Once again, I'm trying to reverse engineer an old tachometer circuit--1970s vintage this time.

There are three diodes on the circuit board, and two have absolutely no markings whatsoever. Can anyone deduce the polarity of the two labeled with question marks in this diagram? [I don't know how to write a schematic. Below is an attempt to show the components in their configuration on the original circuit board.)

old tach circuit

Below is a composite image of the components and the connections on the circuit board. The jumpers are not shown enter image description here

  • \$\begingroup\$ maybe they are not diodes \$\endgroup\$ – jsotola Mar 23 at 23:08
  • \$\begingroup\$ you can use the schematic editor that is built into this website instead of posting a picture of the schematic .... it would make it easier to move the components to "untangle" the schematic \$\endgroup\$ – jsotola Mar 23 at 23:12
  • \$\begingroup\$ OK. Will do. Tomorrow. Long day \$\endgroup\$ – Bill Catlin Mar 24 at 0:50
  • 1
    \$\begingroup\$ IF they are both diodes then both must have the same polarity as you show for D2 - as otherwise the meter cannot conduct current. \$\endgroup\$ – Russell McMahon Mar 24 at 1:02
  • \$\begingroup\$ Can someone post a link to the schematic editor on this site? Not finding it. Thanks. \$\endgroup\$ – Bill Catlin Mar 24 at 16:03

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