(Topic ID: 206902)

WMS sys 3-4 sound board issue

By Theonlylilo

6 years ago


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TDA2002.png
TDA2003.png
Sound 3-4 scheme.pdf (PDF preview)

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#1 6 years ago

Hi everybody!! Accidentally I had a short between pin 4 and 5 of TDA 2002 amplifier.. I replaced it with a new one but now a big annoying buzz comes out from the speaker (a very very big noise). What's happened?
Please help me..
Tanks
Ciao
Loris

#3 6 years ago

I've tried another little speaker.. same problem. And also the socketed ICs like PIA, sound rom and 6808 just to eliminate possible causes.
Volume pot? Capacitors? Please help me to understand how it works and where I can check
I would like to try another TDA ampli... maybe mine is fault. Could it be?

#5 6 years ago

Hi Schwaggs! Thanks also to you for the reply.
The short has not been too long.. the tip of the logic probe has slipped away between pin 4 & 5 on the bench.

Quoted from Schwaggs:

Also make sure you test the board fully mounted to the game with mounting screws in place. I found that if the sound board is not grounded, strong feedback can result.

Correct, but that noise is very strong and tightening all the 4 screws doesn't improve the noise
The board was just recapped. All the electrolytic capacitors I mean. New capacitors, new sockets, new PIA...
Before the short the board working good... I'm stupiiiid!

Sound 3-4 scheme.pdfSound 3-4 scheme.pdf

#7 6 years ago

You're welcome!
In a couple of hours I try to change immediately R18 & R19 both, then I'll let you know!
Meanwhile here a little videoclip of the noise

#8 6 years ago

So, I replaced R18, R19, C14, C33 and C34.. the big humming is more quite, but is still annoying. Tomorrow I will replace C13, R24 and maybe I'll try another amplifier..
The situation is better but not enough..
Other suggestions?

#10 6 years ago

For the moment I'm testing the board at bench. It is not the first time, and with other boards I didn't have this problem. This same board worked fine at bench before the short..
At bench I'm using the pot on the board, bypassing the cabinet pot jumping pin 1 & 2 of J4

#11 6 years ago

Sorry for my stupid question... TDA2002 and TDA2002V are the same thing? Which means the final "V"? Could be this my problem? (I replaced the original TDA2002V with one 2002)

#13 6 years ago

Ok. I've changed C13, C30, C35 & R24.
As before, the noise is different and quiter but always present. I bought a TDA2002 but doesn't work.. so I used again my stored part and.. I'stucked here. So, suggestions?

#16 6 years ago
Quoted from Schwaggs:

It sounds like a typical amp with the gain cranked to "11"

Which is the meaning of this?

Quoted from Schwaggs:

Does the buzzing get quieter when the volume is turned down?
What type of capacitor is C12? Maybe try replacing that cap?

No, turning the pot the buzzing remains the same. The cap is the original 1 mfd, tantalum (but on the schematics is indicated as 0.1 mfd... double )

Quoted from barakandl:

sounds about right to me? Maybe a little bit "tinny" but is it that small speaker you are using on the bench?

Could be but as I said, other working boards have not this marked buzzing. About the "tinny sound"... yes could certainly be the little speaker.

Making an "experiment", raising the value of R19 (220 ohm insted 2.2 for example) the buzzing almost disappears, but the sound volume is halved...

#18 6 years ago

"Spinal Tap", never heard before but... funny!!! I've seen a couple of videos on youtube

So, I pulled out a leg of C12 and the sound (sound+buzz) disappears but not completely. A very slighty sound comes from the speaker approaching the ear). Restoringthe cap in position with finger, the sound is like before, always disturbed.
I've tried another TDA2002 from a working sound board but nothing changed.

About C12... from the components list is a tantulum 1 mfd but on the schematics is a 0.1 mfd. Who is right? I've a spare electrolytic 1 mfd-63V...

A technical question: I said that changing the value of R19 (from 2.2 to 220 ohm) the buzz disappear despite the sound volume halved. Which consequences would this modify has on the circuit? Maybe a 22 ohm...

#20 6 years ago
Quoted from Schwaggs:

You shouldn't have to mess with the gain of the amp to get this to go away. My only concern from ealier posts was that running this board without the remote resistor might be feeding too much gain into the amp but you said you tested other boards this way and the gain wasn't too high in those cases.

I also tested the board and the remote pot doesn't solve the problem. The disturbe comes from the speaker. Here another video where I test a working board.
You can listen than the sound is completely different.

Quoted from Schwaggs:

I wouldn't replace C12 unless it is for a test. You can safely test this with a 1uf electrolytic. That is what system 6 boards use for coupling capacitors. I would hang on the the tantalum cap and put it back in if the 1uf doesn't solve the problem.

Perfect!

Quoted from Schwaggs:

The fact that the buzz is reduced when the cap is removed suggests that the buzz is being introduced before the amp stage. Using your meter set on AC, measure the ripple on the 5V, 12V and -12V supplies.

Could you explain me how to do it? Multimeter set on AC then simply red on +5,+12,-12 and black on ground?

#22 6 years ago

I took the measures, after the fuses:
-12V = nothing
+12V = 27,2VAC
+5V = 10,3VAC
So my doctor, which is the diagnosis?!
What is the meaning of this "test"?
The bridge rectifier is new.. I've already change it, but before making short. I can change it another time, no problem!
Thanks, as usual!!!

#24 6 years ago

Really.. I don't know.. only put the multimeter probes as discussed, setted on AC. On the screen no dots for millivolts but the values of 27.2 & 10.3.. I added "VAC" because I thought the measures are in Volts
What can I do? Maybe I did something wrong with multimeter?

#26 6 years ago

I took a fast picutre to my multimeter and nothing about a range of V or mV... When I come back from work I want to measure the values on the working board for a comparison.
What can you say to me about ripple tension? Why I have to measure this?

WhatsApp Image 2018-01-15 at 13.48.01 (resized).jpegWhatsApp Image 2018-01-15 at 13.48.01 (resized).jpeg

#29 6 years ago
Quoted from barakandl:

Try putting like a 1-10ohm resistor in series with a probe.

I did it, but the values are the same.
Moreover as promised I took measurements on the working board and also in this case, are identical!
Tomorrow I want to replace my old spare tda2002 with a new one, and a new bridge rectifier (on monday many shops are closed).
But I really don't know what I can do or check

1 month later
#30 6 years ago

So guys, here I am with an upgrade. Finally I've checked other components, sometimes changing them but without solving the problem.
Thanks to my "experiment" (see post #16), I saw that changing feedback resistor R19 the sound was getting better. So to have a clean sound I put a 22 ohm resistor instead a 2.2 ohm and all the noise disappeared.
I really don''t know if what I have done is "good and right", but it seems to works fine

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