MegaSTE flashing keyboard light only

[exxos]

Search for the power connector in schematics. Trace the POWERGOOD to the LM556. There you have HALT and RESET.

ahhh.. was looking for something "less complicated" :roll:

Capture.PNG

[Bikerbob]

ok, so pin 5 and 9 are the output for the 556.

at the moment what I have easy at hand is a Multi meter, is there a worthwile test I can do there?

reset.jpg

This is from the manual. it says pin 17 TTL high.. in this instance I assume thats 5v. but if I get the flickering 5v there.. all that shows me is what I already know issue with the reset circuit. How do I fix it?

I have tested the S201.. it works.. I get high and low on pin 8 of the 556. when pushing it.

James

[exxos]

Normally the 10uF caps go bad. Fitting 22uF can be better as well.

[Bikerbob]

OK, found that the 12v line was shorted.

Found that C224 was shorted (from the 12v line)

I have a question though C226 and C227 frank had said should be 60ohm on mine they were about 120ohm

I tested on the other board to compare.. C224 was not shorted.. lol... but also C226 and C227 were 50ohm.

IF I have to replace anyway.. which is better? since its a cap.. I assume higher resistance is an issue? remove both the 470uf and the .1uf and test them? replace?

James

[exxos]

Have you taken C224 out of the MB to test if its shorted or not ?

[Bikerbob]

cap.jpg

well in the board with a OHM meter on either side I get 0.. none of the other ones are doing that. A cap should never have 0 ohms no?

James

[exxos]

cap.jpgwell in the board with a OHM meter on either side I get 0.. none of the other ones are doing that. A cap should never have 0 ohms no?

You are measuring the *entire* 12V rail doing that , not just the cap.. you cannot test stuff like that in circuit.. id be surprised if a 100nF cap was shorted..

[Bikerbob]

I am reading the whole 12v rail measuring from one lead to the other?

OK.. I can remove it. hoping to determine the issue before I heat up the iron.

James

[exxos]

I am reading the whole 12v rail measuring from one lead to the other?

One side is gnd, other 12V.. so *anything* on the 12V rail is of course all connected... measure across 12V on the floppy or HDD power cables..

[Bikerbob]

But 12v is grounded on the board.
Without the PSU connected to the board. So I would think I have to figure out where 12v goes on the board.

James