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Re: Accuphase T-105 FM tuner schematic download?
So far, the meter problem was a defective D'arsonval movement. The manufacturer forgot to solder the bottom spring to the armature connection. Methinks this meter never worked from day one. With 10 power dual loupes, tweezers, a tiny strip of cellophane tape (to temporarily hold the spring where I can get to the inner solder point) and a needle thin soldering iron tip, I managed to make a perfect repair, attaching the meter spring to the armature and restoring functionality.
The next step was meter calibration. I chose to lock in the low and high end of the scale for most absolute accuracy, calibrating in steps from 21dBf, 40, 60, 80 and 100dBf. For the most part, the metering is surprisingly accurate. Then I calibrated the modulation metering (neat feature for a consumer tuner), using a spectrum analyzer to confirm the bandwidth coming out of my HP RF generator as +/-75KHz and setting the meter accordingly.
Sensitivity problem was a bad cable. I did peak up the RF stages for 3dB improvement in gain. Tweaked the phase response of the IFs for .02% THD bottom. Tuner seems to work great and audio response is relatively flat to 18KHz then drops about 40dB in the next KHz. Just finished aligning the LPFs on a spectrum analyzer and lining up both channels with the stored curve.
Pretty nice FM tuner.