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I am sure that there are those... who have lost faith.... :)

Discussion in 'General CB Services Discussion' started by guitar_199, Jul 24, 2021.

  1. Handy Andy

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

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    To add...

    Are you also checking Q401 - why?

    upload_2021-7-25_11-28-13.png

    It's the SQUELCH//Audio/Final Say - amp part of this mess.​

    Q601 is an amp, but also will get a signal (Emitter Leg goes high) that MUTES the audio before it gets to the Audio Amp - especially when things go wrong.

    If you don't get the "Right" voltage, the thing will shut down.

    Up- Down board ok?



    PLL in lock?

    Looks promising...
     
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  2. guitar_199

    guitar_199 Sr. Member

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    One thing that perhaps I did not make clear enough...... C609 is not even present on the board HA!

    What happened....... early last fall ... when I did "recap - phase 1", when I came to C609 I removed it....BUT did NOT have a replacement. The kit may have been short by 1 ( this is a 47uF 50v). I made a "mental note" back then to order some......but had an accompanying "brain fart" and never ordered.

    Now spring ahead to this weekend.... I was just looking the whole board over and came across empty pads where C609 should be.....and it took me a while to remember what happened in phase 1.

    So...... thinking about the real problem... since C609 is missing...it will behave like a totally open electrolytic cap....which means no bypass for the emitter resistor......which I THINK..... will absolutely KILL the audio gain of that transistor..... and this is the stage right ahead of the audio chip.

    So at this time I am thinking that once I get C609 back in the board.... some audio will show up and hopefully I will be set to run the alignment.


    Do you think this sounds reasonable?

    Thanks ALWAYS for any comment! The "more eyes" looking at a problem is always better!!!!! I am grateful for your time spent.... it adds to my knowledge.

    Bob


     
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  3. guitar_199

    guitar_199 Sr. Member

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    I missed this one!

    First a general statement.... I am going to get C609 in and put in before I do too much else. Perhaps I don't really HAVE to...but getting back into a work week.... I won't be able to give the troubleshooting the time it deserves.

    That said.... let me respond to the points that you have made here....

    -------------------------------------------------------------------
    (Responding to THREE points together!)
    "Are you also checking Q401 - why?"
    AND
    "Q601 is an amp, but also will get a signal (Emitter Leg goes high) that MUTES the audio before it gets to the Audio Amp - especially when things go wrong.
    AND
    If you don't get the "Right" voltage, the thing will shut down.
    "
    I have not given it any particular focus..... yet! I see that Q401 is the AGC amp but drives the squelch circuits...and that is a GREAT POINT that this could also be a squelch issue shutting the audio down. Will definitely keep this in mind when I get down to the "detailed troubleshooting"!!!!!
    ------------------------------------------------------------------
    (Responding to TWO points together)
    "Up- Down board ok?"
    AND
    "PLL in lock?"

    I have not inspected them....BUT.... .going on what my eyes tell me.....and both of my two brain cells agree.......... I THINK both are okay. here is my thinking.....

    I had the generator set for Ch20 (27.205 with 30% mod... not too worried about the level at this point... just wanted a signal to drive!). When I am ON Ch20 AM on the radio..... I get OVER S9 on the meter. When I step DOWN to 19.....or UP to 21.... the meter goes to zero. And ANY time I go back to 20..... that strong indication on the meter is there.

    So first... the up down board seems to be working all the way around. It has been MANY years since this radio saw regular use....so it DOES miss a "step" now and then. but if I roll it again.... it gets there. And according to the above.... when the up down board gets me ON to ch 20..... it sees the generator.... so I believe that the up down is working.... just a little dirty from lack of use. IN related news the scan works. When I switch it on to scan... it takes off.

    Next... the fact that I see signal when the radio is tuned to 20....but the meter goes dead when I step off of Ch20....... I am interpreting (for now) that the PLL is locking. I can't SWEAR that I know for certain that an UNLOCK kills RCV in this radio....but I will definitely "keep this point in my head" when I get into the "detailed troubleshooting". but I am taking this as a "hint" that the PLL is probably tracking and locking.
    --------------------------------------------------------------------

    I, too, am encouraged by what I am seeing here! High hopes that I will get this thing going. I have a mic wired for it.......but I am darned if I know what I did with it. better focus on that some too!!!!!!!!!!!!! :rolleyes::rolleyes::rolleyes:

    Thanks as always HA!!!!!

    Bob



     
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  4. Handy Andy

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

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    Boy, I threw a lot at you, and you handled it well...:)

    Yes, I caught that, you didn't have the cap but what you may not know is that circuit will still work - although the audio will sound "tinny and flat" because of another DISC cap C624 that takes over the signal handling - that DISC cap is really a "tone" filter for the MUTE when AGC lets the Squelch Open - keeps the higher pitched tones from the Pop from Ringing in your ears...

    It is a 0.001uf (102 Disc) that may be installed on the FOIL side of the board - by the Emitter Leg of Q601 to foil ground by it.

    Now as far as what I was telling you earlier - the UP/DOWN module, contains the "Enable/Disable" - a lockout feature that prevents unwarranted or accidental "cross channel" engagement of the PLL - when you hit the buttons to change the channel - it's designed to "mute" the output so you don't hear extra noises that many would think that - when they hear it, there's something wrong with the radio.

    Q601 is kinda special this way, you can kill output by raising the Emitter Leg voltage to cut off (Mute) the audio because the potential differences (Voltage Drop to provide work) across the Collector to Emitter is non-existent - can't amplify if it can't drop current across the C to E junction - just sits there.

    Another aspect of Q601 is with Class A and Class AB amplifiers - is the over amplification - where the Base is biased properly for a GIVEN - PRE-SET voltage drop - a narrow range of change can produce the proper input signal level for the Audio amp - they biased it in a way to provide a good pure sound quality many desire to hear in their equipment - but it is also VERY sensitive to voltage changes - it's ability to amplify can get skewed (literally) and you get no audio - but a hot part.

    Leaving it in that state - the part can fail. Why because there is less RESISTANCE from the Emitter to Ground - causing the over amplification - turning a linearly biased audio amp into a simple On/Off switch and if there's no resistance to help with reducing the current across the Transistor - poof - can pop Q601 and you'd have to put in another - died from too much current passing thru it - again Class A and AB amps fail because when you bias the Base - you must take into consideration that not only Collector to Emitter - but the Base ADDS power into the part - need to be careful with that. Happens on Finals too.

    The Up-Down function? Take a look at the Cobra 148 a special switch is installed on the "cam" of the rotary channel BCD - it follows the indentations - if the channel selector is not seated - that switch "locks out" the PLL keeps the "LOCK" line low even if the PLL thinks it's ok.

    Seen several radios that got "face planted" and screwed up the channel selectors - knobs or switches - if the cam or plates are broken - the PLL - because of that damaged cam - is not going to let you do anything.

    So with your radio, I had to ask about the "up and down" - make sure the wire that goes to the AGC return (Q403) - is not left floating or show open - because it is preventing Q601 from opening up and letting audio thru. - When you TX, it should fall to 0 - muting thru Q601 - when you unkey - you should get 1.4~1.5V on Q601 Emitter Leg. - all because the Up-Down button is not depressed - nor are you in TX mode and you're getting RX voltage.

    That Final Say part - it sux...

    IF not, trace back to Q403 - Collector - it is the one that has the wire (trace) that goes to the Up/Down buttons - it's purpose just locks you out (Mutes speaker) in TX mode or when you change channels - nothing else - it should show about 5.5V when everything is good - that places the Emitter Leg of Q601 in a state to provide linear output and you hear audio.

    So if you have RX and show meter, but not audio - Q601 may be looking at Q403 for help to turn on.
    Q403 is looking at the TX mode or Up-Down button cam...

    Best way to verify that is to look for voltages and if they (Q403 and Q601) follow status changes (TX mode = 0) RX mode 5.5V at Q403 - Else Emitter Leg needs to be at 1.4~1.5V at Q601.

    If voltages are off, suspect Q601 and or Q403 to be bad - if you can get Q403 Collector from 5.5 to 0 and back again, the Up-Down buttons can be ruled out - that parts' ok. You'll have to look either at Q403 or Q601 - possibly more towards Q601..
     
    #19 Handy Andy, Jul 26, 2021
    Last edited: Jul 26, 2021
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  5. guitar_199

    guitar_199 Sr. Member

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    Andy, I can't thank you enough. Every nugget you have tossed me here has been copied into a local Word DOC and will be used as guidelines as I step through figuring this out......

    Bob

     
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