A1200 busted. Power not OK

[Doppie1200]

Hi all,

I'm attempting to restore two A1200's I have laying around.
One of them has an Rev 2B mobo that exhibits a problem.

The POWER OK signal is not working. I traced the fault back to U49 that was broken. A previous owner seems to have done something to it.

Unfortunately I can't tell from the scanned scematics what component this actually is.

So the question is; What is component U49 and does anyone know the part's name/number so I can replace it?

Thanks.

[Doppie1200]

I traced it back in a very low tech manner.

Problems manifested it self at diskdrive access. The controller would go wild moving the stepper motor at random.
I measured the VCC an saw spikes (going down).
Then I looked up the scematic and came across the power good circuit near the keyboard MPU. I thought to take a look there out of curiousity. Thinking that should reset the system on powerfail.

On visual inspection I noticed someone had been soldering there. R629 was only just connected to it pads. It seemed to have floated off.

Then I looked at U49 using a magnifier. The solderpad and the trace to the via leading from pin 1 and R629 was missing. The repairman solved this by shorting U49 pin 1 and 2. Also I noticed a crack in the plastic of U49 around pin 1. When I tried the remove the part it broke in half along that crack.

I get the impression that U49 was disabled by shorting 1 and 2. This the power ok signal is always high. Still there might be something else wrong aswell.

[Doppie1200]

Great help everybody. Thanks.

With this highres scan I should be able to replace this part and find a cause to the strange behaviour.

[Doppie1200]

Found it. It does not read RHSVA43A but RH5VA43A. This should be enough information to get the new part.

[Doppie1200]

The component in question holds the reset line active till the voltage reached a certain level. Then it will deactivate reset only to assert it again when the voltage drops below 4.3 Volts.

Since the component seems no longer available I looked up an equivelant. The Max706 seems to do the same. The device has a different form factor so it is no drop in replacement.

This evening I did some patching and got the Max706 wired up. Now the device boots 10/10 which it previously did not. Also the floppy keeps working. I have not seen it fail yet while previously the stepper motor would exhibit random movement some times.

I'm pretty confident the original component has been the victom of some bat patching practice. The component was shorted and all the components got Vcc connected directly to their reset inputs. Probably this was an attempt to fix problems with an accelerator or something. A beefier psu would have been a better idea.

Anyhow the board seems fixed. I'll have to do some more testing in future.

[Doppie1200]

This is the patch. Pin 1,4,5,6, and 8 bend upwards. Pin 2 connected to Vcc. Pin 3 connected to ground and finally pin 7 connected to the reset line. I used R629 for that because the original trace was missing on the PCB. someone has been doing some serious baking here for some reason. I'm glad it still works.