Amiga 4000 Sound Distorted and low

30 April 2011, 19:33

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

stachu100 · 30 April 2011, 20:38

Quote:

Originally Posted by virge

Cable power to drives +11.68v
U402 (LM833) and U403 (LF347M) don´t have any +12v ... only have -12v

Why?

8bitbubsy · 30 April 2011, 20:54

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

30 April 2011, 21:03

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

stachu100 · 30 April 2011, 21:09

Quote:

Originally Posted by virge

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.

01 May 2011, 00:31

anyone repair this if i send it?

Loedown · 01 May 2011, 03:23

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

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

8bitbubsy · 01 May 2011, 13:10

Quote:

Originally Posted by Loedown

It's a DC rail

And?

kipper2k · 01 May 2011, 13:22

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, 14:40

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 · 01 May 2011, 23:52

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, 00:24

Quote:

Originally Posted by 8bitbubsy

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.

02 May 2011, 00:33

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

Tomorrow i will give the results

Best Regards to all.
Pedro Henriques

gehtjanx · 02 May 2011, 00:35

Hope you get it running, good luck

prowler · 02 May 2011, 00:37

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

02 May 2011, 00:41

Thanks prowler

tomorrow i will try again. I dont have the caps here. I will buy it tomorrow

Thank you all
Best Regards
Pedro

02 May 2011, 02:27

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, 03:56

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

My point is that it can swing from a negative value to a positive value rather fast

The voltage doesn't change!

Loedown · 02 May 2011, 11:41

Quote:

Originally Posted by 8bitbubsy

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, 12:02

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?

30 April 2011, 19:33	#1
virge Posts: n/a	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

01 May 2011, 14:40	#10
Hewitson Registered User Join Date: Feb 2007 Location: Melbourne, Australia Age: 41 Posts: 3,773	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 14:46.

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30 April 2011, 20:54	#3
8bitbubsy Registered User Join Date: Sep 2009 Location: Norway Posts: 1,711	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

30 April 2011, 21:03	#4
virge Posts: n/a	These two chips U402 (LM833) and U403 (LF347M) needs +12v to work and don´t have it

01 May 2011, 00:31	#6
virge Posts: n/a	anyone repair this if i send it?

01 May 2011, 13:22	#9
kipper2k Registered User Join Date: Sep 2006 Location: Thunder Bay, Canada Posts: 4,323	Ithink Loedown is saying that DC is a steady voltage and does not cycle at 50cps it requires a clock to create pulses

01 May 2011, 23:52	#11
8bitbubsy Registered User Join Date: Sep 2009 Location: Norway Posts: 1,711	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".

02 May 2011, 00:33	#13
virge Posts: n/a	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

02 May 2011, 00:35	#14
gehtjanx Banned Join Date: Apr 2011 Location: Vienna / Austria Posts: 134	Hope you get it running, good luck

02 May 2011, 00:37	#15
prowler Global Moderator Join Date: Aug 2008 Location: Sidcup, England Posts: 10,300	Stick with it, Pedro. I have a feeling that this fault will be quite simple to rectify once the cause is found.

02 May 2011, 00:41	#16
virge Posts: n/a	Thanks prowler tomorrow i will try again. I dont have the caps here. I will buy it tomorrow Thank you all Best Regards Pedro

02 May 2011, 02:27	#17
virge Posts: n/a	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

02 May 2011, 12:02	#20
8bitbubsy Registered User Join Date: Sep 2009 Location: Norway Posts: 1,711	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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