A500 sound issue

[trydowave]

Hi. I have two A500s running side by side. One sounds normal and the other one has a slight discrepancy in the left channel.

I noticed it in Turrican II and more so in Lotus 3. The beats were weedy and sometimes they seemed to get lower or higher depending where it was in the music. Also it sounded slightly distorted like a speaker had blown (not the case btw)

Has anyone had a problem like this? Is Paula on her death bed

Cheers
Tony

[roy bates]

its probably not paula but the op amp.
or something in that area.

[GadgetUK]

Originally Posted by trydowave

Hi. I have two A500s running side by side. One sounds normal and the other one has a slight discrepancy in the left channel.

I noticed it in Turrican II and more so in Lotus 3. The beats were weedy and sometimes they seemed to get lower or higher depending where it was in the music. Also it sounded slightly distorted like a speaker had blown (not the case btw)

Has anyone had a problem like this? Is Paula on her death bed

Cheers
Tony

My first port of call would be the decoupling caps on audio out. There's one for left and one for right - both 22uF if memory serves, and from what I understand they should have been bipolar due to some reverse bias but commodore put polarised electrolytics in there. Not got diagram to hand but you should look for those caps near the audio RCA jacks.

EDIT: Following on from that and it can only really be the op amp (and associated components) as suggested by Roy.

[trydowave]

thanx for the advice guys. this sounds technical so i had better take it to a professional . dont wanna kill it like my A600.

[TomCrazy]

Like Roy points out, it's more likely to be something wrong with the analog side of things rather than Paula or something earlier in the chain. A blown Paula is not at all as common fault as a problem with the audio filter circuit is.

In order to correct Gadget's post a bit, the 22 µF capacitors in the audio filter section are not decoupling caps, but AC coupling caps. I just took a look at the schematic, and in seems like each 22 µF capacitor is hooked up in parallel with a 330 nF one on the A500.

Also, using bipolar capacitors is not necessary on any Amiga model I have come across so far. This is probably just a rumor that started somewhere and is still living on.
On basically every Amiga model, the input voltages to these AC coupling caps have a DC bias high enough to bring the whole swing above 0V, making it completely unnecessary to use a bipolar capacitor here.
In fact, bipolar capacitors often have a huge ESR in comparison to regular electrolytic capacitors, resulting in a lower volume.

Going back to the audio problem, there might be something wrong with one of the AC coupling capacitors for the left channel, just like Gadget suggested. But there can be something else causing the problem as well.

I would suggest you to ask for help from someone who has the right equipment to track down the problem and fix it for you.
Audio issues are most often quite easy to troubleshoot for someone with some basic electronic knowledge, so you should be able to get this resolved quite easily with just a little bit of help.

I hope you manage to fix the problem.

[Loedown]

Originally Posted by TomCrazy

Also, using bipolar capacitors is not necessary on any Amiga model I have come across so far. This is probably just a rumor that started somewhere and is still living on.
On basically every Amiga model, the input voltages to these AC coupling caps have a DC bias high enough to bring the whole swing above 0V, making it completely unnecessary to use a bipolar capacitor here.
In fact, bipolar capacitors often have a huge ESR in comparison to regular electrolytic capacitors, resulting in a lower volume.

In almost every Amiga I have recapped it's the two capacitors C324 / C334 that cause the most damage. Through the medium of modern engineering it's easy to amplify signals that may not be quite as loud as one desires using an amplifier, something necessary for Amiga musical enjoyment. Stedy did some measurements on those capacitors and found that their voltage differential between their pins was around 300mV which isn't huge but with the LF347 it has a slew rate of 13V / uS which can give rise to sharp and quick transients on those capacitors which will prematurely age them. Bipolar capacitors have the effect that the side that is reversed biased acts as a blocking diode and this mechanism increases the capacitor's life.

I would speculate that the other capacitors within the whole audio chain would suffer less because their voltages are generally lower overall and don't need to be replaced with bipolar types.

I replace C324 / C334 with through hole parts that are shaped to sit on the pads with the body of the capacitor laid over some of the discrete components, on the A4000 boards I also replace the other two 22uF 16V input and output capacitors in the same fashion.