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-   -   Amiga 4000 Sound Distorted and low (http://eab.abime.net/showthread.php?t=59008)

virge 30 April 2011 20:33

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 :crying

Help me.... :help

Best Regards
Pedro

stachu100 30 April 2011 21:38

Quote:

Originally Posted by virge (Post 752976)
Cable power to drives +11.68v
U402 (LM833) and U403 (LF347M) don´t have any +12v ... only have -12v

Why?

8bitbubsy 30 April 2011 21:54

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

virge 30 April 2011 22:03

These two chips U402 (LM833) and U403 (LF347M) needs +12v to work and don´t have it :(

stachu100 30 April 2011 22:09

Quote:

Originally Posted by virge (Post 753008)
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. :great

virge 01 May 2011 01:31

anyone repair this if i send it?

Loedown 01 May 2011 04:23

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 (Post 753002)
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 :nuts

8bitbubsy 01 May 2011 14:10

Quote:

Originally Posted by Loedown (Post 753092)
It's a DC rail :nuts

And?

kipper2k 01 May 2011 14:22

Ithink Loedown is saying that DC is a steady voltage and does not cycle at 50cps it requires a clock to create pulses

Hewitson 01 May 2011 15:40

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.

8bitbubsy 02 May 2011 00:52

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".

FOL 02 May 2011 01:24

Quote:

Originally Posted by 8bitbubsy (Post 753307)
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.

virge 02 May 2011 01:33

I have found that the C405 caps burned. it´s a -22uf i will change the C403 too.

Tomorrow i will give the results :banghead

Best Regards to all.
Pedro Henriques

gehtjanx 02 May 2011 01:35

Hope you get it running, good luck

prowler 02 May 2011 01:37

Stick with it, Pedro. I have a feeling that this fault will be quite simple to rectify once the cause is found.

virge 02 May 2011 01:41

Thanks prowler ;) tomorrow i will try again. I dont have the caps here. I will buy it tomorrow ;)

Thank you all
Best Regards
Pedro

virge 02 May 2011 03:27

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

Hewitson 02 May 2011 04:56

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 (Post 753307)
My point is that it can swing from a negative value to a positive value rather fast

The voltage doesn't change! :banghead

Loedown 02 May 2011 12:41

Quote:

Originally Posted by 8bitbubsy (Post 753002)
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.

8bitbubsy 02 May 2011 13:02

Wow, calm down... I stay corrected, but no need to be mean! :mad
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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