Rockford Fosgate Punch 100x2

[nikb47]

I have been tracing components on this amp trying to find where the high dc offset is coming from. With the amp fully powered up I have found that there are some differences with the MMBT3904Ls at Q211 and Q111

Q211
Pin 1 -12.88dcv
Pin 2 -13.54
Pin 3 -1.849

Q111
Pin 1 -12.88
Pin 2 -13.54
Pin 3 -1.805

Could Q211 be defective causing my dc offset on the right channel of .060v?

 

[nikb47]

nevermind. I just looked at the schematic. Ive got more testing to do.

 

[Perry Babin]

Does gently heating Q202 or Q203 (one at a time) cause the DC offset to go towards 0.000v DC?

 

[nikb47]

Yes heat on Q203 causes offset to drop while heat on Q202 causes offset to climb.

 

[Perry Babin]

They may have mismatched HFE (DC current gain). Replace them with two new transistors from the same batch. You may have to replace one of them (either one) repeatedly until you get a pair matched well enough.

This isn't necessary. 0.06v is insignificant.

 

[nikb47]

Ok I understand now! What is this pair of transistors usually refered to as? I understand that .06v is not significant but wanted to do this more as a learning experience. Thank you.

 

[Perry Babin]

These two transistors are part of the differential amplifier.