cpc 464 short circuit

[retro_4_ever]

hi, i'm looking at a Z70200 (big board) it appears to have a short from the 5 volt rail to ground- i removed the power jack and the single electrolytic cap to rule them out then realized that the line from power jack to tape connector pin 3 is ok. its from the output (pin1 onward) i have gone through all the chips thinking that i would see some difference in resistance on there power lines on a defective one but they all show about 16 ohms . there doesn't appear to be any open circuit resisters. cpu and gate are in sockets so they are removed.. any suggestions or am i missing something ? mark .

[Bryce]

If it's a straight short circuit, something must be getting pretty hot? Are you sure the voltage drop isn't because the PSU can't provide enough current?


Bryce.

[retro_4_ever]

i thought it wise not to power it again until the short is gone, I have another motherboard of the same model that i am using for comparisons . i was using a good supply when it was originally powered it up.
the 5 volt line to ground of the ram chips etc all show next to no resistance (16 ohms) this cant be correct ?



mark

[Bryce]

Check the ceramic capacitors, that the legs of any them aren't shorted. 16ohms sounds touching (oxidised) wires.


Bryce.

[Bing]

What about 24,5 ohm? I 'm in the same situation as was @retro_4_ever.

One LS244 was literally BLOWN (I COULD SEE ITS INTERNALS) cutting the legs didn't worked and the sort is still there. I found some liquid on the board, something sticky, very nasty thing... Let's say "water damage"    all cleaned now.

My multimeter is not precise enough to chase the short, and I'm not the best with the solder in my hands to start desolding chips.

I also tried with:
-ceramic capacitor besides LS244 (not shorted)
-diode (not shorted)
-electrolithic capacitor (not shorted)
-two transistors (not shorted)
-z80 and ula not shorted

What are my best choices?
Keep going with all the ceramic capacitors?
Memory chips? (as I'm afraid of)
Maybe find a precision meter (milions? Tenths of milliohms?)

 

[Bryce]

Try powering it with a 5V source that can't supply more than 2A and check what parts are getting hot. You won't damage it any further doing this. It's highly unlikely that any passive parts (resistors, capacitors) are the problem. RAM is the most likely point of failure.

Bryce.

[retro_4_ever]

i forgot about that motherboard....i will have to dig it out and have a look again .

[Bing]

Try powering it with a 5V source that can't supply more than 2A and check what parts are getting hot. You won't damage it any further doing this. It's highly unlikely that any passive parts (resistors, capacitors) are the problem. RAM is the most likely point of failure.

Bryce.

Thank you!
Thank you!
Thank you!
Thank you!
Thank you!
I did what you said. (First of all, I remove shocketed chips z80 and ula) 
With a 0.745 miliamp load IC114 (74LS373) started to  burn my fingers
I cut the pins and the short has gone.

Now I have to replace it, and see what happens with those memory chips and the rest of the board.
Thank you.

[Bryce]

Well done, one step closer to getting it working.

Bryce.