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Old 30 April 2011, 20:33   #1
virge
 
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Amiga 4000 Sound Distorted and low

Hi All.
I have an Amiga 4000 with low and distorted audio.
Last week the U198 Regulator blows ... i have changed the U198 (7905) and I decided to change all the capacitors.
Them problem now is that sounds distorted and low ... i have to put my Amplifier on max to listening audio on amiga... I have changed to another amplifier and that´s the same.
My problem now it´s the +12v.
The power supply gives me the +11.68v
External Floppy pin 23 +10.99v
Video port pin 22 +10.99v
Cable power to drives +11.68v
U402 (LM833) and U403 (LF347M) don´t have any +12v ... only have -12v

Help me....

Best Regards
Pedro
 
Old 30 April 2011, 21:38   #2
stachu100
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Quote:
Originally Posted by virge View Post
Cable power to drives +11.68v
U402 (LM833) and U403 (LF347M) don´t have any +12v ... only have -12v
Why?
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Old 30 April 2011, 21:54   #3
8bitbubsy
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11.68v is just fine... And the reason you don't measure -12v on the vcc on the opamps is probably because your multimeter (if you used one?) isn't updating >50 times a seond.. :P
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Old 30 April 2011, 22:03   #4
virge
 
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These two chips U402 (LM833) and U403 (LF347M) needs +12v to work and don´t have it
 
Old 30 April 2011, 22:09   #5
stachu100
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Quote:
Originally Posted by virge View Post
These two chips U402 (LM833) and U403 (LF347M) needs +12v to work and don´t have it
I know, but the question is "why +12V is missing".
It can be burned resistor, burned cooper track, defective vias.
Once you will find an answer you will repair your A4000.
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Old 01 May 2011, 01:31   #6
virge
 
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anyone repair this if i send it?
 
Old 01 May 2011, 04:23   #7
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There's a few people around the world who do repairs, you just need to find someone near Portugal. I won't speak for anyone, I'll let them offer.

The reason for your distortion is that one side of the audio is being clipped because of a missing rail.

Quote:
Originally Posted by 8bitbubsy View Post
11.68v is just fine... And the reason you don't measure -12v on the vcc on the opamps is probably because your multimeter (if you used one?) isn't updating >50 times a seond.. :P
It's a DC rail

Last edited by TCD; 01 May 2011 at 04:56. Reason: Back to back posts merged. Use the edit function.
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Old 01 May 2011, 14:10   #8
8bitbubsy
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Quote:
Originally Posted by Loedown View Post
It's a DC rail
And?
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Old 01 May 2011, 14:22   #9
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Ithink Loedown is saying that DC is a steady voltage and does not cycle at 50cps it requires a clock to create pulses
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Old 01 May 2011, 15:40   #10
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We're talking about a DC voltage supply here, not sure what all the confusion is about.. There is no clock or pulses (unless you are referring to the SMPS), and definitely no cycles.

Last edited by Hewitson; 01 May 2011 at 15:46.
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Old 02 May 2011, 00:52   #11
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My point is that it can swing from a negative value to a positive value rather fast, thus it might not be too easy to see if there's -12v going to it. Also, there might be som DC offset making it even more hard to spot a negative number, especially if you have silence with just a little interference.
If you're playing a tune, then it should be rather easy I guess, the multimeter should capture some negative values at some "frames".
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Old 02 May 2011, 01:24   #12
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Quote:
Originally Posted by 8bitbubsy View Post
My point is that it can swing from a negative value to a positive value rather fast, thus it might not be too easy to see if there's -12v going to it. Also, there might be som DC offset making it even more hard to spot a negative number, especially if you have silence with just a little interference.
If you're playing a tune, then it should be rather easy I guess, the multimeter should capture some negative values at some "frames".
Now this is getting confusing.
I think your now talking about Ocilliscope outputs?

From his first posts all his voltages are correct.
Its proberly damage from cap leakage.

The only voltage not mentioned is +5v.
All the other voltages may be a little lower, due to them being under load.
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Old 02 May 2011, 01:33   #13
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I have found that the C405 caps burned. it´s a -22uf i will change the C403 too.

Tomorrow i will give the results

Best Regards to all.
Pedro Henriques
 
Old 02 May 2011, 01:35   #14
gehtjanx
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Hope you get it running, good luck
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Old 02 May 2011, 01:37   #15
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Stick with it, Pedro. I have a feeling that this fault will be quite simple to rectify once the cause is found.
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Old 02 May 2011, 01:41   #16
virge
 
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Thanks prowler tomorrow i will try again. I dont have the caps here. I will buy it tomorrow

Thank you all
Best Regards
Pedro
 
Old 02 May 2011, 03:27   #17
virge
 
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Can someone give me the value of: Amiga 4000 motherboard

C403
C405
C407 ( 47 uF 16 V ? )

Tomorrow i will buy it.

Thanks and sorry for being so boring

:-P

Best Regards
Pedro
 
Old 02 May 2011, 04:56   #18
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Pedro, there is a schematic available here:

http://www.shiftreload.com.au/users/...cs/a4000Rb.zip

C403 = .22uF
C405 = .22uF
C407 = 47uF

Quote:
Originally Posted by 8bitbubsy View Post
My point is that it can swing from a negative value to a positive value rather fast
The voltage doesn't change!
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Old 02 May 2011, 12:41   #19
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Quote:
Originally Posted by 8bitbubsy View Post
11.68v is just fine... And the reason you don't measure -12v on the vcc on the opamps is probably because your multimeter (if you used one?) isn't updating >50 times a seond.. :P
Vcc isn't the output, get your damned terms right and stop sprouting utter bollocks!

http://en.wikipedia.org/wiki/Vcc

Furthermore, if no signal is present on the op amp inputs the output rail, even if it has DC bias will not swing unless the op amps have gone into self oscillation. The op amp LF347 relies on a voltage Vref and if one of the resistors in the voltage divider has gone open or closed circuit then the op amp can swing to its maximum output.
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Old 02 May 2011, 13:02   #20
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Wow, calm down... I stay corrected, but no need to be mean!
There's a reason I say probably when I'm not sure.

So I guess the op-amp takes -12v and +12v as vcc then? So he checked the -12v pin and there was no power?
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