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Sega Rally preventative maintenance & fixes

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lix View Drop Down
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    Posted: 05 Aug 2019 at 3:49pm
I'de bought a Sega Rally twin unit a while back which had various faults but it was otherwise complete.

One monitor chassis was dead, one of the Model 2 boardsets had graphics glitches with polygons randomly dissappearing, and the other boardset refused to boot.

I did a bit of solder rework to the monitor chassis and I briefly got a picture before the image collapsed into a vertical line and faded away, so I gave up on it and replaced it with a known working chassis (easy way out).

The graphics glitches on the Model 2 board were fixed with a single ram chip (did a repair log on that one), and the CPU board not booting was down to two Fujitsu MB3771 Power supply monitor & Watchdog chips failing to pulse the reset line adequately. I fixed that issue by removing both MB3771 chips and hacking in an old school reset mechanism with a 10uf capacitor between ground and the reset line, with the reset line being pulled up to +5v with a resistor (I forget the value, but after the capacitor has discharged when the power is off then there is a long enough low pulse before the resistor brings the capacitor up to charge, not ideal but it worked). Cypress now seem to make the MB3771 chip, so I'll buy some of those and replace my hacks at a later date.



Anyway, I noted a couple of the cab fans were not working but I've only briefly had the machine powered up a few times since owning it, but it was due to see a lot more uptime so I decided to replace the two dead fans today. It turned out that one of the fans merely had a terminal wire pulled out, so put that back on and the fan is working again. Don't know why I didn't check for loose connections earlier but there you go.

The other side of the twin definitely had a bad fan, so I bought some Sunon DP200A fans from RS components. However I actually bought the wrong type, ones with wire leads instead of terminals, but I transfered the terminal blocks over from the dead fans. Correct type order link below:




The fan that needed replacing was at the back, which while fiddly it isn't impossible to swap without stripping the cab down. I removed the back cover on the pedal unit, disconnected all the cables between the bed and the upright monitor stand section. 

Removed the four screws holding the fan onto the case and reached inside so I could lift up the fan and pluck the terminal power cables off the fan and also unscrew the grounding wire from the fan case. Then with the seat up and the footplate removed it was easy enough to reach inside and remove the fan. Putting the new fan in is just a reverse of that procedure.



While I was in that side I noticed that the front case fan was the wrong way round, so both fans were sucking air out of the case and I guess working against each other a bit. So I flipped it around so air comes in through the front and exits via the back of the machine, I believe that's how they're meant to operate.

Then, as part of my preventative maintenance plan I thought I'de test the power supply lines to see if they were at their correct respective voltages of 5 volts and 12 volts. With a multimeter I decided to check the 5 volts on the upper most ROM board in the cage, I momentarily pressed a finger on one of the capacitors on the ROM board and I recieved a jolt of electricty down my arm. I touched it again thinking that it can't be right, but another jolt went down my arm to where I was resting on the metal case. I was literally quite shocked at this so got my neon mains testing screwdriver, and no matter where I touched on the boardset the neon light glowed bright.

Everything was working, the game was playing just fine but it seemed like the game board was operating at mains potential. I then realised that I'de smelt a kind of fishy smell ealier, the kind of smell you get from leaky capacitors when the electrolyte warms up.

Powered the machine off, took the power supply out and I was greeted with a nice amount of corrosion from several leaky caps in the 5 volt filtering section. The electrolyte had spread across into the mains voltage potential area and was conducting towards the low voltage side I assumed. Nice!



I checked the other side and seemingly both power supplies had this issue, plus there was quite a bit of pcb burn from excessive heat within the power supply cage as I guess the fans hadn't been pulling air through for quite some time.



As I already had two exact same PSUs which were in good condition I decided to swap them over, but before I did so I checked the 'good' ones and at least one of them had a minor amount of corrosion and discovered one capacitor had just started leaking. I desoldered all the caps and checked underneath them for signs of green corrosion on the legs or underside of the cap, or any sign of bulging. Got one good cap off the one of the bad supplies and put it on the good PSU.







Put them in and everything was fine, no more glowing neon screwdriver when touching anything. No more fishy smell!

The frame ground of the machine (which is connected to the mains earth) is never tied to the PSU output ground on these machines, I know some machines do have that, but in these PSUs the frame ground is connected to 'MOV' transient voltage suppressors on all the voltage and ground outputs, so the boardset ground is meant to be floating in respect to the case ground potential. Quite surprised I didn't kill the boardsets just by touching them to be honest, but damage can sometimes take a while to manifest on silicon chips so I'll be prepared for future board repairs.

I'de also found some chewed up wires, which must have been damaged by someone putting the footplate or seat base back with the wires trapped, exposing the wires but not breaking them. I just PVC taped them back up for now.



So I will point out, check your fans, make sure they're still spinning, and check your power supplies for voltage variance, leaky mains and leaky caps! Do it at least once a year, or whenever they're coming out of hibernation. If I hadn't have caught these issues in time then I'de probably have had a very dead Sega Rally on my hands again.





Edited by lix - 05 Aug 2019 at 3:54pm
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Post Options Post Options   Thanks (0) Thanks(0)   Quote Hurray Banana Quote  Post ReplyReply Direct Link To This Post Posted: 05 Aug 2019 at 7:50pm
Very nice write up
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Post Options Post Options   Thanks (1) Thanks(1)   Quote Vamino Quote  Post ReplyReply Direct Link To This Post Posted: 05 Aug 2019 at 8:05pm
Nice write up mate. You did well getting the cab back up and running.

A couple of things I'd add to your list is to make sure all of the cab is earthed from top to bottom. On cabs such as a sitdown Scud Race it makes a huge difference. A properly earthed Scud Race cab reduces the risk of getting sound and monitor interferrence when the FFB motor engages.

Also with a cab that has come off site I'd check around for signs of liquid spills. If someone has spilled something on the dash it has a tendancy to run down the back and it may find it's way into conenctors causing all sorts of issues.
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Post Options Post Options   Thanks (0) Thanks(0)   Quote jonwarby Quote  Post ReplyReply Direct Link To This Post Posted: 06 Aug 2019 at 7:42pm
Thanks for sharing this.  Sorting out the fans is definitely a good shout they don't like to run hot.  Reading your story about the PSU has got me thinking it may be time to check mine and replace those caps.
Looking for DDR parts, Sega Rally Seat Art, Cromptons Silver Ski Pusher parts
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