exxos's Atari, Amiga & retro forum

Further to exxos' thread concerning reversed caps, I realised when I was recapping my STE that capacitors C204 and C205 (those near the DMA chip) might also be fitted upside down depending on the motherboard.

If you look at the various STE motherboard revisions on the forum, you will notice that each motherboard seem to have these caps fitted differently.

In the case of the C415 cap, the + and - silkscreen was mistakenly inverted, while the C204 and 205 caps are probably wrongly fitted.

Does someone have investigated a bit more on this matter ?

I measured the voltage across them both as I still have a STE on my bench.


Make of that what you will. Though I guess we cannot assume the actual track layout in the series is identical across boards. Would need investigating as well.

On this other machine they are non-polarised capacitors anyway.

After having looked at my pcb reconstruction (you can see it here : https://easyeda.com/editor#id=e4596c34d ... 207a38e830 ; just abort copper pour computation so you will not wait forever), I confirm that the + sides of those capacitors should be toward south of the motherboard, when they are polarized (mine is NP too, like the second one from exxos)

For the record, those capacitors are satellites of U204 that generate the pair of +12V and -12V rails (see p14 of my schematics) needed by U206 to drive the RS-232 TX pin (p13 of same schematics). (if I understand things correctly). Giving [+12V]--[+C205]--[GND]--[+C204]--[-12V]

I have just checked my first STE motherboard (a non socketed CPU) and both capacitors are NP.

My second board had polarised caps, and when I replaced them I kept the same direction, ie with C204 + north and C205 + south.

If you look at the motherboards, there are 2 + silkscreens, one north and one south.

So, are those 2 + silkscreens mistakes and these caps should have their + towards south ?

:?:

With caps still removed, I would check that

* Pin 1 of U206 will be wired to negative side of C204
* Pin 14 of U206 will be wired to positive side of C205

With caps already installed, it may be trickier to check ?

Just ALWAYS install NP caps as a rule of thumb (in that location) I usually use NP polymer caps.

exxos wrote: 10 Oct 2022 13:50 I measured the voltage across them both as I still have a STE on my bench.

Make of that what you will. Though I guess we cannot assume the actual track layout in the series is identical across boards. Would need investigating as well.

Looks like your caps are both the same orientation and your silkscreen is opposite orientation. Known issue from factory?

BW

Badwolf wrote: 10 Oct 2022 17:30 Looks like your caps are both the same orientation and your silkscreen is opposite orientation. Known issue from factory?

Must be.

Assume it's just the silk screen which is wrong as the schematic looks correct to me.


Pin 1 is -12V. So the + on C204 connects to GND which is correct by the schematic. But on the MB the + on the silk screen is not correct.

But they are NP caps, so the silk screen isn't really relevant anyway. If Atari intended on using NP caps, they should have just left it off the silk screen.

Or maybe it was a mistake and they fitted normal caps originally, then realised it was a mistake with the silkscreen, so then just shoved NP caps in.. ah who the hell knows :roll: