TECH: Comet Sound problem: Only CPU sound rom makes sound?

[mathew...@gmail.com]

My dad got this Comet almost 2 years ago and it's NEVER had all the
sound. We've been buckling down lately and trying to get the thing in
the best working order we can. We're stuck on this sound problem. The
game has always played Music and made general sounds. I'll try to list
all the sounds it makes if it's important. I just ordered the complete
set of new sound roms from John Wart Jr. and replaced them all and
still...we get the exact same thing. Without the extra sound board
installed we get no sound at all. If we install the extra sound board
but not the chip on the main CPU board we get nothing. In an attempt
to figure out wich rom was at fault (because we do get a decent ammount
of sound I thought probably just 1 chip was bad) I started pulling 1
chip at a time. Until I had all 4 chips pulled off the sound board.
We still get all the same sounds.

So apparently the Sound board needs to process everything (because as
previously stated we've tried it without that board) and BUT there's a
problem on that board or somewhere that's causing those 4 chips on it
to not be accessed or something.

Anyone have any ideas?

Thanks as always!
Matt

[K's Arcade]

Did you try turning the mixing pot on the speech daughter board? With
the pot turned in one direction you will only have the sound from the
cpu. Turn it the other way and you will have the speech. Find a balance
where the speech and sounds are about the same volume. If adjusting the
balance pot does not do anything and you still do not have speech then
you will need the speech board repaired or replaced. K

[mathew...@gmail.com]

DOH! with all the reading up I've done on this I should have pointed
out that yes, we've done that. I always skip the small stuff. ;)

[beaver]

Not top posting this time due to various different question. See
below.

Allroy1975_@_yahoo.com wrote:
> the best working order we can. We're stuck on this sound problem. The
> game has always played Music and made general sounds. I'll try to list
> all the sounds it makes if it's important. I just ordered the complete
> set of new sound roms from John Wart Jr. and replaced them all and
> still...we get the exact same thing. Without the extra sound board
> installed we get no sound at all.

Comet is a System 9, a CPU I know quite well. Except for speech, sound
is produced by the D/A on the CPU board. This is then passed through
the auxiliary board, where it is mixed with the speech, and fed back
into the CPU board for amplification to the speaker. Locate U1 on the
speech board. Is there a pulsing on pins 9 and 12? If so, is there
speech output on U1 pin 3? This will tell us if the problem is on the
speech board.

>If we install the extra sound board
> but not the chip on the main CPU board we get nothing. In an attempt
> to figure out wich rom was at fault (because we do get a decent ammount
> of sound I thought probably just 1 chip was bad) I started pulling 1
> chip at a time. Until I had all 4 chips pulled off the sound board.
> We still get all the same sounds.
>
> So apparently the Sound board needs to process everything (because as
> previously stated we've tried it without that board) and BUT there's a
> problem on that board or somewhere that's causing those 4 chips on it
> to not be accessed or something

That is correct. The sound board is needed or the path will be broken

[mathew...@gmail.com]

>
> Comet is a System 9, a CPU I know quite well. Except for speech, sound
> is produced by the D/A on the CPU board. This is then passed through
> the auxiliary board, where it is mixed with the speech, and fed back
> into the CPU board for amplification to the speaker. Locate U1 on the
> speech board. Is there a pulsing on pins 9 and 12? If so, is there
> speech output on U1 pin 3? This will tell us if the problem is on the
> speech board.

do this with a logic probe, right?

[beaver]

Yes, you need at least a logic probe. A meter will be too slow. Let
me know what you find.

Edward Cheung CARGPB26
www.edcheung.com

[mathew...@gmail.com]

alright, I got pulsing on 9 & 12 when there SHOULD be voice, however,
nothing ever pulsed on pin 3. Since you said the speech should come
out on pin3 I'm guessing there's a problem with that U1 chip?

Thanks again Ed.

Matt

[beaver]

Pulsing on 9 and 12 means everything upstream is fine (four sound ROMs,
sound CPU on main board, etc). The problem is on the speech board. U1
is the D/A chip. The output on pin 3 is analog sound. Make sure pin 1
has 5V supply on it. If that is good, put a scope on the analog output
to see if there is anything. If you don't have one, you can rig a
small audio amplifier with speaker to connect to the output pin. Or
you can wire it directly to the amp input on the CPU board at 1J16-2.

If you are lucky, U1 is fine, but you have an problem with the dual
op-amp U3. I repaired a speech board last month with that same
problem. To find out more, you need a scope or a small audio amplifier
as mentioned before.

In any case, speech boards are not expensive. They usually appear on
ebay and have no bidders. I have a tested one that I could sell you
too.

[mathew...@gmail.com]

Ed my man, you are very good indeed. I checked that Pin1 and I was
indeed getting the +5v. I replaced the little dual amp chip at U3.
Now I have almost all the sounds. I'm missing the crowd cheer at the
beginning of the game and I'm pretty sure it's the same sound that
plays when you exit the comet ramp. Not sure what the problem is now,
but I'm a LOT further than I was. You RULE!

thank you VERY VERY much. :)

Matt

[beaver]

I am glad to hear good results!

During the sound test, the CPU plays one or two sounds from each ROM
chip. If you are missing particular sounds, you may have a problem
with one particular chip. To find out more, perform the sound test
with one ROM chip missing. If the result is the same as with all chips
in, you have most likely found the bad chip. Of course it could be a
socket issue, etc. I have not run sound test with ROMs swapped, so I
do not know if you can diagnose a bad chip by swapping ROMs.