Calibrating a Pioneer CT-F1250 cassette deck

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I put a set of ALPS heads in my CT-F1250, to replace the old worn heads. Well, now I'm having a problem adjusting the bias. I've adjusted the azimuth properly and the playback levels and EQ, and that's all peachy. The problem is bias adjustment.

When recording a tape, I change the bias signal generator knob to the bias setting. The deck goes into mono mode, and I adjust the right channel bias setting until the arrows are both lit. Then I record a 3000hz signal from the line input... and the left channel cannot go high enough before running out of potentiometer. The right channel still has a third of its adjustment range left, but the left channel is right at the end and can't go any higher. This means the left channel is under-biased. This is the case for STD, FeCr, and Cr02 tapes. (I don't have metal tapes to test with). Since that left channel is under-biased, anything recorded on the left channel is at significantly lower volume. Which is not fun :(

What would be causing this? I've already checked for tape skew and tape path problems...

Charles.
 
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