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Can this 575 logic board be saved?

jajan547

Well-known member
Whelp, the fresh VRAM showed me that it probably isn't (just?) the vram to blame. The symptoms have changed a bit - the screen will now eventually start to flicker with different colours. If I move the mouse cursor to select a menu (blind) you can see obviously correlated changes to the flickering and bands of colour/black on the screen.


I'm pretty sure that I have verified the continuity on the VRAM SIMMs to what look like sensible chips on the board (CLUT & MEMCjr).

The board boots to this state with and without a RAM SIMM in place. I can hit keys and hear audible responses from the OS error chime and even hit the power button followed by return to power the machine off.

Does anyone have suggestions as to what to try next? Another reflow of something and/or removal / cleaning of the VRAM / RAM SIMM sockets?
About 90% sure U26 may be faulty. Before replacing it altogether remove the chip, clean the pads with braid, followed by some cleaning with alcohol, then reattach the chip and we’ll see if that solves it. I say this because that’s the video RAMDAC chip. It’s possible there may be cap goo from C29 under the chip.
 

Jagmn

Active member
Thanks I'll give the cleaning a go. What hot air settings do you use to remove a chip that size? I've just got a fairly standard temp-controlled station with a few different nozzle sizes.
 

jajan547

Well-known member
Thanks I'll give the cleaning a go. What hot air settings do you use to remove a chip that size? I've just got a fairly standard temp-controlled station with a few different nozzle sizes.
Also when you remove the chip can you upload a photo?
 

Jagmn

Active member
So I haven't tackled U26 yet as I wanted some practice at removal/replacing chips. I did a few SOJs on a scrap board as practice before deciding to tackle the on-board RAM chips on the 575 board (due to the slightly mysterious failures post-reflow attempt). Unsurprisingly I did find and clean up some corrosion, presumably from some leaked electrolytic, around U22 and U19.

Unfortunately I think I have fried some of the chips - I'm back to the death chimes. I have re-checked continuity and for bridges (there was one bridge between address lines due to my fumbling and some solder resist that got corroded away). Is there a way to independently check these chips for proper functionality? They're NEC 424400-70 parts. I've spotted some on ebay.

I'd prefer to get the board back to a functional state before tackling U26 (so I can at least tell if I've fixed or killed it). Though I guess I'll have to admit fudgy-soldering failure at some point :cautious:

As an aside: I assume it isn't a sensible way forward to get the board to boot just with a RAM SIMM? I imagine it'd be possible but would need CAS/RAS signals re-routing and possibly some other dodgy hijacking.
 

jajan547

Well-known member
So I haven't tackled U26 yet as I wanted some practice at removal/replacing chips. I did a few SOJs on a scrap board as practice before deciding to tackle the on-board RAM chips on the 575 board (due to the slightly mysterious failures post-reflow attempt). Unsurprisingly I did find and clean up some corrosion, presumably from some leaked electrolytic, around U22 and U19.

Unfortunately I think I have fried some of the chips - I'm back to the death chimes. I have re-checked continuity and for bridges (there was one bridge between address lines due to my fumbling and some solder resist that got corroded away). Is there a way to independently check these chips for proper functionality? They're NEC 424400-70 parts. I've spotted some on ebay.

I'd prefer to get the board back to a functional state before tackling U26 (so I can at least tell if I've fixed or killed it). Though I guess I'll have to admit fudgy-soldering failure at some point :cautious:

As an aside: I assume it isn't a sensible way forward to get the board to boot just with a RAM SIMM? I imagine it'd be possible but would need CAS/RAS signals re-routing and possibly some other dodgy hijacking.
I think that you can boot by removing the bad RAMs and leaving them blank on the board (not a long term solution but testing works). Unfortunately you have to go one by one and power off and on. I still believe this board is definitely salvageable. So keep practicing and try removing the RAM chips one by one to test which is bad and then when you get comfortable try U26.
 

Jagmn

Active member
Thanks, I'll give that a go. I'm surprised that the board should cope with ad-hoc RAM chip removal; the data lines looked like it was geared up for 32bit read/write access spread across the chips. Is the MEMCjr smarter then that, then?

And any tips on how to prevent chip frying? I removed with 350°C/60% hot air and re-soldered with a temperature controlled iron. The chips were pretty hot after the desoldering.
 

jajan547

Well-known member
Thanks, I'll give that a go. I'm surprised that the board should cope with ad-hoc RAM chip removal; the data lines looked like it was geared up for 32bit read/write access spread across the chips. Is the MEMCjr smarter then that, then?

And any tips on how to prevent chip frying? I removed with 350°C/60% hot air and re-soldered with a temperature controlled iron. The chips were pretty hot after the desoldering.
I mean you can always go lower, sometimes fresh solder helps to remove them quicker so there's less heat, and yes it should work just fine.
 

Jagmn

Active member
Thanks; I'm giving the ram chip removal a go.

Currently I've removed the bottom 5 ram chips as these were the ones I think most likely to be fried. The board isn't booting beyond the death chimes yet. If I remove the last 3 chips, would it be able to boot just from the SIMM?
 

jajan547

Well-known member
Did you go remove a chip test add chip back or just remove all five, should be remove test reflow until you hear no more death chime.
 

Jagmn

Active member
I removed all 5 so far. It's getting to the point where I'm damaging the pads (I've managed to remove 2 and get wires back to where they need to be) so I was hoping to do it in one iteration and maybe later replace the chips with ones bought on ebay.

Looking at the specs you could buy these boards in - perhaps the bottom 5 need to be populated? I think it came in 8mb and 5mb on-board variants.
 

jajan547

Well-known member
I removed all 5 so far. It's getting to the point where I'm damaging the pads (I've managed to remove 2 and get wires back to where they need to be) so I was hoping to do it in one iteration and maybe later replace the chips with ones bought on ebay.

Looking at the specs you could buy these boards in - perhaps the bottom 5 need to be populated? I think it came in 8mb and 5mb on-board variants.
Let me look real quick
 

Jagmn

Active member
Okay; thanks. I'll do some experimentation and re-check what I've got at the moment. I don't think I should have damaged the three remaining chips at the top of the board so there might be something else that I've damaged in the process.
 

jajan547

Well-known member
Okay; thanks. I'll do some experimentation and re-check what I've got at the moment. I don't think I should have damaged the three remaining chips at the top of the board so there might be something else that I've damaged in the process.
Hoping for the best
 

Jagmn

Active member
I think I've verified all of the lines I may have affected with my work. Still have 3 chips in the top-3 ram spaces (nearest the ports) and still have the sad mac chimes.

I'm waiting for some fresh RAM chips to arrive before I continue testing as I don't want to risk damaging the board some more with soldering/desoldering more chips on the same pads.
 

jajan547

Well-known member
I think I've verified all of the lines I may have affected with my work. Still have 3 chips in the top-3 ram spaces (nearest the ports) and still have the sad mac chimes.

I'm waiting for some fresh RAM chips to arrive before I continue testing as I don't want to risk damaging the board some more with soldering/desoldering more chips on the same pads.
Sounds RAM related, so we’ll see when they get here.
 

Jagmn

Active member
Yup!

As the chips are taking their time to arrive, I did some more chip-removal practice on the board with the last 3 dram chips (I think I did it much more cleanly this time).

I decided to test the board just in case it would work with just a SIMM (no on board memory). This is what I observed:
  • No SIMM or MB RAM: Chime followed by silence until manual power off.
  • 4MB SIMM, no MB RAM: Chime followed by ~ 3sec silence followed by death chime.
  • 16MB SIMM, no MB RAM: Chime followed by ~10sec silence followed by death chime.
So it doesn't seem likely that the board would booth without any on-board RAM, unless I have some more self-inflicted faults somewhere.
 
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