Talk Morgan Forums Morgan Sports Cars Classic Morgans (pre-CX) Dreaded PCB Stikes Again

To conclude this one, here's the repair completed by my local electronics engineer for £36.00



As we can see the burnt track has been neatly bridged.



After studying the relay Richard chose to fit an uprated silicon diode in a slightly higer value.



And here's a close up of the uprated silicon diode next to the original, it does look like germanium diodes were originally used on the board.



Germanium diodes have higher reverse leakage current and that increases with temperature, another drawback of germanium diodes is their low reverse blocking voltage that limits its operation to low voltage rectification only. Conversely, silicon diodes can work reliably up to 170 degrees celsius, and the reverse bias saturation current is very high. The forward break-over voltage of the silicon diode is very high, losing roughly 0.7 – 0.9 V with the silicon crystals having the capacity to withstand high temperatures, moreover, the forward voltage drop for silicon diode junction is low.

In summary, silicon diodes are more temperature stable, less leaky and generally way more robust than the germanium diodes it looks like were used on the Morgan dash module PCB, Richard clearly felt switching to a more robust silicon diode of a higher value would offer superior reliability and longevity.

I collected the repaired board from Richard on Friday, fitted it on Saturday and tested it yesterday.



I'm pleased to report all my previously missing switch illumination and tell-tale warning lights have returned, and the speedo continues to work perfectly, for those following this post you'll know it was the loss of my speedo that started all this. However, I'm currently running with one fan speed only as going to fan speed two will blow the 5a fuse 4 for the dash module, I'd say this will be an issue with the relay for fan speed two rather than the blower motor itself. The good news is a Morgan heater is a powerful thing especially now I'm able to open the heater valve fully (more on this later) and fan speed one is more than capable of moving all that hot air, I conclude the loss of a second fan speed is more or less irrelevant.

What's important to me is my speedo works, I've got my indicator tell-tales back, my headlight and dip beam tell-tales have returned and my switches illuminate as they should. Back to the heater, with the dash out I took the opportunity to free up the partially seized heater control mechanism that from day one since I bought Monty has been impossibly stiff to operate, it now has the full range of movement from off to a fully open heater valve that with fan speed one gives furnace like cabin heating performance even when driving with the roof down.

I also removed all three gauges and polished up the bezels which came up like new, located an annoying rattle that actually turned out to be a loose handbrake lever button, and generally made sure all the connectors were good.



Overall I have to say I'm not very impressed with Morgan's dash module PCB, I would not be at all surprised if I suffer further diode failures, fortunately dropping the dash to remove the PCB is easy and now I've learnt the job the next time will be even faster. Having Richard just up the road is also a real bonus, he repairs all sorts of electronics locally and has a very fair charging policy.

As I understand it Morgan was charging £600 for a replacement PCB which is insane and many on TM have reported multiple failures, and of course these PCBs are no longer available so these days the failed one must be repaired. Moving the relays off the board as shown to us by Rog in his excellent post is definitely a good idea, it’s certainly one way to upgrade those weak germanium diodes to something better, but I can't help feeling the ultimate solution would be to delete the PCB entirely as offered by Luigi Borghi with his custom dash module option.



However, given how easy it is to access the PCB and as it only cost me £36 to have it repaired and uprated locally, this proved the best option for me. If it fails again I'll have Richard upgrade all the diodes, and if it fails a third time after that I think I'll be investing in a Luigi Borghi custom wired dash module.

Dave.