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Re: S-85 refurbish: I've got a problem during the RF alignment of bands 1 and 2 - trimcaps don't peak!

 
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Emanuele,

Fantastic pictures, but I do not believe that the C7A tuning cap can be at fault.

Obviously the coil have too much inductance, so it¡¯s core have to be moved OUT of the coil (to the top, or to the bottom, I do not know which one will be best).

Normally, the trimmer cap should be set at it¡¯s center setting, then the core of the coil have to be adjusted to obtain a peak at the low frequency specified in the alignment instructions, then the response should be peaked with the trimmer at the high frequency specified in the alignment instructions.

Then go back to the low frequency with the core, then to the high frequency with the trimmer, and doing this several times until no improvement is measured.

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But Ah¡­ I understand: no alignment instructions in the manual for the CORES of the coils, like if nobody had touched those since the day of the manufacture !!!

AARGH !

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Find a plastic driver and MOVE those cores while observing what happen with your Spectrum Analyser, PLEASE !

BC band: low side at 600kHz (L3 core ??), High side at 1400 kHz (C4 trimmer), then loop again.

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Is it just me, or the two lower band inductors are connected in reverse (one for each other) in the S85 schematic ???

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73, Jacques, VE2JFE in Montreal

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De?: [email protected] <[email protected]> De la part de Emanuele Girlando via groups.io
·¡²Ô±¹´Ç²â¨¦?: 16 f¨¦vrier 2025 16:12
??: [email protected]
Objet?: Re: [HallicraftersRadios] S-85 refurbish: I've got a problem during the RF alignment of bands 1 and 2 - trimcaps don't peak!

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I think it's time to narrow? the investigation down and tackle one problem at a time.

As the "v1 / V2 coupling" and the "not C62" problems are really intriguing but seem loosely related to the "trimcaps don't peak" initial problem, I would put them temporarily a part.

In order to reduce complexity, I would also remove from the table the fact that the problem occurs in both the antenna circuits and the mixer circuits.

I would start the analysis again considering the antenna circuit only.

There will be no shortage of surprises though.

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Since the circuit is difficult to understand, and in fact it seems it shouldn't even work as it is, let's temporarily consider it as a black box and examine its operation as if it were a pure unknown DUT.

The setup is as follow:

The receiver is turned OFF. Band switch on band 1.

BS dial fully CW (-> C5-A fully open that is at minimum capacity.

Tracking generator output set at -40dBm.

Absolute vertical readings are only qualitative measurements.

Relative vertical readings can make some sense.

START 500KHz / STOP 1800KHz - SPAN=1300KHz - 130KHZ/DIV.

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Here is what I get:?

Band 1: main tune dial at 0.54MHz - the bottom of band 1 (C7A fully meshed - maximum capacity)

The peak is a little below 500KHz (in fact not visible) - C4 has no visible effects.

Turning the main dial toward higher frequencies, I see the peak moving right.

At the end:

Band 1: main tune at 1.6MHz - the top of band 1? (C7A fully open - minimum capacity) - C4 fully close (max capacitance)

I expected to see the peak to move up to 1.6MHz. It doesn't happen! It stops a 960KHz (!!) without even reaching the alignment frequency of 1400KHz.

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Band 1: main tune at 1.6MHz - top of band 1 (C7A fully open - minimum capacity) - C4 fully open (min capacitance)

So, turning C4 fully open the peak moved right by about 180KHz reaching 1140KHz, still far away from 1.6MHz (and even below 1400KHz!).

At 1400KHz (the alignment frequency) I cannot never ever have a peak.

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Considerations:

  1. the peak, while correctly starting from the bottom of the band, never ever reaches the top, even when the mail dial is at 1.6MHz; it doesn't even reach the alignment frequency of 1400KHz. This is the problem.
  2. C4 (that I confirm is part no. 44-191) moves the peak left and right of about 180KHz; it would be good if the peak would be within a 180KHz span from 1400MHz but it is not.
  3. Given this data C4 will never ever "peak" at 1400KHz as required by the alignment procedure.

If the peak went up to 1.6MHz, then I would be able to get the peak around 1400KHz as per service procedure and carefully looking for it tweaking C4.

Another possibility is: the span C4 provides is not sufficient. It should be in the order of 450KHz, too much in my opinion.

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Changing figures, the same happens on band 2, 3 and 4 (the lower peak is at the very bottom of the band while the top peak never exceeds 2/3 of the band.

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So the question is: why does the peak stop at 1140KHz without ever reaching the top of the band?
In my opinion the only possible reasons are: "C7-A defective: too high capacitance" or "a coil is seriously defective".

Another alternative: C4 is faulty not allowing the due frequency span,
This is why I started thinking I have to disassemble the ANT unit and check the wiring and all the components one by one.

C7 is my preferred suspect. If for some reason it has too much capacitance, this would affect all bands the way we see.

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Thoughts?

How can I check C7 without destroying it?

May be grease deposited on the plates over time (at eye I don't see anything)?

May be it needs cleaning? can I use IPA? Oil? gasoline?

--

Emanuele (IU1KNR).
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