A1200 crippled audio

Ok so I'm almost sure this will get me the "caps!" answer but it never hurts to verify :)

The problem: Crippled audio from a A1200 board I've been testing

Observation details: Loaded up Pinball Dreams and right away the sound was crippled, crackling like some paper was torn in the background, similar to what one gets when the "tops" of a waveform go beyond the maximum and are clipped.

Finer observation: This somehow seems to affect only one channel, but in the following way: When e.g. the left channel plays, the sound is crippled as described. But there was a section where sound only came from the right channel, and it sounded absolutely fine. Just as soon as the left channel kicked in though, the volume level on the right channel seemed to sink down to practically zero and of course the sound came out all scratchy and crackling and torn, although here and there bits of clean audio could still be heard among the mess.

Tested:
-12 V on the PSU input is fine
+12 V and -12 V on the middle pins of LF347M measure +12.05 and -11.86 respectively, so they're also fine

Is it still possible that the LF347 chip has gone bananas and what's the best way to verify this?
Or
Should I just replace the 22 uF caps?
Or
Something else I have no idea about?

Thanks!

@BLTCON0

its VERY unlikely to be the LF347 unless there has been a very serious sustained spike in the + / - 12v

As you know the likely cause is rthe two 22uf Capacitors that are notorious in dying with exactly the symptoms you describe. As you also know this is a common fault so man up my friend, and get them caps replaced =)

Thanks :) I was concerned the +12 and -12 voltages on the LF347 might measure ok yet there could still be damage to the inputs/outputs.

So two more questions:
a) what's the proper tool for those SMT caps? some precision hot-air device? I have only dealt with through-hole ones so far and also the location behind the keyboard connector of one of these isn't very promising for my standard soldering iron.
b) could I solder regular 'legged' ones (e.g. the ones from an A500) for verification and as a short-term solution until I get a proper replacement set or the pads are too tiny to provide adequate mechanical support?

I should also add that initially the board was tested in a tower configuration with an Indivision AGA and a Lyra II adapter installed, and apart from the sound problems it exhibited serious discoloration on the RGB port (it was simultaneously connected to a CRT via the Indy and an LCD TV via RGB-to-SCART). The picture was perfect via the Indy but discolored on the TV. After I removed the board and tested it on its own without the Indy or the Lyra, the discoloration persisted for a while then went away.
The nature of the discoloration was let's say a distortion of colours towards blue-purple-pink shades and strong quantisation of tones. Looked like as though the green element had gone away completely. Any idea on that?

Hi there BLTCON0,

you could also check R332 and R334 which are behind the audio sockets and between the two 22uf caps. I had a board that sounded much like yours and it turned out that one of the resistors were broken although they both looked physically ok, they are 1K resistors and in a bad place to get to with a soldering iron ie keyboard connector and audio sockets (lots of plastic to avoid).

I used thru hole caps to replace all the electrolytic capacitors, you just form the legs to an L shape to fit where the old cap was, the only downside is that you wont be able to fit a top shield onto the board (if it actually had one). You could always form the legs so that the cap could lay on its side instead of standing up prior to soldering it in place too (I did on a couple of 1200s).

A hot air station with a small diameter nozzle works fine, if you look on fleabay though, there is a nozzle with 2 holes and is adjustable (one fixed and the other can be moved from 5mm to 10mm distance), that is something that I would have liked when I did mine. A lot of folk have just used pliers and twisted the SMT caps a quarter turn to remove the body and used a soldering iron and tweezers to remove the legs from the board, I have never tried it myself though, maybe worth trying on a duff circuit board to get the hang of it (same for using a hot air station).


I don't know much about your colour problems but I wouldn't be surprised if it was also the caps next to the video chip, it makes good sense to replace all the electrolytic capacitors in one go. If there is any signs of leakage, then clean up the board before removing them.

Zetro recommended using a dishwasher set at 50 degrees (no soap etc) and stopping it before the dry cycle starts to prevent further problems, then stand the board on edge and let it dry off for a couple of hours before removing the caps (this is from memory so it may be slightly wrong).

From what I experienced before knowing about removing the leaked residue, is that it can cause the circuit board to overheat in the area where the residue is and actually burn the board (I was using a hot air station).


It is also important to shield nearby plastic parts form either a soldering iron or the hot air if you go that way, I used metal strips cut from a duff computer CD players case and formed them to fit around the keyboard connector, IDE connector and the Audio connectors plus the power connector for the floppy drive and the PCMCIA slot where another cap is. The shields can always be used again in the future and cost nothing to make.

Realistically, if I didn't already have the thru hole capacitors then I would have definitely gone for this :-

http://www.amibay.com/showthread.php?t=4400

The right parts for the job and I would also recommend a hot air station even a second hand one, they are pretty amazing things to use :thumbsup:

"caps!"


:lol:

"lock"

:) Sorry, could not resist!

Just a thought, you tired wiggling the phon socket. mine did as described, just a loose solder joint in the end

When replacing the audio caps, I would suggest using non-polarized caps

Commodore made a mistake in the audio circuit - the audio circuit by its design alternates voltage from -12v to +12v (thus you need atleast a 25v rated component) Using an eletrolytic capacitor in this circuit is elementary wrong by design and will reduce the life span considerably.

You should look at replacing this with a non-polarized / bipolar capacitor

Farnell do stock through-hole 22uf BiPolar (non polarized) 35v Capcitors (35v would be at the top I would want to go as the potential charge would be 35volts @ 22uf which is more than is needed for the circuit.

I would recommend these for those whom have yet to build up skills in the SMD soldering - also its wise to mention that while you might find some on eBay with a good enough look... you may struggle to find a 22uf @ 25volt SMD based bipolar / non-polarized capacitor. (the highest Farnell stock are 16v in the SMD range which is two low.)

Thanks people, I've got some homework to do with the extensive info provided here! I'll update as results come by.

JuvUK wins the "most unexpected reply" prize :lol:

Update:
The voltages on the left-channel side of LF347 aren't good. As I had no idea what they should be, I measured them against my A1200D (where all pins are +2.46 V at idle) and I get:

Pin12 (AVREF - IN4+): 2.46 V (therefore seems ok)
Pin13 (Paula pin34 - IN4-): 0.27 V (??)
Pin14 (output to Pin3 - OUT4): 11.69 V (??)

(pins 1-3 out of order too, 11.25, 11.25 & 11.65 V, but they're further down in the schematics so could be a result of the above)

On the schematics there's R321 = 1.5 K between Pin13 & Pin14 but the actual resistor is marked 681 aka 680 Ohms, what's with that? Mistake in the schematics?
Anyway it indeed measures 675 Ohms as should on my A1200D, but open loop on the 'bad' board. Also, continuity between pins 13 & 14 and R321's pads is perfect, as is between the adjacent C321 capacitor, so I'm hoping it's just the resistor. Will replace and see.

(see photo) this I totally didn't observe when initially inspecting the board, but only after the measurements indicated something's wrong. The photo is bad, but a 'scratch-type' deformity can be seen on R321, making the 681 marking a bit illegible. R331 (its right-channel twin) looks and measures perfect on the other hand. Sign of burnt resistor?