Time Warp
Williams, 1979Hi 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
Pin 4 is the output to the speakers and pin 5 is +12v. Since the other end of the speaker goes to ground did you kill the speaker by shorting 12v across it? Do you have another speaker to try?
There are two transistors that do some amplification between the digital to analogue converter and the tda2003 amp, i wouldn't think a pin 4 to 5 short on the tda2003 would cause them to fail tho.
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?
Quoted from Theonlylilo: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?
Is it possible you burned the feedback resistors? I don't have schematics for that sound board but I suspect it may be similar to the system 6 circuit. There may be a 220 ohm on pin 4 that connects to a capacitor back to pin 2 and a 2.2 ohm from the 220 ohm to ground. Depending on how long the pin 4-5 short was in place, these resistors would have been fried.
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.
There should be a large (470uf) cap on pin 4. Does that look OK?
Have you recapped the board? Might be a good idea to recap the whole thing.
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!
Thanks for the schematic!
Check the feedback resistor network R18 and R19 and less likely R24.
Also check C13 and C14 to make sure they are OK. I suspect they are, especially if you used 16V or higher caps there.
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
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?
Have you tested the board in the game? What are you using for the remote volume control? Is it possible the gain is just too high using only the volume adjustment on the sound board?
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
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)
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?
sounds about right to me? Maybe a little bit "tinny" but is it that small speaker you are using on the bench?
It sounds like a typical amp with the gain cranked to "11". Does the buzzing get quieter when the volume is turned down?
What type of capacitor is C12? Maybe try replacing that cap?
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...
Quoted from Theonlylilo:Which is the meaning of this?
Its a joke from Spinal Tap, a "comedy" from way back.
Quoted from Theonlylilo: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
Then the buzzing is not from the volume being turned up too loud.
Quoted from Theonlylilo: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...
Try pulling one leg of C12 and see if the buzzing goes away. If it is still there, the problem is in the amp stage.
"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...
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 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.
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.
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?
You have it right. Basically what you are doing is measuring how much ripple voltage is making it past the power supply filter caps (on all voltages) and voltage regulator (on the 5v supply only).
Might be a good idea to test the bridge rectifier diodes too.
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!!!
Are you sure about the range of those AC measurements? Are you sure the meter wasn't reading millivolts? As in .0272 for the 12V supply? 27.2 volts of AC on that supply does not seem possible. You should see less than .25V of AC or 250 millivolts with proper operating filter capacitors.
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?
Quoted from Theonlylilo: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?
Look very closely at your meter, it should have a range indicator. Usually "V" or "mV"
I have seen similar looking meters that can give bad VAC readings on a test like that. Try putting like a 1-10ohm resistor in series with a probe.
Quoted from Theonlylilo: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?
If you have ripple voltage on your power supply, that ripple (slight variation in power supply output voltage) can be directly translated into noise on the amplifier output.
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
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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