Re: DM501A Can't find main board serial number

Rick
 

Because the display was stable showing -.0064 I was able to check the BCD for each digit. On my scope it showed D1 was 0040 and D2 was 0240. D3 & D4 were 0000. That all decodes to 0064. The sign symbol is receiving +2.5v. I think the display is valid.

So I went back to the diagnostic tree in the manual and went in the direction of verifying that relay K1408 was open and then checking R1511, R1408 and trimmer R1405. R1408 was good. R1511 is supposed to be 5% 2.7ohm . It was 25% off so replaced it. That didn't help zeroing out the display. No change.

The R1405 trimmer does trim full scale +12v to - 12v but the meter was not very stable towards +12v. The power supply procedures have you check only the -12v side which was fine but not the +12v. I decided to use the scope and while it is +12v it has a very clean 2vpp square wave on it. I wasn't expecting to see that. R1405 trims between +12v and -12v. So the square wave is bleeding onto the wiper. The square wave almost disappears when getting to the far -12v side.

I need to understand where the square wave is coming from. I suppose it could be coming from the wiper but then I would think it would bleed onto the -12v side and it doesn't. It only shows on the +12v side. The next thing to check in diag. tree is U1601, the analog section of A/D Converter. Among other branches, the trimmer is connected to the Hi-Q GND pin 2 on U1601. I don't really know how that works. I wish I knew what the hell I was doing!

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