Taking Readings at the OD Entrance Pot

[kd]

In the "You Choose........Ford or Tag" thread there was mention of the proper way to take readings at the 100k pot for the OD entrance. Someone said it should be taken with the amp on and the OD on. Does this mean the amp is powered up to get a correct reading?

Thanks,
Ken

[butwhatif]

If u r referring to the slider to ground dc resistance reading, it should be taken w/the amp off, otherwise the grid leak voltage will interfere with your meter reading. u r lookin for a dcr reading around 24k ohms as a starting point.

[martin manning]

The catch is that if the power is off, and the OD relay is wired so that it must be energized to engage the OD, the other end of the 220k resistor at the top of the trimmer will be grounded, and therefore paralleled with the pot's element. An actual setting of 24k in this case would read ((220+76)*24)/(220+76+24)=22.2. The grid leak current is possibly the reason why the measurements posted didn't make sense.

[kd]

Thanks guys, that clears it up. I understand now what was being said.

Ken

[butwhatif]

I don't think most people unsolder the circuit to make this adjustment, as well it has been measured in d specimens 'in circuit' . scientifically, the real dcr would be resultant from all paths to ground, but real situation it is usually done in circuit. i do this in circ as well--- just sayin

[martin manning]

Yea, understood. The number really needs to be accompanied by all the other relevant info to be meaningful. You could power the amp and make the measurement in standby and/or with V2 pulled. That would get the actual trimmer setting, no?

[heisthl]

The catch is that if the power is off, and the OD relay is wired so that it must be energized to engage the OD, the other end of the 220k resistor at the top of the trimmer will be grounded, and therefore paralleled with the pot's element.

Just another reason to wire the relay so the default is OD on.

[odourboy]

The catch is that if the power is off, and the OD relay is wired so that it must be energized to engage the OD, the other end of the 220k resistor at the top of the trimmer will be grounded, and therefore paralleled with the pot's element.

Just another reason to wire the relay so the default is OD on.

Come on guys - just take your measurement with the amp on Standby if you feel the need to energize the relay.