Problems with your machine in general.
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Post by exxos » Tue Aug 22, 2017 10:43 pm

I had a motherboard sent to me from a chap in the USA. Apparently he was fitting a CT60 and managed to kill the motherboard while swapping PSU's about :(

Which PSU exactly caused the issue is unknown as there was ATX and PICO PSU's involved. Though it was one reason I was motivated to do some reviews on PICO PSU's to test the regulation out. I have already seen a lot of PSU's spiking up to several volts even with new PSU's. So be warned!

The machine came to me with a whole host of mini nightmares to go through. One of which was a failed NVRAM replacement. It seems the guy tried to cut the pins as close to the motherboard as possible to remove the old NVRAM. Though doing that means the pins are stuck in the PCB holes and nothing to really get any heat onto them with, and nothing to pull to get the pins out! This turned into a nightmare to fix.

Normally you cut the pins as high up as possible, heat the pin up, pull it out. Simple stuff. Though with the pin basically being chewed up, in the hole, top and bottom. well, lets just say it took "some time" to correct it. I also found a track had been ripped up at some point (wasn't me!) so had to repair that also.
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In the end a socket was fitted and the track repaired.

I also had to remove a bodged clock patch install and do some repairs to the motherboard to get it back to "stock".

On power up...
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Total chaos! Oddly though, the machine did actually seem to be running as I could near the floppy drive loading stuff up off my boot floppy! So the machine was running, just no video on RGB or RF :(

After some scoping about I suspected the Videl was chucking out data, but there was nothing on the output of the video DAC. So.. Out it came..
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.and in with the new..
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On power up...
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Nothing had changed! crap!!

Much pondering later it seemed Videl was suspect in the data it was feeding the Video DAC. I did a lot of comparisons between my spare Falcon and this machine and while it was hard to compare as the faulty machine had basically a black screen, I did suspect the Videl wasn't outputting what it should be.

I also back tracked to COMBEL and traced all the video load/select lines were triggering, and they was. So COMBEL was telling the Videl to load data, just it wasn't getting the the Video DAC :( So this means the only likely cause is the Videl chip is dead.

So, Sometime later.. A new Videl IC turned up... So out with the old...
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.. and in with the new..
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I had never attempted to change a IC of that size before, so wasn't really expecting anything "good" to come out of it.

I did ponder about using a stencil to apply solder paste, but decided to paste it by hand and add some extra flux to the pads. Its a technique I developed over time as I never liked the apparent "easy" methods of "flux and drag" with a soldering iron. It looks easy, but dragging the iron oven pins can bend them. It can damage "cheap" PCB pads. The problem I always had was solder would always end up behind a couple of pins and you could hardly ever see it. More to the point, its pretty impossible to get the solder "un-stuck" regardless of the amount of flux or heat used. The only option was to remove the IC and clean it up and start again.

So what I did , which is what I do mostly now, is to run SMT paste around the edges of the IC pads and add flux on the middle areas of the pads. Then plonk the IC on top and heat the sucker up. The flux melts and covers the PCB pads and the IC legs, then the SMT paste melts and have a easy flow between the IC legs and pads and it gets "sucked" into place by capillary action.

The good thing here, SMT paste only gets on the bottom of the IC pins and there is practically no risk of solder creeping up the legs and shorting them out. Probably the worst case here is the need to add a little more paste and re-heat if the SMT paste was applied to thin on some pads. Its better to add solder later than try and get excess off!

So power up we shall...and..
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"Another successful repair" as we used to say in my old workplace :)

Of course its going to have my clock buffer patch added soon along with a new AXIAL CAP to replace the aged one.

With the PSU being the likely cause of killing this falcon, a quality input capacitor will help smooth out all the nasty spikes in those "cheap PSU's" and help protect the motherboard from them.

Considering the Videl is right next to the PSU connector, there is no great surprise that it was killed first. Of course the owner of this board really, got away lightly as the PSU could have blown a lot more IC's and the board would have been a total write-off!

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