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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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Hello Emanuele,

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Did you see any part number on the Main Tuning Capacitor ?(C7) ??

It should end with 341¡­

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As you seems to have a LCR meter, can you also measure C7A (the section of the main tuning capacitor that tunes the Antenna Coils ??

It can be easy with L3 being removed.

Please measure the fully meshed and fully open values.

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And, being there, can you also measure the inductance of the two L3 windings, please ??

From those measures, we can try to make sense of what is happening.

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It will be also good if you can take pictures of the L3 coil, seen from the top, and also from the bottom.

Can you see something in the center hole, or not ?

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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¨¦?: 19 f¨¦vrier 2025 17:59
??: [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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Don,

no idea about the run of this unit. The serial on paper is almost impossible to read; looking carefully it seems read "serial. 196479". No other info.

Here is an info briefing:

As arrived, the radio looked fully original.

I recapped it following this rule: do not touch anything in RF sections (ANT, MIX, OSC) unless something proves to be really faulty. Main filter caps were retained as they measured good and perform well. Checked all resistors and replaced when out of value (above the required tolerance).

At the end of the job all voltages where very close to the ones indicated in the service manual. One exception: V1pin6 measures 140V instead of 125Volts.

V1 and V2 measured "good" and "very good" respectively on my simple emission tube tester.

As I said multiple times the radio works, but:

  1. IF aligned seamlessly;
  2. bands 1 & 2 trimcaps don't peak during alignment;
  3. sensitivity close to zero in the lower part of band1; can hear stations on the higher part of the band;
  4. bands 3&4 work much better and the trimcaps peak during alignment (even if we have seen the ANT and MIX peaks behave like the ones in band 1&2. May be the trimcaps range is much larger allowing the tanks to reach the alignment frequency. Still I have to check if sensitivity remains constant all over the bands..
  5. while keeping reasoning about the RF circuits I replaced R26 as you can see in my previous posts pictures;
  6. once again I can assure C5 was fully OPEN while performing all test;
  7. both the variable capacitors seems in good shape; nothing prevent them from traveling the entire route, in one direction or the other.

Furthermore, it appears that no one here can fully understand how these strange RF ANT and MIX circuits really work, making fault finding process close to impossible - really frustrating. What surprises me a lot is that in more than 70? years no one has ever raised a hand on the subject: we all were not able to find any reference to the problem...

IMHO, the best way to represent what's going on is the following picture:

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Today I torn down L3/C4 assembly. Here it is.

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In DC, using 4 wire measurement procedure, the first coil (pins C and D) measures 5ohm, while the second (pins A and B) measures 1.5ohms, all in line with what is reported in the S-40B service manual for the same L3 (even if the part numbers differ...).

The TRIMCAP, measured with an LCR meter, ranges from 1.7pF to 30pF - good to me.

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Now I have to go.

Tomorrow I am going to check the ANT and MIX wiring... and S1 wafers positioning...

--

Emanuele (IU1KNR).
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