Topic: Snow Bros Repair Log  (Read 1966 times)

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Offline Womble

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Snow Bros Repair Log
« on: January 02, 2013, 04:57:30 AM »
I bought this board in eBay ages ago, probably at least a year, and have only just managed to get round to it.



It turned out to be a pretty simple fix, well about 4 simple fixes in all. The seller sold it as dead, saying it used to boot up with a gfx fault until recently when a part fell off the board. The part was not described in the auction but it was included in the sale.

On arrival the board wasn't totally dead, as it did give this on the screen...



...followed by this...



...which repeats for a few cycles then the screen just sticks at this...



... plain background with random blocks.

I was expecting the loose part to be a capacitor of some sort as they are often knocked off boards, it turned out to be a small 5 pin SIP module with all its legs snapped off, it sounds ominous as these are usually custom parts, until you see where it came from...



... it is actually a "support package" for the LA4460 amplifier chip, containing the capacitors and resistors the amp needs to work correctly, the board is actually laid out for the PX4460 support pack, or discrete components so I could have put normal parts in. I also could have attempted to repair the original PX4460 but that is a bit hit and miss. The only place online I could find that sold them was andysarcade.net, and as I know Andy and was due to meet up with him in Melbourne as part of the Game Masters exhibition I asked him to bring one out with him, and this is it...



As its sole purpose is to support the amplifier chip the chances that this falling off the board being the cause of the board completely dying is effectively zero, without this SIP package the amplifier would not work so there would be no sound but it has no bearing on the rest of the circuitry. Still as the amp chip is likely to be unhappy without it (potentially oscillating and getting very hot) I removed the leg stubs from the old one and installed the replacement...



...unsurprisingly enough this had no effect on the fact the board was dead. The legs on this type of component are clamped on and soldered on one side inside the package, this one isn't in perfect condition as you can see but it does the job.

As the board was clearly not booting, although it was probably trying, the first thing was to point the scope at the data lines on the system RAM, ROMs, which are daisy-chained off the 68000 CPU. There are two ROMs and two RAMs, each providing half of the 16Bits of every read/write cycle of the 16 bit CPU. These looked horrible...



... this sawtooth pattern is not what healthy logic transitions should look like and can be a sign of bus contention, ie more than one device trying to talk on the bus at the same time. On the main CPU data bus there will be the CPU itself, some RAM, some ROM and buffers all of which have to be controlled so that when one device has control of the bus, all other devices stay silent for the alloted period of time. Its possible that one of the RAMs, or ROMs had died in a way that it was no longer able to shut up, but as all 16 data pins across the bus where showing this pattern it would take two identical faults on either both RAMs, both ROMs, or a single fault on the CPU itself, all fairly unlikely, the most likely was the output enable system was not working correctly and the /OE pins on the devices were not being controlled. So I pulled both system ROMs from their sockets...



... leaving the RAM and the two SRAMs below and the CPU below that, and powered it up. With no ROMs the system should do nothing at all, and the data bus should be silent, which is what I got, along with a white screen. This doesn't actually prove much but it allows me to check that all the data bus lines looked the same while the board was quiet.

Once the main ROMs were reinstalled on the board I got this - a working board, kinda...



... the main fault was down to oxidised contacts in the ROM sockets and the reseat scratched through this oxide layer and restored the contact. Had this not changed anything the next stage would have been to dump the ROMs to confirm they actually contained the correct data, but as the board booted they clearly did.

The next fault was that the gfx colour was wrong, somewhat muted and very flickery, more evident at some stages of the attract mode cycle than others...



Having worked on Snow Bros boards before I have seen this fault multiple times and it has always been the same problem, a bad 41464 DRAM chip in what I assume is the palette RAM section of the board.



Each chip has 4 output pins and on boards that show this fault you will find one with its outputs stone dead, working along the chips with the scope I started with IC26, all its output pins looked like this...



...healthy transitions between logic high and logic low.

On IC25 and IC23 they were the same, but on IC24...



...they were tied high and barely twitching.



... despite the input pins and control lines functioning normally.

So it came off the board...



... and a known good chip soldered in.

This fixed the washed out colours and most of the flickering...



... but it wasn't 100% yet.

The upper sections of the background in the above photo are corrupted, some of the text was still flikering madly and on the title screen there was a block of grey where there shouldn't be anything.



As the corruption was neat rather than a mess of pixels the most likely cause is a fault with the address bus which from these DRAMs led straight back to the IC18 custom chip, a big fat custom part. This is what you hope not to track back to as they are likely to be only found on boards from this manufacturer and this brief era, however it turned out that this fault was very sensitive to touch. When the chip was pressed the fault would clear, only to return when the pressure was released, classic bad joint symptoms. Looking at the pins under magnification showed this...



... the three pins to the right of pin "1" have hairline cracks detaching them from the board.



With a dab of flux and some fine solder I re-flowed these joints and restored the connections, close inspection of the rest of the IC showed no other bad joints, and the board was fixed.





Yay!
« Last Edit: January 02, 2013, 05:14:09 AM by Womble »
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Offline Panther

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Re: Snow Bros Repair Log
« Reply #1 on: January 07, 2013, 11:11:07 AM »
Nice one  :)

Not a game I've ever played !