Reply To: Strategy for Changing Capacitors and Trim Pots on FM Tuner

#10330
Glitch
BRONZE Member
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    I’ve been able to do a full adjustment of the FM tuner on a BM6000. I more or less gave up on trying to follow the exact directions in the service manual since it was not clear to me how to translate things like the signal levels, etc. to the equipment that I have. Instead, I used the service manual as a guide, along with the schematics, theoretical descriptions of FM tuner operation, and (better) explanations from service manuals for other (non-B&O) equipment to interpret the “spirit of the adjustment”.

    To relate the signal levels to my equipment, I made measurements of the other four Beomasters that I have that (I think) still had their original adjustments. I took notes and used the trends from the sample set to setup the tuner that I replaced the caps and trim pots on. This was a bit challenging since all four tuners were setup somewhat differently.

    My newly adjusted tuner now sounds as good as the best of the other tuners and quite a bit better than the worst of the other tuners. I have no idea if it is actually adjusted “to specification”. It sounds “good enough” that any misadjustment isn’t readily noticeable.

    I was able to do most of the adjustments with a digital oscilloscope, a multimeter, a TinySA, and a handful of attenuators. I needed one other piece of equipment to set the Channel Separation. The service manual called for a stereo encoder. I used a cheap FM transmitter, a waveform generator and attenuators to get the signal down to a proper level.

    FM_Transmitter

    I’m happy with the results. I’m happier that I didn’t have to spend more than the receiver is worth on equipment to fix the receiver.

    I still want to refine and improve my adjustment procedure. One of the things I still don’t understand is what they are trying to convey in this paragraph of the manual

    FM_Adjustment

    What “signal” do they mean in the 2nd harmonic section and what is one supposed to adjust to get the correct waveform?

    Glitch