I am after a resistor For my A1200 mobo..?

[Steve T]

Originally Posted by spannernick

Ok will not do that then,I will buy the proper capacitor for it then,It can wait,its my backup board anyway.Time for some googling I think..

Thanks for all the help.

GL, yeah its all too easy to make a bigger mess than you started with, tearing up pads and traces... helps to practice on a dead board with similar components... your component is probably glued down too.

[Hanzu]

Originally Posted by spannernick

Here are 2 pictures ,can you spot it..??

I also can spot something and maybe something new too:

In the right bottom edge of your first attached picture look at silkscreen E232C and right below a via with copper color. I wonder why there is copper in it this via should look like a black hole and open since the modulator has only 3 pins next to it. Also it looks like the track to E232C is cut from its via, but that is something that can do trick in the picture and you should inspect yourself.

In the picture it looks like C30C is first getting very hot and then cracked in the middle saving PCB from burning.

I also have REV 2B that I bought faulty from ebay.uk and it was badly burnt around C30B loosing its front pad completely. I have added new bigger and higher voltage 470nF, but now in power up my C30B burns a mark to my fingertip in just 2-3 seconds after power up. So indeed it draws a lot of current for some reason.

So now that I know spannernick, me and this third fellow... https://www.reddit.com/r/amiga/comme...0_help_needed/ ...all have REV 2Bs and burnt or burning C30B. This must be related to REV 2B itself. Cannot be a coincidence that all are same and not the most common revision.

When powered off and measuring over C30B or C30C or C30D there is only 18 ohms, so something is short circuited somewhere. I'm going to change C30B and measure it on the table if not short circtuited then C30D if no help in that then one by one those Zetro mentioned.

I have to also mention in this REV 2B everything else looks good visually. No other burnt marks, matte grey solders or caps leaks, but someone has seriously mistreated PCMCIA slot. Some pins are bent but not short circuit and the front half of the plastic is ripped off. IIRC ROMs were also in wrong sockets which probably just means someone tried to do some weak testing with this one and made a mistake in replacing those. CN13 (keyboard connector) white top frame was missing, happily I managed to buy NOS spare for it.

spannernick did you solve your problem already? You seem to be missing keyboard connector top part too, but at least you have that original REV 2B mouse port PCB.

A little detail with no importance: Your keyboard controller chip resonator is red and mine is blue.

[Hanzu]

Originally Posted by Zetr0

@Spanners

if you want a cheap set of mono-ceramic caps - here is a good one from eBay


10x 470nF 0.47uF 0.47µF 10% 16V 0805 SMD Ceramic Capacitors
I am not affiliated with this sale


While 16v is best, you can go up to about 25 volt 0.47uF (470nF) - I hope that helps

Why not higher than 25 volt specs? Not working? Bigger voltage tolerance also offers bigger case and so reaches better over burnt pads and tracks to be soldered to some track or end that has survived.

[Zetr0]

@Hanzu

Going off specification assumes knowledge of the circuit - because I didn't design the circuit I do not have full knowledge of the reliance and tolerance of components on that circuit.

With that I generally insist like for like components - the only time of go - off specification is quality of a component - such that replacing a 85c rated capacitor with a 105c (high tolerance to heat).

When it comes to voltage monolithic capacitors (in series) can only charge to what they are being fed or pulled from so looking at the current at ( 25v * 0.47uF ) is not to dissimilar to ( 16v * 0.47uF ) - however if you decide to use a 50v unit this is way off the specification and most likely the tolerances of the embedded circuits.

It might be easier if I show this metric - so we use coulombs

16 volts * 0.47 uF = ( 7.52 × 10-6 coulombs )
25 volts * 0.47 uF = ( 1.175 × 10-5 coulombs )
50 volts * 0.47 uF = ( 2.35 × 10-5 coulombs )

The higher voltage rating we go the higher the potential that is offered on the circuit - and this can fall outside of the tolerance of the circuit and possibly in the worst case scenario damage components with higher current than what is expected.

To be humbly honest my friend, personally and professionally I think its best to replace burnt pads and damaged tracks - it only takes a bit of practice and a little investment in time and some money for epoxy based copper track and pad repair.