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

  • Topics Started 9
  • Total Posts 81

I’ve made some progress with my quest to properly set-up FM tuner boards. I decided to experiment on the the tuner from my most beat up BM6000 (~5.5 of 1o condition). This receiver seems to be becoming my “parts machine” (or garage machine) as the best parts slowly migrate to my nicer machines. If I am successful with this experiment, I’ll use what I learn in the restoration of the nicer BM6000’s and BM8000’s.

I’ve worked through replacing all of the capacitors and most of the trim pots. I replaced only the trim pots related to a specific adjustment in the service manual at a time. I noted the settings of the old trim pots and checked/documented the “before” measurements of the circuit. For some of the measurements, I tried tweaking the old trim pots to learn the effects before replacing them. I skipped the steps that involved adjusting the inductors.


In general, I was able to make measurements and adjustments that “seem” like they are correct and consistent with what I’ve researched online. However, I don’t have the experience to know for sure if they are actually right.

Tuner/IF Adjustment:  Using the TinySA and an oscilloscope, this is what I got for a IF curve.

Here is the “before” capture with the OLD trim pots at the LOW end of the frequency band, 85.5 MHz. I don’t have an “after” capture for this since this measurement is adjusted only with inductors.


Here is the “before” capture with the OLD trim pots at the HIGH end of the frequency band, 108.0 MHz.


Here is the “after” capture with the NEW trim pots at the HIGH end of the frequency band, 108.0 MHz.


I was able to see the amplitude of the IF curve change as I moved each trim pot and adjusting to a point where it peaked seems to make sense. I wasn’t able to change the symmetry of the curve significantly. I did note that the “centering” of the IF curve was sensitive to the tuning knob position. I tried to position the knob half-way between where the display changed between 0.1 MHz below and 0.1 MHz above the intended frequency. I don’t know it was the right thing to do.

When I did these adjustments, I did not pay much attention to the “blip” near the center of the IF curve. I originally just thought it was an anomaly (random noise) in my setup. This may actually be a feature of the TinySA and be a “marker” at the center of the sweep. Unfortunately, the documentation for the TinySA is sparse and the firmware is still somewhat fluid, so some educated guessing is needed. I have some more research to do here.

If the “blip” is a center marker, would it make sense to center the marker with the tuner knob, then try to adjust for symmetry with the trim pots?

Another area that I have questions relates to an earlier question about the “mV EMF” level. I noted that some of the settings seems to be pretty robust to the input signal level. Other settings, like the “AGC” and “Opening of the Stereo Decoder” were sensitive to the input signal level. I think that the interpretation of the meaning of Vac is what is messing me up, or at least is adding to the error. There is a 3 dBm difference between Vrms and Vpeak, and 9 dBm between Vrms and Vp2p. Any comments on how this should be interpreted would be appreciated.

How I handled this was to note the signal level needed to get the correct voltage measurement with the old trim pot, then initially set the new trim pot to match. Afterward, I then played around with resetting the voltages based on a Vrms assumption of the signal level. I am not very confident that this is giving me a 100% correct setting.

In general, the FM tuner successfully receives stations and sounds similar to what it did before I installed the new parts and readjusted. Some aspects of the sound quality actually seem slightly improved. However, I still don’t think that it is exactly right. On some stations, on particular songs, there seems to be a slight fuzziness or distortion. I’ve also noticed this on one of my other tuners (that I haven’t touched). Any ideas on what I could be experiencing?


As a side note, I tried experimenting with using the “3C” method from the original post. For some of the measurements, it provided a results that were pretty close to the final setting. The FM would “work” but could easily be improved with a proper adjustment.

Here is my 108 MHz IF curve with setting R1-R4 with the “3C” method.


I think that I actually just got lucky with this result. I tried to see if the procedure was repeatable and the second time was an epic fail.