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Womble
2nd June 2009, 07:28 PM
Picked up a couple of faulty Snow Bros boards a while ago. :118:

One had missing chips so I focused on the complete one initially, the seller had sold them both as having gfx issues and stated that neither had been hacked around, not quite sure what he calls the fact of logic chip hacked off the second board, old age probably.

Anyway - the fault on the 1st board was that the colours were messed up and the screen was very very flickery.

http://i40.photobucket.com/albums/e230/Womble76/sb1.jpg

Hard to catch "flicker on photos" but you can see the raster scanning down the face changing the colour from blue to pink, thats flicker!

http://i40.photobucket.com/albums/e230/Womble76/sb2.jpg

The game looks ok but the colours are washed out and that damn flickering....

http://i40.photobucket.com/albums/e230/Womble76/sb3.jpg

I went over the board looking for damage and found nothing, despite the dust it is a very nice board. The game was running fine and the sounds were good so I didn't bother to dump the 4 ROMs, just reseated them.

Had a poke around the board with the scope and found that the majority of the chips had what looked like an unstable logic 1 state - the high logic level was pulsing in time with the screen flickering, by looking away from the screens but watching the monitor and the scope screen out of the corner of my eyeI could see that the images were exactly in time with each other.

At that point I thought it would be an easy fix, all I had to do was find the chip with normal inputs and bouncy outputs and that should be the problem - nice idea except I couldn’t find a chip like that. I was somewhat concerned by the NEC D41101C chip by the custom IC, I struggled to find out what it was. It looked rather like a SRAM chip and if I assumed the pinout was the fairly standard SRAM pinout then it did have a few outputs bouncing around, however it also had an "address pin" that looked exactly like a clock signal which kinda buggered my theory. Turns out it is something to do with the video, a "910*8 NTSC line buffer", either way when I had the correct pinout it was clear that its outputs were ok, it was the input signals that were bouncy and that clock signal was indeed a clock signal.

I really dislike having a board that is doing bad things when there is a fat custom chip present, this one is a 120pin surface mount chip that I couldn’t find any info on. Thankfully customs are pretty robust so I left it alone and hoped it was not the culprit. Thats about all you can do with customs, leave them alone and look for other issues, if you find none then you end up back at the custom again.

Poking around the two Sharp LH5126-12 chips at 6N and 7N I found what looked like a fault but turns out to be totally normal. Address pins A8, A9 and A10, along with data pin D7 were all tied low. This is very unusual as it effectively removes a large lump of the RAM space and partially buggers the output. The other RAMs at H6 and H7 are used normally.

Sticking with the RAM I hit the 4 DRAM chips at A2, B2, A4, B4 - the 1st arcade board I have seen with DRAM instead of SRAM. It needs more support circuitry but the capacity is a lot higher, probably why DRAM has since taken over as the standard RAM chippery.

At this point I made a stupid mistake, arcade boards usually have the chips lined up so that when the JAMMA connector is on the left hand edge (how my bench is set up) the chip markings are the right way up, this board is the reverse. I read the chips as being 4164 chips - very very common chips from the early to mid 80s, so I pulled out the pinouts for them and went round the chip. It didn’t make much sense, pin 1 on 4164s are not used, yet pin one on these chips was happily active. For some reason this didn’t ring alarm bells, and as I have a nasty habit of counting up from 1 to find a pin on side A and down from the 5V pin to find a pin on side B I also did not spot that 4164s have 16 pins and these buggers had 18. 4164s are 1 bit wide chips, each address location is only 1 bit, so they are usually used in banks of 8, each chip contributing 1 bit to the byte being read.

These chips were actually 41464s a very different animal, an easy mistake when reading upside down at an angle I guess. These chips are more modern than the 4164s, slightly more inline with the vintage of the board actually. They are 4 bit wide chips, ie 4 data output pins rather than the single one on a 4164.

Once I actually applied the correct pinout to the chip I was trying to troubleshoot on the scope it made a whole lot more sense. Hitting the 4 data lines on all four chips I found one chip that had very unhappy looking outputs. Because of the flickering it was hard to get a good zoomed in view of the data lines, on the good chips with the best scope settings I could find, the data line is frenetically busy,

http://i40.photobucket.com/albums/e230/Womble76/sb4.jpg

... on the suspect chip it was very quiet, a lot of bumping as per all the other chips, but not much activity.

http://i40.photobucket.com/albums/e230/Womble76/sb5.jpg

So I desoldered that chip and powered the board back up without it.

http://i40.photobucket.com/albums/e230/Womble76/sb6.jpg

Nothing changed, the bouncing was still there, the colours were not great and the screen was flickery, this was good news. If the chip was doing something then the board should behave differently when it was physically removed. This suggested that the crap colours and flickering are what the board does when one RAM chip is missing, either physically missing, or still there but knackered.

Found a guy on eBay in the US who happened to be selling 4 of these chips at $2 each, so I bought the lot, soldered in a socket and waited for the parcel. It arrived today and when I dropped in a new chip the problem....

.... vanished!

http://i40.photobucket.com/albums/e230/Womble76/sb7.jpg

http://i40.photobucket.com/albums/e230/Womble76/sb8.jpg

http://i40.photobucket.com/albums/e230/Womble76/sb9.jpg

http://i40.photobucket.com/albums/e230/Womble76/sb10.jpg

The bouncing logic issues have gone away too, I can only guess the board strobes through each chip in turn and the lack of a working chip at that location causes major instability, am surprised that it affects most of the board though, it can't be a short in the chip from the CAS or RAS signal as the instability was there when the chip wasn't

Just got to fix the second one now, handy that it is almost the same fault :)

stu
2nd June 2009, 08:16 PM
Awesome game Womble!!!

Thanks for the log!

Womble
2nd June 2009, 09:18 PM
Thanks, am looking fwd to a good blast on it - my wife loves games like this too.

Decided to have a go at the second board and it wasn't quite the same issue, will post it as a reply to this thread as there is a limit to the number of photos you can have in one post.

So - the second board also had "gfx issues", when it 1st arrived and I plugged in it did this - the game ran but everything was red and flickering like mad.

http://i40.photobucket.com/albums/e230/Womble76/sb11.jpg

The RGB lines connect back to 3 resistor networks strung across two 74HC273s, at 9N and 9M, one of which had been removed rather violently by a previous owner so no surprise the colours were borked. Despite the way it looks all the plate-through holes were thankfully still connected to their tracks. The chips ground pin was still in its hole and had been cut off the chip when they gave up on it. This board is a pain to work on, the copper ground and 5V plane must be massive compared to my other SB board, its almost impossible to get things off this one.

http://i40.photobucket.com/albums/e230/Womble76/sb12.jpg

The existing HC273 tested as OK but I fitted a matching pair of new chips and brought a bit more life back to the board, a nice sepia tone to it all, plus flickering.

http://i40.photobucket.com/albums/e230/Womble76/sb13.jpg

Hitting the output pins on the 41464 chips as per the 1st board I found the chip at 4A had all 4 outputs stuck wobbling around high, so whipped it out, fitted a socket and a new chip.

http://i40.photobucket.com/albums/e230/Womble76/sb15.jpg

I was expecting the board to fire up perfectly, except it didn't. The flickering was gone but everything was still sepia, a distinct lack of blue!

I put the scope on the blue jamma pin and got nothing, the red and green were happily buzzing away. The blue pad on the JAMMA connector connects straight to the far end pin on resistor network RA5 at 10N which connects to the HC273s. On closer inspection it looks like this has had a bit of a whack. There was a small crack in the black housing by the +5V bus pin, which when poked with a screwdriver, flaked off.

I desoldered the whole thing and the bus pin (that connects to 5V) fell off. I went over my scrap boards and could not find another one that was the right number of pins and the right rating, nor could I actually confirm anywhere that this is a normal resistor network - am not sure it is based on how it is connected in this circuit, so I got my mini files out and attempted to mend it. I filed just enough of the black coating to expose some of the metal pad on the internal structure. Refitted the component to the board minus the bus pin, then took an ofcut from an LED leg and bent it through the PCB hole so it lined up flush with the exposed inner metal on RA5. Soldered the old LED leg to the PCB then soldered the end to the broken edge of RA5.

http://i40.photobucket.com/albums/e230/Womble76/sb16.jpg

A quick trim with the wire cutters and powered her back up.

We have blue!!:D

In fact we have a perfect Snow Bros board :)

http://i40.photobucket.com/albums/e230/Womble76/sb17.jpg

I put a drop of PCB lacquer over the repaired RA5 to prevent it from oxidising as there was only a tiny bit of metal to work with originally, I won't be lucky again.

Pash
2nd June 2009, 09:29 PM
Great work there Womble getting not one but both boards going again. I've got a Snow Bros board in my collection, got mine in a bulk lot of non working boards and bugger me it worked. Love the game.

Again well done mate, good write up also.

VectorGlow
3rd June 2009, 03:43 AM
Excellent repair log Womble. :D

I've got a Snow Bros board that I partly fixed months ago - had loads of faults, all the 41464's were faulty, along with the CPU and a few other ICs. Must have been connected up wrong or been subjected to an over voltage.

It now plays pretty much blind - the remaining fault appears to be the D42101-C - it's a 24-pin 'skinny' DIP near the main custom - I've seen this 42101 referred to as RAM and a line buffer. Can't find any datasheet for it though I'm afraid. Womble, do you (or does anyone) know if this can be subbed with some kind of equivalent?

Womble
3rd June 2009, 05:39 AM
Yeh I struggled to find info on it, apparently it is a 910*8 NTSC line buffer, not surprising your board plays blind as is part of the video system. If you google for the chip using the above description you will find a datasheet for it, I might have kept the one I found actually, will let you know.

VectorGlow
3rd June 2009, 05:44 AM
Thanks very much. :)

I tried Googling for a datasheet some time ago but didn't find one, will try again, but if you stumble across yours please let me know. :)

Womble
3rd June 2009, 10:25 AM
Seek and ye shall find!

The D41101 may be interchangeable with the D42101. Based on what I saw on the scope the pinout for the D42 makes perfect sense when applied to the D41 and seems to do the same thing too. I was working from the D42101 datasheet, only just found the D41101 pdf this morning :)

VectorGlow
4th June 2009, 06:42 PM
I did PM you a reply, but just a quick reply here to say a public thank you. You're a credit to the arcade community. :D

VectorGlow
6th June 2009, 09:56 PM
Hope it's okay to tag a bit of repair info onto your thread Womble, but it seems daft to start a new thread. In fact, might be worth keeping all repair threads separated by the game, ie one thread per game, so merge existing ones and just append to any others irrespective of the thread starter. Anyhow, I'm rambling and it's up to the esteemed mods to decide whether my suggestion is a good one or not. :D

On with the motley:

Symptom: thin, broken up, shimmering horizontal lines through various parts of the display. Shows up best in test mode where most of the test grid is blue (should be white) with a few parts being white, also parts of the colour grid has shimmering lines, etc.
Diagnosis: Found that prodding the clock input (pin 8) on the D41101C-1 at location IC22 with a logic probe stopped the problem completely, but looking at it with my ?scope?s probe made it worse, although the signal looked okay. This clock signal also goes to the Pandora SMT custom chip and the two LS273?s at locations IC11 and IC12. It comes from pin 8 of the LS04 at location IC44 where it goes via a 33 ohm resistor before going to the aforementioned ICs. Piggy-backed a good LS04 on the suspect one and the fault went away.
Cure: Replaced faulty LS04 at location IC44

VectorGlow
7th June 2009, 01:12 AM
A bit more info re the 42101/41101 - according to an electronics contact of mine (who also repairs arcade boards in his spare time) this is a FIFO. There's no drop in modern replacement part, but if an adaptor was made up the following SMT FIFO should work:

http://uk.farnell.com/integrated-device-technology/idt72221l15pfg/fifo-1kx9-smd-72221-tqfp32/dp/1217995

Apparently it might also need some glue logic to convert the control signals.

As an aside, dropped a 41101 into my newly repaired Snow Bros board and it works just fine in place of the 42101 that had died a death. The 41101 is also used on Kaneko's "Air Buster", as is the 5-pin PX4460 just above the amp on Snow Bros.

Hewitson
13th June 2009, 11:47 PM
Not sure where in melbourne you are from Womble but Semtronics in Nunawading is a great source for resistor networks. It would be nice to replace that one.