Speedo and tacho not working - Fix

[ianelvar]

Well, this is how I fixed mine - hopefully it helps a few more people down the line. Having done a bit of reading I suspect a lot of people have suffered from this very problem, but I have yet to see a fix posted, so hopefully I\'ve filled a gap.

As you can see in this thead:

http://m.rx7club.com/forum/showthread.php?p=9967007

post number 10 by MR_Rick shows the internals of the tacho and the broken components that lead to instrument failure. The big electrolytic capacitor and the four Zener diodes, plus one of the smaller caps all needed replacing in mine, but I\'d imagine may vary instrument to instrument. Spotting component failure is often easier than you might think as they often leave telltales. The caps will either mushroom up on the top of the can - if they\'re convex they\'re broken - or you will see black marks on the solder joints below where the magic smoke has escaped. Broken zeners and transistors can be spotted similarly by black marks on their solder pads as well as cracked casings and discolouration on the PCB beneath. To save the pain of trying to read the zener values, left to right, they are 4.7, 5.1 and 5.1 in the picture mentioned above. These appear to be used as crude voltage regulators for the speedo and tacho, hence the failure of both as the speedo feeds the tacho. The 1000u cap is vulnerable, as are any of the other can type caps. Replace as you deem fit for a few pence worth of components and enjoy a working rev counter and speedo again. I\'d recommend 1.3W zeners and 105 degree caps. Mount the zeners away from the board a little to aid cooling.

For those not familiar with PCB work, replacing a component can be tricky, so I\'ll give the following tips:

Use at least a 50W iron. Some of the solder pads are on large areas of copper and sink a lot of heat away, so this is an absolute necessity.

Clip the components off their leads and unsolder them seperately. Mazda saw fit to bend the leads over onto the pads so there\'s an art to unsoldering them with the component still attached. Make life easy for yourself.

Do not pull hard on component legs! Wait until the solder is molten right the way through the hole and it will come out quite nicely. Pulling legs out will result in the plating in the via coming out with it and ruining the PCB (unless you\'re very clever).

If the solder doesn\'t want to melt nicely, add some fresh. This might seem counter intuitive adding solder to something you\'re trying to remove, but fresh flux and non-oxidized solder helps transfer heat.