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.

[Matt_H]

On this image, betweeen Lisa, Budgie, and the Keyboard MPU. Looks like a transistor or a relay.

Can someone confirm?

[Plaz]

U49 is a surface mount transistor. The D3 A1 code on it might be able to help us track a replacement. Many times a normal size transistor can be substituded for the repair if you can find the specs for the old one and get the legs soldered in correctly. It's probably an NPN high frequency transistor if I had to guess at it, but it could just as easily be a PNP. What make you think that it bad?. You didn't give much detail. Does "Broken" really mean cracked, busted, snapped off the mother board some how? I'll do a bit of searching and see if I can come up with the specs and a replacement number. (If some one doesn't beat me to it). If you have any other old pc boards lying around, you might look to see if they have the same transistor you can pull and use. But doing surface mount can be tricky.

BTW, taking a closer look at the pins on that, it might even be a voltage regulator..... now scanning more docs....

Plaz

[Acill]

D3 BCW33 is the type this is IF it is in fact a SMD Transistor. Here are the specs for one of them. Its a common part so you should get it. What I would wonder though, if its in fact bad how did it go bad. They dont go bad often.

Manufacturer: PHILIPS

Manufacturer Part Number: BCW33.

Description: TRANSISTOR, NPN SOT-23

Transistor type:Small Signal General Purpose

Transistor polarity:NPN

Voltage, Vceo:32V

Current, Ic max:100mA

Power, Ptot:250mW

hfe, min:420

Current, Ic hfe:2mA

Voltage, Vcbo:32V

SurfaceMountDevice:1

Current, Ic continuous a max:100mA

Marking, SMD:D3

NF, max:10dB

ft, min:100MHz

ft, typ:300MHz

It could also be a voltage regulator and the info on it makes it very hard to tack down.

[Plaz]

@acill

That was one possibility, but check the datasheet. The pins don't match. It has the layout of a voltage regulator. BUT it's always hard to tell with some of these smt's.

Plaz

[Acill]

Got me before I had a chance to finish. I think now that I look at it more it is a regulator. Again i would look more into WHY this went bad if its bad. What you need to do is see what ppower is on the input and the output. The power in is the 3 pin part. The middle is ground, the output is the single pin. I am betting its a 5V out. You need schematics for this. If you can find a matching part that outputs the same voltage it will work. The exact part isnt that big a deal.

[Plaz]

Ok, here's the likely suspect...2n2222a Transistor in a SOT-89 case. And here's a possible replacment if you solder in the correct pins to matching points on the mother board....Radio Shack 2n2222 transistor in a TO-92 case. And here's a nice page with SMD info and soldering technics... SMD Help. Just ask if you need more.

Plaz

[MigaMan]

Usually the only way to test a transistor or volt. regulator is if it is out of circuit.  How did you trace it back to that part?  

[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.

[elginseam]

Have downloaded a scanned A1200 schematic from http://www.acc.umu.se/~patrikax/amiga/others/schematics/
(A1200R2_0009a.png if you download the A1200r2.zip file)
It looks like U49 is not a transistor as they all have a Qxxx prefix and the circuit symbol is triangular not transistor shaped.
The part reference from the schematic is RHSVA43A if that helps.

[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.

[Chain]

 great!
im happy when i can see these repair skills  :pint:

[MASACREWILL]

..I imagine a sci-fi paralel version of history (future), when the world is ruled by Amiga users (well, those like doppie1200) from Amiga.org after some global PC related catastrophe..  :lol:  :pint: