# G0704: Intermittent erratic RPM display



## sru_tx (Feb 21, 2020)

I have only found one thread on this topic. It was a long time ago and the OP has moved on to better tools. In case someone else has this issue...

Problem:
The motor would run nice and steady but the RPM readout would show a steady number for a while then it would flicker, numbers get dim, all 8's, 2's, then back to displaying.  It was annoying to say the least.  

Solution:
After poking around I found that the 5V power supply to the display board was bad.  It was below 3V and fluctuating. I tracked the supply to the power board and U1 is a 78L05, 5V regulator.  It was really hot.  I found that the part was mounted backwards!  I also saw some solder joints on the display and power boards that had flux on them.  Typically manufacturers will clean their boards after assembly. I have a suspicion that the previous owner had this display problem and touched up solder joints hoping to find a cold joint.  Not sure who installed the regulator incorrectly, manufacturer or the previous owner. I replaced U1 with a 7805, higher wattage.  

Result:
Bob's your uncle.  5V is solid.  Display is fixed.


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## markba633csi (Feb 21, 2020)

78L05 is a light duty part, only good for 0.1 amp (100 mA) at most
7805 can do an amp or more depending on the heatsinking
The input and output pins are reversed between the two parts- the factory probably made the mistake when they built it
-mark


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## sru_tx (Feb 21, 2020)

I misread the pinout the other day. You're right, the I/O pins are reversed for the 7805 vs 78L05.  My bad.  So the original part was installed correctly, however, it was still bad.  The 7805 in the TO220 pkg is much more robust.  And for only $1 I've got my display back.  happy camper.


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