Reverse engineering a crossover

[vdH_83]

Do you have an LCR meter or DATS or other way of measuring inductance

I do now See below for final version of the schematic. Next step is measuring the speaker. (this might take a while, my RME-interface is gone for a few weeks)

 

[vdH_83]

I've been measuring the components on the PCB with DATS and some irregularities came up.

1) When I measure the 10R-J resistor ( in the mid-range path) it gave a resistance of 8ohm. Can this be caused by something on the board, or is the actual resistance 8ohm. See picture.

2) When I measure the 25uF capacitor it gives a value of 346,9 uF. Why is this?

 

[terryforsythe]

1) When I measure the 10R-J resistor ( in the mid-range path) it gave a resistance of 8ohm. Can this be caused by something on the board, or is the actual resistance 8ohm. See picture.

The resistance measurement can be affected by other components on board if your meter is measuring using an AC signal. I have the same issue with my LCR meter, and thus don't use it to measure the resistance of resistors in a circuit. (In this regard, the resistance measurements on your inductors also may be off.)

If you have a meter that measures resistance using DC, that should work. If not, I recommend un-soldering one of the resistor's leads from the pc board and take your measurement.

EDIT: If you do measure the resistor in circuit using DC, it may take a while for the reading to stabilize due to the capacitors in the circuit. They will draw current and throw off the reading until they charge up.

 

[Zapper]

Dear Zapper, please see picture below. Just checked the PCB for the seventh time. Im pretty sure this is it Used DATS tonight and the inductors are measured now. I'll upload the DEF schematic right away.

View attachment 337740

vdH_83,

Thanks for clarifying, but those are the exact same schematic, just drawn differently. Both have a 4-way junction between Medium L, 10R, Red L, and 25uF. Red L connects to a second 25uF to GND and the speaker(-) in both versions. 25uF connects to a second Red L to GND and speaker(+) in both versions. Both version have the same netlist. Your revised schematic looks better and is easier to understand though. One schematic style point: avoid 4 way junctions unless you use a dot to mark a connection, because you can't be sure whether the two wires cross or join. The CTO of one chip design company I worked for went as far as mandating that no 4-way connections were to be used in schematics, dot or no dot, because he didn't think it was clear enough.

 

[Zapper]

The resistance measurement can be affected by other components on board if your meter is measuring using an AC signal. I have the same issue with my LCR meter, and thus don't use it to measure the resistance of resistors in a circuit. (In this regard, the resistance measurements on your inductors also may be off.)

If you have a meter that measures resistance using DC, that should work. If not, I recommend un-soldering one of the resistor's leads from the pc board and take your measurement.

EDIT: If you do measure the resistor in circuit using DC, it may take a while for the reading to stabilize due to the capacitors in the circuit. They will draw current and throw off the reading until they charge up.

I agree. To measure the red inductors and the 25uF caps it's also necessary to unsolder one end of the device, because parallel paths will make the in-situ measurement inaccurate.

 

[terryforsythe]

I agree. To measure the red inductors and the 25uF caps it's also necessary to unsolder one end of the device, because parallel paths wil

That certainly is best.

Also, you may want to check to see how accurate your meter is for measuring an inductor's DC resistance. Some may be good, but some (like mine) do not do very well at measuring inductor DC resistance. Mine does not do a good job separating out the resistance from the inductive reactance - it displays a value, but it is not very accurate. So, for inductor DC resistance measurements I use a meter that uses DC for its measurements.