Modified Audio Filter

[CHRIS-F]

Hi Guys, whilst giving my Amiga 500+ a good clean and peroxide treatment, I noticed one of the diodes in the Audio filter section had been modified, I was wondering was this a stock mod or has someone done this, if so what could the reason be? I'm not sure of the history of this model as I gave my original A500 away years ago and then someone gave me this A500+ which sat in my loft for years, don't remember who gave it to me.

Regards,
Chris.

[jimbob]

This is normal. Those 2 diodes etc are the battery charging circuit. There must have been an error in the PCB layout because all 500+ have that diode soldered in that way.

The battery doesn't look bad but the solder connections above and to the left are pretty green so there has been some battery leak and corrosion, you should remove or replace the battery. It's a very mild case of batteryitis so you should be fine to just clean it up a bit, check out the Gary chip too for signs of corrosion.

[CHRIS-F]

Originally Posted by jimbob

This is normal. Those 2 diodes etc are the battery charging circuit. There must have been an error in the PCB layout because all 500+ have that diode soldered in that way.

The battery doesn't look bad but the solder connections above and to the left are pretty green so there has been some battery leak and corrosion, you should remove or replace the battery. It's a very mild case of batteryitis so you should be fine to just clean it up a bit, check out the Gary chip too for signs of corrosion.

Thanks for the conformation :-)

About the battery, I can never remember which way round it is are the green batteries NiCad and the blue NiMh or have I got that backwards? I'm just wondering how to go about neutralising the green stuff on the near by resistors and chip legs, depending on what you read some people say use vinegar some say baking soda, I'm not to bothered about replacing the battery I may just leave it out, but wouldn't mind cleaning the damage.

Regards,
Chris.

[Jope]

Blue = NiCd, green = NiMH.

Definitely take the old battery out in any case, replacing it might not be that relevant.

Baking soda mixed into a paste with water is a nice buffer solution that will neutralize it.

[Daedalus]

Originally Posted by Jope

Definitely take the old battery out in any case, replacing it might not be that relevant.

Yes, very important as it will only get worse. You don't need the battery if you're not worried about the system clock losing time when the computer's turned off.

Baking soda mixed into a paste with water is a nice buffer solution that will neutralize it.

Well, baking soda (sodium bicarbonate) is alkaline and would make a good buffer solution for neutralising battery acid... Only the electrolyte leaked from NiCd batteries is actually alkaline (potassium hydroxide), despite being commonly referred to as battery "acid". Therefore to neutralise it you actually need a weak acid. I use lemon juice (citric acid) to neutralise the electrolyte, followed by 99% IPA, but others use vinegar (acetic acid) instead with similar results.

[Jope]

It's not neutralizing by being the opposite ph, it is acting as a buffer solution.

[Daedalus]

Yes, but is buffering the solution not keeping the electrolyte at pretty much the same pH? Why wouldn't you want to reduce the pH to neutral?

[Jope]

Baking soda is around ph 8,3. Put enough of that on it and the strong base electrolyte will become near 8,3, as the baking soda mixture buffers the stronger base so that the ph doesn't stray too far from 8,3.

Feel free to use whatever works for you.

[CHRIS-F]

Thanks guys,

Just a couple of quick questions.

1. After it's neutralised what's 99%IPA is it isopropyl alcohol?
2. After it's cleaned up is there anyway to protect the chip legs, tinning maybe? or a lacquer? or is it just best left alone?

Thanks again,
Chris.

[idrougge]

1. Yes.
2. You can use your mother's nail polish.

[Jope]

There are also PCB laquer sprays, but clear nail polish is quite good for small touch ups.

[jimbob]

Originally Posted by jimbob

This is normal. Those 2 diodes etc are the battery charging circuit. There must have been an error in the PCB layout because all 500+ have that diode soldered in that way.

Not exactly true. Both these boards are rev8a A500+, same code numbers for the artwork etc but as you can see the one on the right has the layout error corrected so the parts are inserted as per the silk screen.

The sticker for the one on the left says 90/91, the fixed one is 91/92.

So it is worth double checking this if you are fixing up rotten stuff in this area.