A1200 weak crackly sound - both channels went overnight!

[roy bates]

Originally Posted by 8bitbubsy

First I removed all the solder and components, then I used ... *drumroll*... Some kind of liquid you use to remove things that get stuck on your oven. It's a liquid that is kind of rough, so it scratches off the outer layer.
I just thought I'd go wild and try this since I saw the bottle, and it worked very well. You need to scrub like a maniac, but it works. It's not something I'd do on a PCB that works, but this mobo was toast anyways.

It was not a too clean job, but it wasn't dirty or messy either.

i dont know if youve heard of this,but i use "scotchbrite"i think thats what its called,its an abrasive pad(not steel wool)i use it alot to clean up traces that have broken, works great.

its used in the car trade for cleaning up the aluminium doors,i dont know where to get it now though,i had a box full of the stuff it lasts forever.

[8bitbubsy]

roy bates: Not heard of it, thanks for the tip!

Anyways, I'll just post this again for Photon so it's on the new page:

[roy bates]

actaully there is something i do alot,im assuming these are multi layer baords,what i do is if i cant tell where the traces go between layers,to make life a bit easer on myself,i hold it up to a "strong light" you can see where they go,between layers.

i have done it this way alot,and it does work.

[Photon]

Cool Bubsy! You mean you unsoldered all the components, chips etc? Cor!

But my problem right now is a bit different. See the blue circle in my photo above? That soldered via goes into the board, but I don't know where the internal layer it's attached to goes.

To ensure the damaged area inside is isolated, I'd like to cut off the trace that you can see going upwards between opamp pins 12 and 13, but first I need to know where it goes inside.


Edit: shining a light through the PCB only reveals a circle of light around the via. Does that mean it "goes to substrate"? Why would any trace go there? I might be shining through an internal copper layer, but I don't think that's possible?

Alternately, it's just a test point.

Edit: checked the schematic: C711+pad to substrate is the "noise" node. Strange that it's not there for the other channel.

[8bitbubsy]

ok, I'll grab my multimeter and find out where that via goes to inside the board.
EDIT: Sorry, can't find any points where it is connected to :/

[Photon]

Yeah, same here, seems to be some noise suppressor. Other end of C711 goes to some via under Lisa, made me stop bothering about it. It's probably designed this way and quite ok. It measures the same as the A1200 now, unlike before, so I don't have to cut it off.

Thanks for the photo btw, it helped me sort out an irrelevant matter with the transistor to the U28 (line driver) just now!

So I did another clean with alcohol and scotchbrite (or was it scotch and alcoholbrite?? it's all a haze...) and measured, comparing with my cherry A1200 mobo.

VREF is fine and dandy, -12V is 11.21V, weeelll...might be the best I'm gonna make it. Some input to U28 pin 3 is not 11.8V like it's supposed to be, but no wonder as it comes from the opamp pin which is cut off from +12V at the moment.

So it all seems to check out, tomorrow the brand new opamp goes in and I reconstruct the R/C331 connections and finally the replacement caps for C303/C304 at are now removed go in again.

Night

[Photon]

Hey, hey, I saved the world today...
Everybody's happy now.
The bad thing's gone away,
And everybody's happy now.
The good thing's here to stay
Please let it stay.

I fucking did it (Nope, didn't save a world but at least an Amiga mobo )

I was really skeptical about this as there was clearly damage inside and I wasn't sure I'd be able to isolate it... but with the new opamp and some reconstruction I have my A1700 working with sound again!

(Well the actual work performed won't impress a normal solderperson but why I'm so happy is that I defied my chickenshit ways and trusted my deductions and measurements enough to make the correct solution by cutting up my computer. With all the help and suggestions given by seasoned Amigans and walls of text by me so far, I thought I might as well share what I did to solve this unusual (perhaps?) variant of the A1200 leaky caps no sound issue.)

Unsoldered 220pF cap, solderbraid, brand spanking new opamp, and SOLDERHONEY!



Opamp in place using solder honey, pincer and 'poke tool' (screwdriver on top) to hold it down when you let go, honey is sticky!



Opamp soldered by heating its legs, showing also replacement for cut off -12V, C302, R302 (purple wire is from -12V over by the line driver)


Whaddya mean it looks improvised!? Haha, that resistor is huge but I chose to only use new components from my assortment boxes so... It will be properly hotglued and isolated with nice blue paper tape for that professional touch tho ahaha

[Edit: looks like joint point between 220pF cap and resistor is touching top of an SMD cap on the mobo. That is just the angle, it was floating in air when I took the shot. Second short purple wire looks like it's touch bottom of another SMD cap but it's not. The reconstruction for C302/R302 is resting on the RAM chips and not touching anything else. Also, I cut the -12V coming from the via at top where it say E301 for good measure, since the solderpad for R302 on the mobo wasn't very nice.]


Anyway. All in all it seems fine, ran two demos and it's lovely to hear sound again -12V dropped from -11.21 to -10.86 under load (after the 10 ohm resistor, mind) but I think that's quite okay.

O K COMPUTER


Babboon makes victory dance...

[prowler]

Well done, mate!

It's really great to see you succeed after all that hard work and perseverance.