Bias current stuck at 0 on left side

rulerboyz

AK Member
Bias current stuck at 0 on left side (solved)

I've run into another bias stuck at zero situation while trying to set the bias on the Kenwood KR-4140. The right side is holding it's bias at 10mv, but when I test the voltage drop across the output resistors on the left side I get a reading that quickly drops to zero after I apply the meter leads. I tried comparing the voltage drops across other resistors, diodes and collector-emitter of the neighboring transistors and right and left seem pretty close. The collector-emitter voltages for the power transistors are all close to 25V each.

Is it possible that this is a bad trimpot? Before I had the dim light bulb tester I did have that cooking resistor problem which might have taken a toll on the trimpot. Any way to test to see if a pot is still good?
 
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Yes, I get a non-zero reading (for example 15mv) that counts down to zero while my leads are on the resistor. Every time I put the multimeter leads on it I get the same behavior. This is in fact the same behavior I was seeing on my Rotel power amplifier, which is still awaiting repair in a storage box.

I just realized that you can test the Ohm value of your trimpots as though they were regular resistors. This goes back to what Echo Wars was saying to Hpsenicka about the fact that a 5ohm trimpot can be substituted for a 4.7ohm one, or a 200 ohm for a 150 ohm because trimpots are basically resistors which can be adjusted over a range of resistance to get the value you need.


Update:

I've cleaned up some possible solder bridges and tested various other things and am now getting the bias to set at 10mv on both sides. Bias problem solved.
 
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