Another cluster repaired and packaged up ready to ship back to it's owner.
Another unique fault. The primary issue was the odo display would be backlit, but blank for several minutes (or more) before any mileage data appeared. Strangely, if the cluster was switched off, then immediately back on, the data would appear after a much shorter delay. This pointed to maybe a failed capacitor not charging/discharging as it should. As we know, leaking/failed electrolytic capacitors are often an issue with FD clusters, so seemed a reasonable hypothesis. So, 'scope and multimeter probes in hand, together with a sketched circuit diagram I have painfully derived from the PCB tracks, the challenge was on to find the cause of the issue.
The first clue was that the reset pin on the NEC microprocessor chip (which processes speed data to calculate mileage) was being held low (zero volts, so switching it off) and when the pin went high (5 volts), the processor would spring to life and the data would then appear on the odo. By why/how was the reset pin being held low for so long? The circuit diagram revealed 7 components involved in producing the reset signal. One was an electrolytic capacitor. A 'scope trace showed this to be charging very, very slowly on power up. When charged enough, it operated a Schmitt trigger to send a clean high (5v) signal to the processor to start processing data for the display chip. So, cap changed,
but no improvement! Next, what components were involved in charging the cap? A few resistors and a diode array (essentially 2 diodes in a TO-92 "transistor plastic package). Resistors rarely fail, and when they do, there are usually visual signs, so the array, DA2, was a potential problem. In circuit testing showed one of the diodes was suspect. I had a used, known good, spare in stock, so changed it out and that fixed the fault.
There is now the usual slight delay as the processor starts to operate, as seen in this video of the CPU board on my test box. The processor is held off deliberately for a short period at "ignition on" to allow the voltage to stabilise.
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Here is the section of the PCB after changing C6 and DA2. Note the main solder (and critical) contacts are on the rear of the PCB. The front pads are simply plated through holes.
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Note the early signs of black corrosion, caused by the large cap (C3) in the photo starting to leak it's electrolyte. The customer agreed I change this cap out and clean up the corrosion. In doing this, I noted ZD3 (a zener diode) looked pretty badly corroded!
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So that was changed as a precaution. I have seen this component fail on two previous repair jobs!
That's much better, new cap and diode in place, with electrolyte corrosion cleaned.
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Back side soldering of DA2 and C6....
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There were a few other, minor, items to fix (blown warning lamp, broken trip button...) but all good on final test:
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