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UPD2824 talk

Discussion in 'CB Radio Modifications' started by loosecannon, Mar 16, 2019.

  1. loosecannon

    loosecannon Sr. Member

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    I have been restoring a President AX-144 lately, and that usually leads me to stare at schematics in order to gather ideas on possible upgrades or mods i might like to try out.

    yes, there is more mod info out there for this board than most others thanks to our friend Handy Andy, and Andy just let me say that all your past work is still appreciated.

    one of the main things Andy said that i picked up on was just how many different versions of this board there are, and with that how different each version can be from others.

    There are SOO many differences! some versions have mike gains, some don't, and even the ones that don't have mike gains differ from each other in the mic circuits.

    then you have parts in the radio that aren't the ones listed in the schematic for your particular radio.
    for example, TR25 and TR26 are mic amp transistors.
    every schematic i looked at showed TR25 to be a 2SC945 and TR26 to be a 2SC1312 (might have that number wrong).
    in my AX144, they are both 2SC1312.
    i know, not a huge difference but it does illustrate how difficult it can be to decipher just what you do and don't want to mess with inside one of these beasts.

    SO!
    I stated all that just to get it out of the way that just because something is being discussed in this thread as a mod to a UPD2824 SSB chassis, that it may or may not be something you want to do to your particular radio.

    I also want to keep this very specific in order to keep the posts short and to the point.
    again, these stories have mostly all been told already.



    To start, i would like to look at R501, which is a 100 ohm resistor mounted on the mic jack PC board, OF SOME MODELS!

    it doesn't seem to be there on the PC122 versions.

    so, why is this resistor there?
    is it just to guard against high volumes on power mikes?
    is it there because it was cheaper than putting a coil there, and hey, it's a wire wound resistor!

    does anyone see any benefit to replacing it with a jumper and a ferrite bead?
    i can't imagine that 100 ohms is having much effect on mic audio level, but that again begs the question of why it's there.



    maybe i am missing what it's purpose is.
    LC
     

  2. ExitThirteen

    ExitThirteen Grumpy and Cranky

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    The idea of having the wirewound 100 ohm resistor inline with the audio path is that it can suppress RF from getting into the audio path and making its way to the mic audio amp, where it could cause all kinds of havoc. If it's a wirewound resistor, I'd leave it there, as it does serve a purpose. I don't know why more models didn't implement this, to be truthful.


    ~Cheers~
     
    sp5it likes this.
  3. loosecannon

    loosecannon Sr. Member

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    thanks for the verification Xit!

    I just had this feeling that the wirewound aspect was an important one.

    Right now i am looking at the AM modulator section, and since it is basically identical to the old galaxy style modulators (because galaxy copied this design so obviously), i am going to change out TR43 and TR44 for a TIP42 and a TIP36 respectively.

    just because it's something to do.
    LC
     
  4. Handy Andy

    Handy Andy Do Your Research First, Then Decide...

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    There are some other scenarios too...

    Note it's location in the circuit.

    For the rest of those technically curious...
    MicJack.jpg

    They placed it IN SERIES with the Mic gain, so the Mic Gain pot, being what it is,, when you use it's full rotation - it doesn't pass all the current the microphone has to give. Why? Well, a power mike can blow the substrate on the Mic Gain (yes there is that much power possible) so they placed a dropping resistor and use it as an RF choke. It limits current for a reason. Mostly for the Mic Gain control itself.

    • Sure there are Tonal and Voice level quality issues, but these things are setup for "most scenarios" - if you wish - CHANGE IT! Just don't be surprised if problems develop.

    Wirewound offers a level of choke and provides current limiting on the source side of the pot, not the ground or wiper side. That way, if you have static, a poke can shoot thru and into the Mic gain pot. although not a big deal, you do have mics out there that are dynamic that share case to shield - which means if the operator is at one potential - the radio can operate at another - hence the Positive Or Negative Ground issue these things have. To provide some current limiting and RF choke in one package may sound like a cheap way out - but it does work.

    Now, I'm not the guy that built this radio, but from my own repairs of similar setups, blown mic pots are rare but not impossible. I've seen D104's pop open the substrate from people that mod these things. (Pic available on request...) Poor Amp shielding and RF burns from the operators - can also wreak havoc on these too.
     
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  5. loosecannon

    loosecannon Sr. Member

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    thanks for the reply Andy.

    i'm going to be leaving it in place.

    I think im going to do the assymetrical modulation mod to it, but im going to use a trimmer in place of the resistor initially so i can set the negative peaks right where i want them and then measure the resistance.

    this is how i usually do it when i do that mod.

    I upped the value of C118 from .047 to .1, and i think im going to up C129 just a tiny bit also.

    the next question is why did they use only a 330uF cap for the output coupling of the audio amp?
    i would like to try a 1000uF there unless someone can see a reason why it would be a bad move.
    LC
     
  6. Handy Andy

    Handy Andy Do Your Research First, Then Decide...

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    I used to up the auntie on that for those that liked to use external "stereo" quality speakers to get home-sound for better fidelity of the receive audio (which is why those other mods are inclusive with this) - but as you will find out the STABILITY of the audio amp is the one in question.

    For as you raise the uF values, so does it's bandwidth. This then equates to lowering of impedance expected. You may start out limited bandwidth - but as you open up that throttle thru using larger value caps, the expected impedance drops - quickly - past 470uF - to less than 2 ohms - generating an instability issue.

    The typical AM only types used a range from 0.22uF (224) ~ 0.1uF (104) to as low as 0.047 (473) as well as an ohmic value of 1 ohm to 2.2 ohm to "balance out" the bandwidth to an expected impedance - and putting all of this in with-and- thru a Reactance Modulator or Speaker - both are inductive and can generate an EMF (EMP more likely) due to the mis-match - the Inductance is fighting against the extra bandwidth and high-frequency rolloff filters usually solve this.

    So the use of Cap resistor combos to reduce bandwidth to match impedance better and keep the inductive reactance in balance - much like tuning of coil - a cap can be use to "sharpen" as well as change the way the skirt appears to the next stage as well as unto itself in regards to imbalance and frequency response issues around bandwidth.
     
  7. Handy Andy

    Handy Andy Do Your Research First, Then Decide...

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    Compare the uPD 1182 to say a uPD 1242 and you'll see what I mean...
    UPD1182.jpg

    Although essentially the same, Note pin 3 and the "bootstrap" value and type it is...

    Now equate that to the output cap values...

    In the PC122, R166 and C123 are used to improve the bandwidth...
     

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