A1200 audio problem

[elspero]

So, my A1200 arrived, but the audio isn't working properly:
low sound, distorted, popping sounds, you name it.
Both RCA connectors and RF connection have the same problem.

I replaced C303, C304, C324 and C334, but nothing has changed.

The power brick is working properly, the -12V is supplied.

I need the readings of C324 and C334: in my board they both sign -7v, is this correct or should it be -12v? In case the -7v is incorrect, what could be the cause?

C303 and C304 are both at +12v.

The board is a 1D.1 revison.

Any idea?

[JLPedro]

Maybe the Paula chip is faulty, or not getting enough voltage. Did you noticed any other symptom as mouse erratic moving...

[kipper2k]

Paula sounds suspect to me too, I'll meter my board when i get home and see what voltages i read

[elspero]

@JLPedro: the mouse is working fine, no problem at all.

@kipper2k: Thank you!

How can I check if Paula is working fine?

[stachu100]

It must be -12V, not -7V.
Please check your PSU or try on another PSU.

[kipper2k]

Originally Posted by elspero

@JLPedro: the mouse is working fine, no problem at all.

@kipper2k: Thank you!

How can I check if Paula is working fine?

When i had similar issues i just removed Paula, soldered a PLCC socket and used a Paula from an old board, that worked for me, but i wouldn't go that far yet.

[elspero]

Originally Posted by stachu100

It must be -12V, not -7V.
Please check your PSU or try on another PSU.

I tested the PSU, and it supplies the -12v correctly.
I tried with another PSU, same results.

So, looks like something is wrong between C324, C334 and Paula?
The tracing on the board are difficult to follow, does anyone have a list of how the components are chained together starting from Paula and going to the RCA outputs?

[kipper2k]

Heres Stedy's website with the schematics...

http://www.ianstedman.co.uk/Amiga/sc...chematics.html

[elspero]

Thank you for the schematics, kipper2k.
Truth to be said, I don't really know how to read them 100%.

I replaced C306, but nothing has changed.
I tested R324, R334, R327, R337 and everything looks good.
U15 to R302 is ok. R302 is ok.

I am stuck. What now?

[kipper2k]

Originally Posted by elspero

Thank you for the schematics, kipper2k.
Truth to be said, I don't really know how to read them 100%.

I replaced C306, but nothing has changed.
I tested R324, R334, R327, R337 and everything looks good.
U15 to R302 is ok. R302 is ok.

I am stuck. What now?

there is a way you can jump the traces and feed the output directly to the L/R capacitor , that will prove good/bad traces. Theres a thread on it somewhere, i'll try to track it down in a bit

[kipper2k]

What was the rating on the c324 and c334 that you used as replacements? They need to be 22uf @25v

I think all the caps on the audio circuit should be 25v

heres the digikey number for the 22uf i use

http://search.digikey.com/scripts/Dk...e=PCE3009CT-ND

[elspero]

I used electrolitic, not SMD, 22uf 25v.

[kipper2k]

k, gotta ask, did you put them in the right way ?, can you take a close up of the board

[elspero]

Yeah, the polarity is right, I checked it before and after soldering, using a picture as reference.

[elspero]

If the op-amp readings of this site: http://amiga.serveftp.net/audio_repair.html apply to the A1200 too, I think I found what's wrong. The readings of the LF347 with the A1200 powered on are completely off.
ie, where it says 2.5v, it's -4,5v.
But from where does the op-amp get the power?

[kipper2k]

Pin 4 is +5v and pin 11 is -v, which i am assuming is being fed from the -12v rail and maybe fed through a voltage divider to supply -5v to it. The outputs for the left and right channel are pins 1 and 7 respectively.

For both channels to produce a noisy output then chances are the problem exists before c324 and c334, either originating from the LF347 or the Paula chip itself.

I would at this stage begin to think its Paula as that would be the weakest link in the chain as regards sensitivity to frying. Its almost at the point of swapping out components. One thing you can try... If you have a macro option on a digital camera, take some real close up shots and examine the components and board for micro fractures, you will be amazed at just how much you can blow up the board with todays cameras.

[Loedown]

Originally Posted by kipper2k

you will be amazed at just how much you can blow up the board with todays cameras.

He wants to fix it Kipper, not blow it up

Ok, let's get a few things straight, arbitarily changing caps solves nothing, poking around when you really don't know what you're doing solves nothing. The Amiga audio circuit isn't complex and it isn't difficult to comprehend. C324 & C334 should be NON POLARISED PARTS as is even stated in the diagram I have ( little NP after the parts ). Some simple voltage checks will give you the answers you need. First point of call, check your voltage rails for + / - 12V, then check your Vref voltage this should be very close or exactly 2.5 volts. These first points give a starting basis that things are ok. If all that is good then start fiddling around with the op amps and checking their voltages. Check the voltages on the pins of the actual op amp LF347. Ignore the MPF102 FETs, they are just used to engage the filter.

[elspero]

@Loedown:
The +12v and -12v, tested on R301 and R302 are 11.75v and -12.11v
AVREF tested on C306 is 2.42v

The voltages on the op amp LF347 are:
(1)-4,96 (2)-4,97 (3)-4,45 (4)-5,54 (5)-4,46 (6)-4,95 (7)-4,96
(14)-4,53 (13)-0,36 (12)+2,41 (11)-12 (10)+2,41 (9)-0,42 (8)-4,55

The Pin 4 should be +12v, right?

[kipper2k]

hows your soldering skills, a replacement LF347 is $1.09...

http://search.digikey.com/scripts/Dk...=497-2963-1-ND


and a replacement PLCC Paula at http://www.vesalia.de/ is e8.90,

if you have the tools its a 30 minute job.

btw, did it look like the capacitors had already been changed, it is possible a trace has been damaged out of sight once the caps are on the board, but again it has to be a trace thats common to both outputs thats why i am still leaning towards Paula now.

[kipper2k]

Max rated voltage in for the LF347 is +/- 18v. Pin 4 is the +ve input


www.national.com/ds/LF/LF347.pdf