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..
