Atari mini Trakball opto repair

Thought this might be of use to anyone with a busted mini Trakball, as used on the cocktail version of Missile Command and others.

One of the Trakballs on my MC has been very erratic since I bought it.  Symptoms were direction changes on-screen even when the ball was continually moved in one direction.  Both x and y were affected.

The problem was tracked down to the 2 optical encoder boards.  Each board has two infra red transmitters and receivers inside a black, plastic enclosure:



While the enclosure may look like an all-in-one custom part, it isn't and its innards can be replaced.

Drop in replacement parts can be found at Maplin - see part numbers CH10L and CH11M.  Here they are:



Unsolder the 8 legs from the board and remove the optical switch enclosure.  Looking at the base, you'll see that the rx/tx parts are slotted into holes with a small plastic clip preventing them from being pulled out.  Now the tricky bit: the clip needs to be held aside just far enough to allow the part behind it to be yanked out by the legs.  I did this by getting a thumbnail behind the clip to hold it back while grabbing the legs with a pair of pliers held in the other hand.  Get it wrong and you'll be pulling component legs off at this point!  Don't panic: you'll almost always get 4 attempts as the first break usually leaves a stump which you can still grab onto.  Once down to the last stump, snapping the clip off would probably be safer than taking the risk of not being able to remove the part.

One receiver removed:



Got 'em all:



Now slot the new parts in.  The receivers connect to the outer pins on the board's connector and live inside the smallest part of the enclosure:



Make sure the components are inserted the right way around with the raised 'bobble' on each facing towards the middle of the enclosure.  There are grooves on each hole, so it would take a determined effort to put one in back-to-front.



The transmitters should have red dots on the back of the legs.  Alternatively, when the parts are held with the raised part facing towards you and the legs pointing down, the right leg of the transmitter is longer than the left.

All parts in situ:



Make sure the parts are pushed down far enough for the clips to pop back into place.

Now solder the assembled enclosure back onto the PCB and pop it back in the game for testing.

After surgery, my Missile Command Trakball was as good as new.

--
Dave



Edited by DaveSpicer - 05 October 2010 at 7:37pm

Nice one!! very handy 

Top post Mr Spicer :)  

Edited by Aran - 06 October 2010 at 5:42pm

There may be some variation.  Not sure about trackballs, but I've seen mice with opto-couplers which have one receiver and one transmitter molded into a single, U-shaped part.  I think the upright Missile Command uses the same board as above, in which case off to Maplin...

Any trackball with opto couplers in it could be repaired by sticking the seperate parts directly on the board as long as you get the polarity right.  Worst case, the resistor on the LED side may need changing if the current rating is different on the replacement parts.  Having the enclosure presumably helps to cut out ambient light and makes sure the parts are positioned properly.

Assuming you have the same opto board as above, you can do some simple tests to track down problems:

1) Check the resistance between the ground and outer pins on the opto board with everything wired in, but the game off.  With the connector at the back of the board, the ground is the left-hand one of the center 2 pins.  You should see 3.3K on a Missile Command.  If not, start pulling things apart and testing step by step to find a short, duff connection or bad resistor.
2) If step 1 was ok, turn on the game and monitor the voltage on each of the outer pins of the opto board in turn.  You should be able to make both pins change between 0 and close to 5 volts by slowly turning the appropriate roller.  If you can't, there's probably an IR LED or receiver fault and you need to swap the parts out.  Might as well do the lot while you're there rather than trying to fault find any further.

--
Dave



Edited by DaveSpicer - 28 October 2010 at 7:59pm

I think these might work ?

The key is keep the clip clear of the opto part and pull firmly but slowly.  They fit very snugly and at first I did wonder whether they were glued in.  Something else you could try is screwing a long, small-diameter self-tapper into the clear casing to give a firm anchor point.

The housing keeps the optos nicely aligned and, as you said, probably helps to cut out ambient light.  They ought to work just fine if put straight on the board and I've seen old ball mice done this way.

I'll have to have a better look at the parts you quoted tomorrow.  The transmitter has the same If as the Maplin ones, so ought to be okay.  The forward current is easily tweaked by changing the resistor on the opto board if need be.

--
Dave

Hi,

Hmmm, forgot about this, sorry.  My previous post didn't involve an engaged brain because the max If value is pretty much irrelevant unless it's unexpectedly small.

The datasheets aren't amazingly helpful for those parts, lacking any guidance as to typical values.  However, I'd be inclined to try them as you can easily tweak the brightness.  The legs do appear to be the right way around, although be aware that the receiver has the long/short legs switched when compared to my parts pictured earlier.  Make sure you have some way to tell them apart before removing the packaging!

Suggested course of action based on thinking about it a bit:
(background info in a followup post so as not to get bogged down)

- Swap the parts in.
- Monitor the outputs from the opto board with the game plugged in and powered.
- If you can shield the board as much as possible, you should see 3.3-5 volts and close to zero volts as you turn the rollers by hand.  The game will do *something* if the difference between the two is at least 2 volts or so, but the results may be erratic.

Possible failures...

Output stuck at zero probably means the transmitter/receiver are back to front (or I was mistaken about the leg order).
"On" voltage too low could be a bad connection or insufficient transmitter brightness.  Power down and test the resistance from the outputs to ground on the PCB.  You should see 3.3K on a Missile Command, not sure about others.  Waggle the loom and connectors to make sure you can't make the value change.  On mine the opto connectors were really loose and I bent the pins a little to make a more solid connection.  Also check that you have a good 5 volts at the board.

Assuming these checks didn't find anything, you could decrease the value of the resistor on the opto board to increase the transmitter brightness.  Temporarily holding/clipping another 100 ohms in parallel will double the forward current and would be a good first step to see if it makes any difference.

--
Dave

[Edited to fix 'transmitter/emitter'.. should have said 'transmitter/receiver']



Edited by DaveSpicer - 07 March 2011 at 9:01pm

Excellent info Dave, thanks !!!

A quick bump for this old thread as it helped me out today... After telling someone I only had one game down at the moment, during a game on Centipede this afternoon the up/down motion of the trackball stopped working. Doh! A google search pointed me back here, I removed the Optos from the boards and found I had one faulty receiver when comparing it to the other working one, after quick trip to Maplin and a bit of soldering Centi's working fine again. The Maplin part numbers are still current, if you're lucky they may have one of each in stock!

Edited by smarty - 19 October 2014 at 4:23pm