Questions:
My last concern is the size of the R4 10K resistor, smaller size could mean that it will go smoking away.
I have no match for this one. I don't know the wattage of the bigger and of this one.
Also, in order to replace the two ICs involved in the jump wire, SN7406N is avilable, but MC74HC14AN looks way lessa vailable.
Is there a newer perfect replacement for this one?
I tested the BIOS on another 286 and it POSTS up to a certain point.
I get codes and that is enough to know it can work.
I checked the CPU Vcc and the one that is getting 3.74V on this board, gets the full 5.15V required.
The cpu is actually dead as per seller's prognosis or this is a motherboard issue.
I'm pretty sure it's the cpu at this point.
When I hit reset nothing happens (it has to be drawn HIGH), and it just loses "Frame" on POST card, no reset happening.
I'll desolder and socket the cpu. Maybe I'll try it on another board, but I also have plenty of PLC68 286...
No, don't use the 74HC series. It has different logic levels than old TTL. The 74HC14 also has a push-pull output, where as the 7406 has an open collector. A modern equivalent, which is still available, is the 74LS06.
No, don't use the 74HC series. It has different logic levels than old TTL. The 74HC14 also has a push-pull output, where as the 7406 has an open collector. A modern equivalent, which is still available, is the 74LS06.The OP is not proposing to use a 74HC14 to replace a 7406. Both types of chip are already on the board and both are to be replaced.
Obviously the battery leaked, and is missing completely now. A via has been drilled out. That really could be an issue.
Also several traces show damage from leakage, you certainly need to take care of that.
No battery at all might be an issue as well.
No, don't use the 74HC series. It has different logic levels than old TTL. The 74HC14 also has a push-pull output, where as the 7406 has an open collector. A modern equivalent, which is still available, is the 74LS06.The OP is not proposing to use a 74HC14 to replace a 7406. Both types of chip are already on the board and both are to be replaced.
Yes, exactly.
If nothing works I'll buy new chips.
The cpu was proven to be functional. But the 2nd Vcc input was below minimum required (+5V).
I tested the smal 104 cermaic capacitor the +5V was coming from (a small trace from hole to pin), and it was a 130nF /0.1% /2.4ohms ESR, the V was 5.xxV (as intended).
Question:
if it wasn't being siphoned by this cap, why Vcc is lower on the cpu? What else could be affecting it?
Vss were = 0V.
https://pdf1.alldatasheet.com/datasheet-pdf/view/1284116/AMD/80286.html
The cpu was proven to be functional. But the 2nd Vcc input was below minimum required (+5V).
I tested the smal 104 cermaic capacitor the +5V was coming from (a small trace from hole to pin), and it was a 130nF /0.1% /2.4ohms ESR, the V was 5.xxV (as intended).
Question:
if it wasn't being siphoned by this cap, why Vcc is lower on the cpu? What else could be affecting it?
Vss were = 0V.
https://pdf1.alldatasheet.com/datasheet-pdf/view/1284116/AMD/80286.html
No, because I don't know how. I have to ask for your help on this (or link a guide).
I'd look it up by myself but no clue what I'd be looking at.
PSU is working 100%, AFAIK. I could check voltages once again to rule that out.
No, because I don't know how. I have to ask for your help on this (or link a guide).
I'd look it up by myself but no clue what I'd be looking at.
PSU is working 100%, AFAIK. I could check voltages once again to rule that out.
the point is just to get a realistic load on the rail and see if the voltage stays stable and doesn't drop
I don't know in this case if the rail is directly provided by the PSU or regulated on the board itself
ideally you would use an electronic load connected to the rail and increase the current until you either reach the expected load or the voltage starts to drop too much (indicating failure)
a barebones electronic load can be had for relatively low money from the usual places
alternatively, you can also use an adequately sized resistor (in both value and power dissipation) connected to the rail to draw some current
So the only fault you are aware of is a missing voltage rail? So one VCC is good, ones missing? If you added a socket for the cpu then check your soldering work and that the problematic VCC is a low ohms connection and not shorted anywhere (to ground or a neighboring pin for instance). Then trace the VCC back to the power connector. You can also see if that rail is used elsewhere or other power rails are missing.
Perhaps a good time to take some better images of the board.