1040 STF : 2 Chips OK, 6 Chips NOK

[Marsupilami]

Hi,

I must have missed something in the upgrade process from 2 TOS 102 to 6 chip TOS 104 on my STF.

Before : Works fine with Tos102fr


After : White screen, the ST seems to boot loop.


Oh and also tried to remove the 74LS11N : Same problem.
The most surprising thing is that I didn't install the sockets, they were there from the beginning.
So it "should" work.

Any idea ?
Thanks

PS : And B.T.W., i saw this very helpfull post wich describes the opposite (6 to 2). I don't understant why because you can't find 2 chips TOS upgrades anywhere

[PhilC]

Just looking at the CE pad, is the 256k one made properly? Bit difficult to see.

Other than that, presuming the roms are all good, which I would trust to be as its exxos roms, then with a multimeter check that things like 5v, 0v, ce or etc are all connected on each rom. Because it works in two chip mode I'd expect the lines to the cpu to be ok but maybe there is a problem with one of more sockets. Try cleaning them with IPA.

Other than that it's a case of metering them all out and looking for shorts or breaks. Possibly just a dry or corroded joint due to age.

[DoG]

Looks like there is a bent pin on C137 in the top right corner? On top of the "T" so to speak. Otherwise check the solder blob on CE as Phil said.

[Marsupilami]

Thanks.

I added more pictures, the 256k is ok and the pin too.



I will check the voltages with my multimeter.
I have this schematics for a rev C, mine is rev D so i hope it's the good one.


Oh and B.T.W, should the 74LS11 be removed (again) or not ?

[PaulJ]

Doesn't matter. All depends on the 256k/1m solder jumpers. U63 pin 14 sure looks like its tucked under...

[DoG]

Doesn't matter. All depends on the 256k/1m solder jumpers. U63 pin 14 sure looks like its tucked under...

That what was I ment in previous post. Wrote the wrong part from the silk screen. Ment U63. NOT C137!

[rubber_jonnie]

Agree with the others on pin 14 of U63. Looking at the pic, A16 looks like it may be shorted to a small pad/via in the middle, and whilst I'm not 100% sure that it matters because the pic gets fuzzy when I zoom in, I'd be inclined to clean it up and re solder it a bit motre neatly.

[Marsupilami]

Yes, you were all right : U63 pin 14 wasn't in the right place. Now it is.

And i have almost the same behavior. The difference is that it flickers very quickly at boot loop with an increasing number of bomb (4 then maybe 5 then maybe 6...). It's hard to see because it's very quick.

Zoom on A16 : Is it a problem ?

[rubber_jonnie]

Ok, so looking closer the top pads are ok, but I think you have a solder whisker on the bottom, which I've indicated below:

Possible_Short.JPG (155.08 KiB) Viewed 615 times

I'd suggest you clean this up and see if it helps.

[Marsupilami]

Ok so...

I cleaned up a little bit the U16 & 17 :


Then I did the 28x6 soldering (again) under the motherboard
Then I tried and... it was different (even worse) but still not functional.

So i was fed up with it : I decided to remove the 6 chips and go back to 2, that's how i discovered a broken leg on the U48 (I didn't find the missing part).


I put everything back together and tried something :


And guess what ? It works !
Although I don't know what action repaired the ST

Anyway : Thanks alot !