Things get darker and darker.
- Coils are not open.
- TRIMCAPs are not shorted.
- I confirm problem in band 1&2, bands 3&4 worked like a charm.
Started from scratch.
Test gears setup:
The Spectrum Analyzer is an Advantest 3361B (0.1-3600Mhz) driven by a 10MHz EXT REF coming from a rubidium reference oscillator. I performed the calibration procedures before using it.
The RF probe is an active probe able to reach 500Mhz. Its input is very high-Z. Loosely coupled (gimmick cap) to T3-pin2 (3rd IF output, just before the detector..).
Tracking generator output connected to V8-pin2 trough a 2000pF cap as per service manual.
NOTE: in all the following test the vertical measures a purely qualitative. Horizontal measures are quantitative measurements.
I performed a visual IF alignment using the SA tracking generator.
?
?
The IF cans peaked @452KHz, the BW is 5.9KHz.
The IF strip seems ok to me. Do you agree?
?
Local oscillator actual frequency test:
I relocated the probe input to loosely couple ?(gimmick cap) the local oscillator.
Sig-gen (HP 8640B) @1400KHz, AM modulated @30% connected to ANT by a RTMA dummy antenna. Used as a reference for dial setting.
Here is the plot of the local oscillator signal in the Band 1 tuned @1.4MHz. It seems ok to me (1400+452= 1852 KHZ):
?
Sig-gen (HP 8640B) @4000KHz, AM modulated @30% connected to ANT by a RTMA dummy antenna. Used as a reference for dial setting.
Here is the plot of the local oscillator signal in the Band 2 tuned @4MHz. It seems ok to me (4000+452=4452 KHZ):
?
I think these tests can get the "image" frequency topic out of discussion.
Do you agree?
I then tried to isolate the problem stressing the ANT resonant circuit alone, with the following setup:
The S-85 was turned OFF.
Band spread dial fully clockwise in all tests.
NOTE: in these last tests the right marker is where the peak should be.
?
On band 1, with the Main Tuning dial set to 1.4Mhz (as per service manual), i've got the following results:
??????????? TRIMCAP C4 fully engaged (higher C):?????????????????????????????????????????????????????????????????????????????????? TRIMCAP C4 fully released (lower C):
?
The peak is lower then the test frequency between 370 and 220 KHz - not normal!
The steep left slope and the asymmetry in general are also suspect. In fact, listening on band 1, all signals (and the noise as well) tend to disappear below .5MHz.
?
On band 2, with the Main Tuning dial set to 4Mhz (as per service manual), I've got the following results:
??????????????????????? TRIMCAP C3 fully engaged (higher C):???????????????????????????????????????????????????????????????????????????????????????????????????????????? TRIMCAP C3 fully released (lower C):
The peak is lower then the test frequency between 1MHz and .7MHz - not normal!
At least the slope is symmetric.
?
Removing V1 produced no significant change.
?
Conclusions:
- the tanks do resonate.. but at the wrong frequency. In both cases the peaks always stay far below the tuning frequencies.
- Band 1 also suffer strong slope asymmetry. Listening the radio it looses sensitivity below .5MHz. Why is that?
- The reasonable performances I get from the receiver are due to "partial" signal passing through even if the corresponding tank doesn't resonate correctly.
HELP!
Emanuele (IU1KNR).
?
Emanuele Girlando