2230 numerical readout error and power supply noise


John Brown
 

This 2230 seems to work OK (analog section - don't know about storage).


But the readout of volt/div is wrong on some settings; always going to the next higher reading according to the analog voltage level supplied by the voltage divider. In other words 2mV/div almost always reads .2V/div which is the next higher voltage range the divider provides to the digital section. 2mV measures 2.267V (which is within the proper range 2.104 to 2.340) but the readout displays .2V (which is the next range 2.340 to 2.502).


I at first thought it was the 4 finger contacts on the drum switch; but they measure good and consistent and the voltages out of the switches also measure good - well within the range listed for each setting.


Then I scoped the switch output and found a 20ish KHz spike very noisy about 20mV. My theory now is this spike is fooling the digital decoding. Yes?


Going to the power supply all the positive supplies have some level of the 20Khz noise. The negative supplies not so much.


The 100V supply has a 250mV spike. 30V supply a 50mV spike. I replaced both filter caps (C954 and C956) with no change. The 8.6V supply a 30mV and the 5V supply a 20mV. Replacing caps made no difference.


I will keep plugging away. All power supply measurement has been at the final filter cap for that supply (diagram 9)


Isn't this in fact way too much noise on the supplies? Whether or not the cause of the bad display readouts.


Thanks
John

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