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RCI 2950 Bias Problem

Robb

Robb

Honorary Member Silent Key
Dec 18, 2008
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Silicon Valley CA, Storm Lake IA
Having a problem with a RCI 2950.

The bias voltage is too high, and the driver bias is real low.

Driver bias is around 120-130 mA, and the final bias is around 9 mA.

Is this a regulator circuit problem; or is it a common fault with this radio near the finals?
 

robb, if that 2950 has the bipolar finals in it as opposed to the newer mosfet versions, then chances are you have a blown bias diode.

they are those little thingys mounted to the driver and finals.

it would be D92 for the driver, and D90 and D91 for the finals.

when they go, they can cause the "very high bias current" symptom.

check them with your diode test function on your DMM.

not sure where to get them, but they are all MV1Y diodes, which is typical.

if i were you, i would first remove the leads of the diodes from the board, test them, and whether or not you find a bad one, test the driver and finals to make sure they are still good.
my guess is that they are.

if none of this turns up a bad diode or transistor, then i would suspect the VR itself.
LC
 
They are varistor aren't they? ie a specialized resister designed to change resistance according to temperature.If i were him i would also check the specific VR pots involved,as they have a habit of shorting or opening and or becoming intermitent warlock35po
 
Warlock35po said:
They are varistor aren't they? ie a specialized resister designed to change resistance according to temperature.If i were him i would also check the specific VR pots involved,as they have a habit of shorting or opening and or becoming intermitent warlock35po
Click to expand...

No, the MV-1Y is a thermal diode whose conduction changes with temperature. It is used to protect a transistor that gets hot from going into thermal runaway and destroying itself. It is different from a varistor because it only conducts in one direction, while a varistor conducts equally in either direction. BTW are you sure a carbon VR used to set driver or final bias can short out ?

- 399
 
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