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Old 23 June 2010, 01:44   #1
jimbob005
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need help on a4000 audio

hi all
I know zetr0 say he would have a look for me but i was thinking if its something easy i could do it my self so i started ..

now i found this on the net
http://amiga.serveftp.net/audio_repair.html

then got out my multimeter and started testing the LF347 and i was getting -9.8 on pin 1,2,3,5,6,7,8,9,10,12,13,14 on pin 4 was +12 and 11 was -12

but i tested the vref i changed the caps and still getting -12 from it

so i tested the ohms on pin 13-14 and got nothing as i was thing that the lf347 was gone any was i had another one that tested ok so i desolderd it and poped the other one on and fixed the broken pads and tracks

but now still no sound but im now getting +12 on pins 1,2,3,5,6,7,8,9,10,12,13,14 still the same on pins 4 and 11

any one help?
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Old 23 June 2010, 01:50   #2
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Hiyas Jimbob,

Good on you for having a go m8!

alas if you have followed that guid and reading the above its quite likely Paula has died... does the mouse work and the joy-1 firebutton ? I believe thats all configured to Paula.

I will hit up some specs and see if I have any ideas =)
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Old 23 June 2010, 01:52   #3
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mouse and joy ports are ok
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Old 23 June 2010, 03:19   #4
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I am hitting up the specs now my friend...

I would say the lynch-pin of the audio circuit is U402: LF347 (JFET Quad OpAmp) - its possible this is grouding out in some way on the tracks...

Its also possible that U403: LM833 (Dual OpAmp) feading each of the Audio outputs coule be frazzled.

something tells me this could be a grounding issue or a damaged track.

good luck m8
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Old 23 June 2010, 10:30   #5
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I had this problem a while back and it took some time to fix. I replaced all the caps and both U402 and U403. I also had a damaged track on the bottom of the motherboard. If I were to have this problem again I would start with checking all the tracks in the area.

The guide you are referring to was very helpful, and the fellow who wrote it also helped me immensely by email.
 
Old 23 June 2010, 19:10   #6
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would it be posable to remake this off board and just take the relavie signals from the board to power it???
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Old 23 June 2010, 19:18   #7
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@James

you sure can - all you need is 1 quad op-amp and 1 dual opamp + sundry discretes, do you need a schematic of the existing circuit?
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Old 23 June 2010, 19:19   #8
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like this http://amiga.serveftp.net/images/Aud...atic-large.jpg
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Old 23 June 2010, 23:12   #9
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yep thats the one =)
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Old 24 June 2010, 23:40   #10
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hurrray i got sound not very good sound but i got it coming from the left and the right

iv changed the u402 and the u403 found the track from u400 the right in there was nothing so i poped a bit of jumper wire from r440
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Old 29 June 2010, 18:43   #11
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still not got any where with this iv got sound but very bad been on this .. iv spent meny hours trying to find the problem and carnt see anything
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Old 30 June 2010, 01:21   #12
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There was a schematic with all the voltages written down, audio is very simple, there's nothing complex about it. Check your rails properly, check the biasing circuit, follow the sound source logically and have a constant sound in both channels, so that whatever is the issue, it can be seen and followed, not just random music playing.
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Old 30 June 2010, 02:33   #13
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@Jimbob005

its quite likely you have interference on the circuit of the motherboard, it might help to seperate the Left/Right Audio out from Paula but cutting the tracks on the motherboard.

This is an extreme solution so only do simple light cuts that could easily be bridged with solder later.
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