(Topic ID: 248886)

Help identify a component


By Nihonmasa

75 days ago



Topic Stats

  • 11 posts
  • 4 Pinsiders participating
  • Latest reply 74 days ago by vec-tor
  • No one calls this topic a favorite

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There have been 4 images uploaded to this topic. (View topic image gallery).

S&T_SoundSettings.jpg
RapidFire_SoundSettings.jpg
2019-08-06_22h13_52 (resized).png
2019-08-06_21h44_45 (resized).png

#1 75 days ago

Hey there,

I am needing some help to identify the sound amp on a Vector. Have very low volume for voices.
Is it the U18? TDA2002?

Help very much appreciated to be able to order a replacement

#2 75 days ago

Hi,

Yes : I confirm that the U18 (component having 5 pins) is the audio amplifier.
TDA2002 is an obsolete component, but can still be found on some electronic shop.

But, if only voices are low, may be the TDA2002 is not the culprit.
Indeed : looking at the schematics, you can see that the voice is generated by the TMS5200 chip.
2019-08-06_21h44_45 (resized).png

Before to reach the audio amp, the signal goes thru a low pass filter (U13) and a pre-amp (VCA - Voltage Controled Amp. - U14).
Then the resulting signal is mixed with the sound source.
As you can see, half of U13/U14 is used for "voice" signal (top part of schematic), and the other half is used for "sounds" (bottom part of schematic).

You can try to change U18, but if the other part ("sounds") works fine, then, I doubt that this chip is defective.
Most probably there is a problem before, at the LPF or VCA. May be the VCA is not doing the correct amplification, or a defective capacitor in the audio (voice) line.

#3 74 days ago

Thanks a lot!
It is indeed only the voices.
Will try to change the 3 components then (U13/14/18).
Hope it'll fix the issue.
Sound fx are just fine, but voices are very faint.
Already played with the knobs with no luck.

#4 74 days ago

For information, TDA2002 can be replaced by a TDA2003 (still widely available) - 10W (a bit better than the TDA2002 - 8 W). Or even, a TDA2030 (18 W !!!). Both are pin compatible.
But as, sound fx are working fine, sure the TDA2002 is not the problem.

To understand the conception of the sounds generators of the Vector, take a look at this schematics:
2019-08-06_22h13_52 (resized).png

On yellow, you can see the generators : there is two main parts, the first is the "voice" produced by the TMS5200, and the second is the "sound/effects" produced by the PSG AY-3-819x (an additional DAC AD558 is also used, but mixed with the AY outputs).
So, you have two lines : one for "voice", one for "sound/effects"

First, both lines goes thru a LPF filter, marked with orange dash.

Then each line is pre-amplified by a separate VCA (amplifier controlled by voltage input), marked with blue dash.

At the end, both signals are mixed and sent to the TDA2002 amplifier, marked with purple dash.

Both VCA use a voltage as input, to control the amplification.
The source can be (selected by a strap), either an external potentiometer, or an internal DAC (marked with green dash).

I'm not an expert with this Bally sound board, but I see that there is some test points on it.
May be a good start for investigation, is to check if the voltage at the VCA test point (filled with cyan, on this schematic) is correct.
If the board is configured for external control, then changing the potentiometer should change the voltage.
Hope this will help for fixing this issue.

#5 74 days ago

I have had the preamps go bad.

#6 74 days ago

As usual noflip95 provides excellent clear explanations

One thing to note is the electrolytic capacitors on these boards are a common failure point. Have you changed them all? There are three 1uF DC offset capacitors in the speech path at C19, C24 and C25 which if any are going open circuit will cause loss of volume. Also there's two 330uF capacitors at C37 and C38 in the voltage doubler circuit used to generate the -5V power supply for the TMS5200 speech chip.

Quoted from noflip95:

Both VCA use a voltage as input, to control the amplification.
The source can be (selected by a strap), either an external potentiometer, or an internal DAC (marked with green dash).

Although it's supported (if jumpered) to adjust the volumes via software (internal DAC), Bally never used it because you couldn't properly play back audio while adjusting the software volume options to hear the effect so it was cumbersome to do. All Bally S&T are jumpered for external potentiometer volume adjustment.

#7 74 days ago
Quoted from Quench:

Although it's supported (if jumpered) to adjust the volumes via software (internal DAC), Bally never used it because you couldn't properly play back audio while adjusting the software volume options to hear the effect so it was cumbersome to do. All Bally S&T are jumpered for external potentiometer volume adjustment.

Except one OEM Bally Rapid Fire.

#8 74 days ago

Capacitors could indeed be the issue! Should have thought of it. Will start there.

@noflip95 thank you very much for those explanations and screenshot! Much appreciated!

#9 74 days ago
Quoted from vec-tor:

Except one OEM Bally Rapid Fire.

Cheers, learn something new every day. The Rapid Fire manual says:

RapidFire_SoundSettings.jpg

Meanwhile other S&T games of that era say:

S&T_SoundSettings.jpg

#10 74 days ago

Full capkit done: voices back to normal!
This teaches me to look at the simple stuff first amd never forget the capacitors!

Now on to solving my Gorgar sound issue...

#11 74 days ago
Quoted from Nihonmasa:

Full capkit done: voices back to normal!
This teaches me to look at the simple stuff first amd never forget the capacitors!
Now on to solving my Gorgar sound issue...

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