...
Pin 1 of the MB3712 generates the waveform as I can see it through my oscope, voltage varies between 6V and 8V. Going though the U27 cap doesn't change it, looks like the voltage is too high (6V when the pot is at the minimum)
I have a working bootleg which is almost identical, and I have the same waveform generated from MB3712 pin 1, but the difference lies after the U27 cap : the voltage goes from 6/8V to 0.5/1.5V), thus allowing the sound to be heard perfectly ...

What am I missing there ? All the caps are good (I changed U27 a few times to be sure, and the others were tested good too), the pot is good, the wavefrom is there but apparently too high, what could cause this ?

Thanks

Hello Runik,

Did you ever solve your sound issue? I have already replaced the MB3712 on mine along with the caps. Everything else appears fine but I am still learning.

Thanks.

 

[CanuckAB]

I would also like to know the answer to that. I have a Pengo without sound, yet signals on my 'scope

Thanks in advance!

 

[Runik]

Hello Runik,

Did you ever solve your sound issue? I have already replaced the MB3712 on mine along with the caps. Everything else appears fine but I am still learning.

Thanks.

Actually I haven't looked into it since last time ...
I didn't replace the MB3712 with the bootleg one as I want to keep this one as a working reference, I have yet to find a replacement on a scrap board ...

Looks like we're quite a few with the same problem though

 

[AndyMcDandyCDN]

Actually I haven't looked into it since last time ...
I didn't replace the MB3712 with the bootleg one as I want to keep this one as a working reference, I have yet to find a replacement on a scrap board ...

Looks like we're quite a few with the same problem though

So I did some further investigation and found four bad resistors, R29-R32. I replaced them so now the connected IC is getting data signals again but still no sound. I went through the Pengo manual again and noticed it references IC51 & IC70 as cause for no sound. Used my probe tester and I think my IC70 might be the cause but I am still learning so not sure what each pin should read. I will attach the file in case anyone wishes to look at it.

 

[AndyMcDandyCDN]

Hello everyone, just an update on my progress to get the sound working.

I noticed C23 had no logic probe activity on the audio amplifier side but did have a low on the opposite side. Also noticed the same with C24.

Replaced both and at first both had logic probe activity on both sides of the cap. Still no sound. When I turned the board back on I noticed that now C22, 23, 24, were now all acting the same way, Logic probe activity on one side but not the other. Replaced all three and the new MB3712 (I had bought a 2pk) just incase my soldering damaged it which lead to these cap issues then powered on the board. C23 and C24 doing the same thing. Used volt meter and there was no voltage on both of them.

Replaced C23/24 and once again, strange happenings. Voltage reading with C22/23 but none on C24. However, logic probe detects nothing on amplifier side of C22/23 and high for both side of C24.

I'm stumped. Even my electronics friend who gave me the needed caps for C23 is stumped. Should I try using a jumper wire to bypass those three caps just to see what happens or would that ruin the MB3712 chip?

 

[ArcadeTechGW]

Don't bypass the caps.

Did you lift a pad or a via when you soldered?

 

[AndyMcDandyCDN]

Not that I noticed.

 

[DogP]

Are you trying to use a logic probe to check an analog audio signal? If so, that's definitely the wrong tool for the job... I'd suggest making an audio probe. Using an amplified speaker (I use one like this: https://www.radioshack.com/products/radioshack-mini-audio-amplifier , but you could use an old set of computer speakers for example), connect the GND to GND on your PCB, and the audio input to the various points in the audio circuit that you want to check (you can cut off the end of the cable, adapt from a 3.5mm plug, alligator clip to the plug, etc). If you think you have sound at C23, touch the probe there and you should be able to hear it. If not, move back further in the circuit and hopefully you'll find where it's lost (or find that it's not coming out of the source at all).

A logic probe (as the name implies) is for checking logic levels. An audio signal that's AC coupled through a capacitor isn't a logic 1 or 0.

DogP

 

[AndyMcDandyCDN]

The only old set of speakers I have that I no longer use which have a 3.5mm plug are these little square ones that also have a USB plug for power. Would they work?

 

[DogP]

Yeah, pretty much any powered speaker that takes in unamplified audio should work.

DogP

 

[AndyMcDandyCDN]

Ok. I will try and find them tomorrow and give your suggestion a try. Ty.

 

[AndyMcDandyCDN]

Yeah, pretty much any powered speaker that takes in unamplified audio should work.

DogP

You are a genius. I found those USB powered speakers with the 3.5mm jack and soldered the wires to a couple of spring clips. I did as you said, connected one to the ground and the other to C23. On the side I figured was dead which is side going to the MB3712, nothing, but on the other side of C23, perfect audio.

 

[DogP]

You are a genius. I found those USB powered speakers with the 3.5mm jack and soldered the wires to a couple of spring clips. I did as you said, connected one to the ground and the other to C23. On the side I figured was dead which is side going to the MB3712, nothing, but on the other side of C23, perfect audio.

Excellent! So either C23 is completely open circuit, or U87 (MB3712 amp) is dead and killing the signal on that side of the cap. If you touch your finger to the pins of the amp, do you get any buzzing sound out of the main cabinet speaker? If so, the amp is probably working fine.

I know you said you've replaced the caps and amp, but if you have signal on one side of the cap, but not the amp side, and the only thing at that point is a cap and amp, the problem pretty much has to be one or the other.

DogP

 

[AndyMcDandyCDN]

Excellent! So either C23 is completely open circuit, or U87 (MB3712 amp) is dead and killing the signal on that side of the cap. If you touch your finger to the pins of the amp, do you get any buzzing sound out of the main cabinet speaker? If so, the amp is probably working fine.

I know you said you've replaced the caps and amp, but if you have signal on one side of the cap, but not the amp side, and the only thing at that point is a cap and amp, the problem pretty much has to be one or the other.

DogP

I was afraid you would say it has to be one of those two. I ran my finger across the pins and nothing. I guess it is possible that both MB3712 chips were bad since I bought a two pack from ebay and tried both while going through like 4 caps.

I'll try and find another supplier. Hopefully I have better luck.

Thank you so much for your help and I'm looking forward to trying that speaker trick on my Amidar and DK Jr boards.

 

[DogP]

Rather than running your finger across them, try putting your finger across all the pins at once... and if they're dry, maybe lick them first (or run them under water if you want to be civilized ;-) ).

But yeah, there's not much to the circuit... personally, I'd pull C23 and test it, since it's really easy to check (multimeter on capacitance test, even if the reading is pretty far off, at least make sure it's still a capacitor and not an open circuit).


I assume you don't still have your original amp? And I assume you've confirmed that +12V is good at pin 2 of the amplifier?

DogP

 

[AndyMcDandyCDN]

Nothing with wet finger across all pins. Voltage reading is 14v.