A4000 sound problems

[abraXXious]

Hi guys,

I have been restoring an A4000D for the last few weeks and found it completed last night. So I set everything up on its new table/home, plugged in speakers and was greeted by some horrible sounds.

I am getting static from the audio outputs. Sometimes soft, sometimes loud scratching noises. If I use Eagleplayer etc to generate sound, the sound comes out on the right RCA, but very low in volume, but the left RCA is just pretty much static all the time.

I have checked connections, changed cables/speakers etc, so thats not the problem.

Now, I did a little research on the 'net and there seems to be one of two problems it could be.

[1] - the PSU cannot cope with the load it is now under.

I am running the A4000D, Cyberstorm Mk III with 128mb onboard, one internal floppy drive, a scsi 10,000 rpm 76 gb hd. A cybervision 64/3d with the optional scandoubler module, an IDE DVDburner, a DKB3128 with 128 mb onboard, and Adriane II ethernet card plus the A4000 has 16 mb fast on its mb.

So it is possible the psu cannot cope. Strange that everything else works perfectly except the sound, but from what Ive read that is very possible.

[2] - The A4000D MB has dead or failing caps. This is also possible, but the motherboard looks very clean, no signs of leakage or black spots etc.

Any tests anyone could suggest, or experience in similar matters etc would be very helpful.

Thanks,


Steven.

 

[AmiNeo]

Hey, seems like you've done a bit of homework. These are the most likely causes which you have identified. Look into getting a modded PC PSU for it so you can be sure shes getting enough juice...

From what I have learned so far from Zetr0, Merlin and the guys, the -12V (negetive 12 volts) line is essential to the audio circuit and it will be the first thing to show signs if something isnt working right with capacitors or power supply, so if you're still having the same problems after getting a better PSU it's most likely that a failing capacitor which could be in the audio circuit or indeed, as experience has recently taught me, the power area.

If a modded PSU doesnt solve the problem then I'd look into having all the capacitors replaced.

In the mean time, open her up and check for any obvious signs of capacitor failure such as fluid around or discolouration of the solder joints, swollen tops on the capacitors etc. Ofcourse if theres no obvious signs its also possible that the capacitors have just dried up being electrolytic. This is a common problem with many amigas and one that has recently been a pain in my butt.

Let us know how it goes

 

[Tomse]

I just redid the schematic for the A2000 (during my vacation in france) and checked the A4000 schematic, they are pretty much the same, and quite simple.

1)
first.. if you have a CD player with audio out and a play button, connect the audio to CN404 and see if you get audio out, without playing other sounds (at first)
if everything is good, try playing sounds in addition to the CD audio. if you hear distinct scratching from the amiga audio but not from the CD we should look at U402.
if it's already connected, remove the audio connector and try again


2)
if you have access to a voltmeter or an oscilloscope, check if there is activity on Paula when playing sounds/music

Paula
pin 33/34 (right/left)
pin 37 gnd

a mid step would be to measure CN404 (after removing any connector)
pin 1 = left
pin 2 = gnd
pin 3 = right
if you don't see anything it could be U402, otherwise it could be U403


if you blindly want to replace components and caps doesn't work.
take a look at replacing
U402 and U403 which are the operation amplifiers

let me hear how it goes (evt PM me if I don't answer in the post, sometimes the gazillion posts in here tend to drown)

NOTE: using a voltmeter for signals like audio can be tricky. you shouldn't actually see a stable voltage but just alternating voltages. if you see a stable voltage, something is wrong (or you don't play sounds)

the U403 and U404 each have 4 op-amps inside, so the IC circuit could very well be defective.