What are R177 and R176 for?

[Roger_S]

I seem to have some kind of defect on my other a4000 mainboard. I have 2 small crisped chips close to the keyboard connector. They are marked R177 and R176, the writing on them is 200. The miggy is working as it should and I think it has something to do with the external floppy drive.

When I replace these 200's with new once, the solder immediatley starts to melt when power is turned on, so I think there is some other defect as well.
I'm searching the schematics for clues but I can't find them.

does anyone have any idea?

[djbase]

Maybe bad capacitors around? Try to clean the pcb.

[yaqube]

Those two resistors limit maximum current for U198 which is -5V voltage regulator (and also decimates power to relieve U198). This voltage regulator is fed from -12V power supply line.

Please measure and report the voltage on all three pins of U198 (7905 part type) so we would be able to give you a clue what has been broken.

[Roger_S]

And the answer is: 0 from all legs

btw i measure 12v between the resitors with the 200 print.

[Zac67]

If the solder melts the current is way too high - there must be a short somewhere. Either the 7905 is dead or the short is in the -5V circuitry.

The two resistors in parallel limit the current to less than 7V/10Ohm=.7A. Power over the Rs would be .35A*7V=2.45W each - too little to melt the solder. The voltage regulator must be dead (increases the power to .6A*12V=7.2W each - I guess the PSU maxes at 1A).

Remove the 7905 and check whether there's a short on the -5V track before replacing it.

[Roger_S]

without the 7905 it also melts the solder. so there must be a shortcut somewhere. I have already replaced all caps on the mainboard, so that can't be it.

[Zac67]

:-? WTF
Check whether C199 gets hot / is short - it seems the only other way to put load on the resistors. Have never had a ceramic capacitor die on me, only electrolytic ones.

[yaqube]

There might be two causes:

* Broken U198 (short circuit inside the device between pin 2 and 1 - it happens sometimes but very rarely. To verify this you need to remove this IC from the PCB (or at least lift pin 2). If the voltage across resistors R176/R177 is zero so the U198 is broken.

* Shorted C199. The same as above: remove it and check the R177/176 voltage.

There might be also a short circuit across PCB traces. Careful visual inspection could help to reveal it.

Please keep in mind that the case of U198 in not connected to ground but to pin 2 which is powered from -12V power supply line through R176/177 resistor pair.

[Roger_S]

I checked again with another mobo and found that there is a track underneath r177 that is missing on the board, it burned away.

I going to send the board to an amiga repair center, see if they can fix it before i make a mess of it.

Thanks for the help!