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Old Wax Caps in Schematic, What Are They For ?

Discussion in 'General Technical Questions and Answers' started by K2WH, Oct 12, 2019.

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  1. K2WH

    K2WH Premium Subscriber Volunteer Moderator QRZ Page

    I have attached a rough schematic of the front end area of a Hallicrafters SX-17. The capacitor ID is C47, C49 and C50.

    All 3 are .05uF and I believe they are bypass capacitors.

    I have been replacing these wax caps and have all of them replaced with new except these three which are virtually inaccessible.

    Only one end is accessible, the other is fed through the chassis below the main tuning cap and cannot be reached let alone replaced.

    My question: I am doing an RF alignment and am having difficulty in aligning the main dial with the actual received signal on a few of the 6 bands available.

    I suspect these caps if they have turned into resistors are affecting my efforts to calibrate the dial with the actual frequency. What are these caps doing in the circuit and could the be affecting or hindering my attempts to get the dial in line with the injected signal?

    What if I just snip them out of the circuit ?

    Circuit diagram, you may need to blow it up, they are at the cold end of the series coils.

    Capture.JPG
     
    Last edited: Oct 12, 2019
  2. K9STH

    K9STH Platinum Subscriber Volunteer Moderator Platinum Subscriber QRZ Page

    They are bypass capacitors and really need to remain in the circuit. However, replacement capacitors and not the original, leaky, wax types.

    Also, if they are leaky, and there is like a 99.999% chance that they are leaky, they can definitely have an adverse effect on the AVC.

    Now, are you aligning the tunable oscillator on the correct side? Some bands use high side injection and some bands use low side injection. If you are aligning the circuits on the "wrong" side, you will definitely have problems.

    Glen, K9STH
     
  3. K2WH

    K2WH Premium Subscriber Volunteer Moderator QRZ Page

    That's what I thought they were.

    As to the high side vs. the low side, I am following the step by step tuning procedure in the Hallicrafters manual.

    The only warning they give is to make sure you are not tuning the radio up on the "image" which is about 1kc below the fundamental.

    Please explain the high side vs. the low side ?

    Bill
    K2WH
     
  4. AI3V

    AI3V Ham Member QRZ Page

    The local oscillator (lo)is separated from the frequency you wish to receive (rf) by the " intermediate frequency"(if) of the reveiver.

    This is the basic "superheterodyne" pricipal.

    https://en.m.wikipedia.org/wiki/Superheterodyne_receiver

    You can have the lo higher in frequency than the rf, or lower in frequency than the rf

    When you put these 2 signals into a "mixer" you get all sorts of signals on the output of the mixer

    You get the:
    Rf
    Lo
    Rf+lo
    Rf-lo

    You also get many more.

    One of the things that happen is a reception of a unwanted rf signal at the "image" frequency. The first, and usually most troublesome image frequency is at the rf + 2 times the if frequency

    .......

    The caution of image and high side/low side are mostly so that the tuned circuits "track"

    The multiple sections of the main tuning capacitor , and the associated coils, need to be tuned with this tracking in mind.

    Rege
     
  5. AA7EJ

    AA7EJ Ham Member QRZ Page

    "I have been replacing these wax caps and have all of them replaced with new except these three which are virtually inaccessible."

    In general - capacitors , irregardless of technology / how they are made, fail same way as any other electronic component.
    Resistors go open - too much current, capacitors short - too much voltage etc.

    Unless they are "liquid electrolytic " technology type and really very old or long unused , capacitors do not need to be replaced.

    Most of old electrolytic can be revived using process called reforming. ( nothing to do with politics or religion).
    Most bypass capacitors are not electrolytic.

    73 Shirley
     
  6. K9STH

    K9STH Platinum Subscriber Volunteer Moderator Platinum Subscriber QRZ Page

    EJ:

    If you really want to take a chance on "reforming" electrolytic capacitors, then go ahead and do so. However, in most cases, even if the capacitor can be "reformed", it is then turned into a "time bomb" waiting to go off (short, etc.) and when it does go off, it usually takes other components along with it and, often, those components are a LOT more expensive than spending a dollar, or two, on a new capacitor.

    Paper type capacitors, of which wax types are the worst, severely degrade over the years and become much more like a resistor than a capacitor. Basically, "leaky". Leaky capacitors play havoc with AVC / AGC circuits, cause tubes to draw more current, and generally severely degrade performance. A unit "may" appear to be working fine with the old capacitors. But, if the leaky capacitors are replaced with new capacitors (and not old but never installed capacitors) it is often like night and day with the improved performance of the unit.

    Replacing the leaky capacitors will reduce the load on the power supply and that, usually, also reduces the heat generated within the unit. Heat is the single most damaging thing in electronic equipment.


    WH:

    As described by 3V, with any heterodyne unit, the intermediate frequency is generated with a mixing of the local oscillator and the incoming signal. This i.f. consists of a signal on both sides (upper and lower frequency sides) equal to the separation of the LO frequency and the incoming signal. Due to the fact the in most general coverage, especially older, equipment, the tuning, of the LO, is not linear, as you get higher in frequency, it takes less movement of the variable capacitor to get the same frequency coverage.

    As such, in getting the dial calibration to accurately reflect the changing LO frequency, the LO has to be either above, or below, the incoming signal frequency as when the dial was calibrated. Depending on the "wrong" side of the LO, changing frequency can result in either too much change when the tuning knob is turned or too little change.

    A "rule of thumb", but definitely not always the case, is for the lower frequency bands to have the LO on the "high side" of the incoming signal and for the lower frequency bands to have the LO on the "low side" of the incoming signal. For the lower frequency bands this is to get the tuning range sufficiently adequate to cover the desired frequency range and for the higher frequency bands primarily for stability. Frankly, with tunable oscillators, the less stable the frequency the higher the frequency.

    Glen, K9STH
     
    AI3V likes this.
  7. K7GQ

    K7GQ Subscriber QRZ Page

    I've gotta to think about this for a moment ...
     
    Last edited: Oct 13, 2019
  8. K9STH

    K9STH Platinum Subscriber Volunteer Moderator Platinum Subscriber QRZ Page

    GQ:

    For example, the AM broadcast band: If the LO is on the low side, to get even a 455 kHz i.f. (very common), the LO would have to be between 85 kHz and 1245 kHz to cover 540 kHz to 1700 kHz. To get that kind of range, with a single band switch position, would require a capacitor with a very large range of capacitance. However, with high side injection, the LO has to cover 995 kHz to 2155 kHz. That range requires a much smaller variable capacitor. Therefore, virtually all, but not every, receiver that covers the broadcast band uses high side injection. It is the same with even lower frequencies. In fact, below the broadcast band, one would have to have a negative frequency LO to reach the lower frequencies. As such, high side injection.

    Many receivers, even below around 5 MHz, often use high side injection. Then, between around 5 MHz and around 15 MHz, either high side or low side is often used. Above around 10 MHz, usually, but not always, low side injection is used because of the stability factor. There are receivers that do use high side injection above 10 MHz but those are not all that common.

    Now, we are talking about receivers that use "free running" LO. With digital technology, high side injection, is used, by some receivers, all the way to even the GHz range. This is because the stability of virtually all digital LO is considerably better than a free running LO.

    Glen, K9STH
     
  9. WB2WIK

    WB2WIK Platinum Subscriber Platinum Subscriber QRZ Page

    The question was:
    Old Wax Caps in Schematic, What Are They For?
    Simple answer: They store old wax, of course!:)
     

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