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Another PC122XL TX Problem
- Thread starter Low_Boy
- Start date
Greetings!
This is a post just with some notes on a patient I had to do one time that would RX the world just fine but not TX.
The final solution was to replace the AM/SSB mode switch on the front panel - one pole was gone.
But here is how I tracked it down.
As you know, petty much every CB and Galaxy made, uses one single amp for the Mic audio - but to toggle the modes, the SSB side routes audio differently than the AM only. IT (SSB) has to go thru the AN 612 to mix audio with the IF for it (the one from the Clarifier which slides that IF the audio uses up and down in a window) - which uses several transistors and diodes to steer power only to the sections to make that mode work - others need to stay off.
I got the radio to TX light - but just not transmit. So I went probing for voltages...
Final And driver are ok, at least as far as I could go with this test...
RX works, SSB RX weak but it did too.
Checked PA - PA did have audio - but no receive - checked front panel and in discussion with owner found they dropped the radio onto the floor and it quit working - so it means physical damage so the front panel bezel got pulled and started checking pots - all ok switches not so much...but I thought I needed to fix more of a mainboard problem so I just went back to the main PCB and did some more work...
Found out that RX voltage - didn't fall to zero when in TX mode - even in SSB mode - went back to locate routings and JV24 and JV19 a pair of lines - one supplied SSB and AM mode power to the AN612 - seemed to work all were steady.
Checked JV24 and JV19 - found JV-24 to have 8 volts all the time - no matter which mode I was in.
Traced voltage back to JV26 then onto small ribbon cable that is in front of the board - but not the ribbon cable that was at the front of the board. Traced it back to the front panel switch - and found that the mode switch didn't toggle all contacts so although I'd get a receiver - SSB didn't "work" per-se - just it caught AM signals all the time hence the weak performance.
@Low_Boy - look for TR (or Q) 19, see if it and JV24 / JV 19 are ok, JV 19 sends audio from the MIC amp INTO the AN612 when the SSB mode - TX or RX - but does not use power - so that JV24 line should not have any power on the line in SSB mode - it turns off TR19 - it shunts power from the Mic amp in AM mode - but lets the AM regulator "Get it" instead.
If any mod's like swapped out resistors or changes to let more audio get thru to AM regulator or SSB modes - this section can also not work correctly because of the RATIOS the resistor values and cap values are.
IN AM mode the AN 612's pin one is GROUNDED directly thru Pin 1 and TR/Q19 takes care of that - it removes the signal going to the AN612's pin 1. But does not turn off the AN612 so whatever is present on pin 3 - is the CLARIFIER IF that then goes to the IF filter BPF - this affects both RX and TX. Mode section switch on the front panel, this AM mode switch affects BOTH RX and TX mode and AM mode is then toggled RX and TX to route accordingly - it affects 8Volt constant.
So you may have a TX light - but your AM and SSB modes still go back to that front panel and there may be more damage in that area then we originally thought. So the opposite may also be true as in TR19 takes all the audio OFF the mic line - because it is on when it should be off. Else mods were done back at the mic amp "split" to let more audio thru to AM regulator - but because the AM mode is on and in any SSB mode - this may affect the Audio so check JV24 - jumper with the red arrow - shorter one right next to the resistor (3.3K) if this turns of Q19 all the time it can mean that other sections are turned on when they shouldn't.
You may have an TX light - but is your RX side turning off properly?
This is a post just with some notes on a patient I had to do one time that would RX the world just fine but not TX.
The final solution was to replace the AM/SSB mode switch on the front panel - one pole was gone.
But here is how I tracked it down.
As you know, petty much every CB and Galaxy made, uses one single amp for the Mic audio - but to toggle the modes, the SSB side routes audio differently than the AM only. IT (SSB) has to go thru the AN 612 to mix audio with the IF for it (the one from the Clarifier which slides that IF the audio uses up and down in a window) - which uses several transistors and diodes to steer power only to the sections to make that mode work - others need to stay off.
I got the radio to TX light - but just not transmit. So I went probing for voltages...
Final And driver are ok, at least as far as I could go with this test...
RX works, SSB RX weak but it did too.
Checked PA - PA did have audio - but no receive - checked front panel and in discussion with owner found they dropped the radio onto the floor and it quit working - so it means physical damage so the front panel bezel got pulled and started checking pots - all ok switches not so much...but I thought I needed to fix more of a mainboard problem so I just went back to the main PCB and did some more work...
Found out that RX voltage - didn't fall to zero when in TX mode - even in SSB mode - went back to locate routings and JV24 and JV19 a pair of lines - one supplied SSB and AM mode power to the AN612 - seemed to work all were steady.
Checked JV24 and JV19 - found JV-24 to have 8 volts all the time - no matter which mode I was in.
Traced voltage back to JV26 then onto small ribbon cable that is in front of the board - but not the ribbon cable that was at the front of the board. Traced it back to the front panel switch - and found that the mode switch didn't toggle all contacts so although I'd get a receiver - SSB didn't "work" per-se - just it caught AM signals all the time hence the weak performance.
@Low_Boy - look for TR (or Q) 19, see if it and JV24 / JV 19 are ok, JV 19 sends audio from the MIC amp INTO the AN612 when the SSB mode - TX or RX - but does not use power - so that JV24 line should not have any power on the line in SSB mode - it turns off TR19 - it shunts power from the Mic amp in AM mode - but lets the AM regulator "Get it" instead.
If any mod's like swapped out resistors or changes to let more audio get thru to AM regulator or SSB modes - this section can also not work correctly because of the RATIOS the resistor values and cap values are.
IN AM mode the AN 612's pin one is GROUNDED directly thru Pin 1 and TR/Q19 takes care of that - it removes the signal going to the AN612's pin 1. But does not turn off the AN612 so whatever is present on pin 3 - is the CLARIFIER IF that then goes to the IF filter BPF - this affects both RX and TX. Mode section switch on the front panel, this AM mode switch affects BOTH RX and TX mode and AM mode is then toggled RX and TX to route accordingly - it affects 8Volt constant.
So you may have a TX light - but your AM and SSB modes still go back to that front panel and there may be more damage in that area then we originally thought. So the opposite may also be true as in TR19 takes all the audio OFF the mic line - because it is on when it should be off. Else mods were done back at the mic amp "split" to let more audio thru to AM regulator - but because the AM mode is on and in any SSB mode - this may affect the Audio so check JV24 - jumper with the red arrow - shorter one right next to the resistor (3.3K) if this turns of Q19 all the time it can mean that other sections are turned on when they shouldn't.
You may have an TX light - but is your RX side turning off properly?
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May explain why on a steady sound I get the first initial blast of audio then it stops.
Just thinking that does not sound right If I say audio ten times I will get ten swings of the needle. If I say Aaaaaaauuuuuuuddddddiiiiiiio I will get modulation on the first A then it drops off. Does it in AM and SSB. Andy I will check what you just posted and get back to you.
Just thinking that does not sound right If I say audio ten times I will get ten swings of the needle. If I say Aaaaaaauuuuuuuddddddiiiiiiio I will get modulation on the first A then it drops off. Does it in AM and SSB. Andy I will check what you just posted and get back to you.
I have a hunch that a cap that switches over from RX to TX - and back again - it causing it to discharge and act as the "power supply" but there's nothing to sustain it - so it falls to zero. At least it should - acts like timing function to help the Mod amp do it's job...but it may not work because when you pulled the limiter - which this thing powers - you still had the cutoff. So you said you recapped it - please double check the work...
Works off of 8V constant, C113 - down by the ALC/AMC control pots - if it's shorted, the 8V constant pulls the TR/Q 28 right back up to ON - so if you haven't replaced it - try putting in a new one and see if this works.
Works off of 8V constant, C113 - down by the ALC/AMC control pots - if it's shorted, the 8V constant pulls the TR/Q 28 right back up to ON - so if you haven't replaced it - try putting in a new one and see if this works.
Don't give up - it's starting to work. We just have to find the hiccup - and when you have it seems to work then cuts out, I'm looking at obvious stuff, onto the more exotic - these radios - although fun, are also a drain on ones' resources. So this is a special one and I'd hate to keep having you to go thru all these different approaches like you would if you took a shotgun to it - hence Shotgun approach...But much of what we've looked thru and checked are ok from it's own standpoint.
So when you have time to spend on it, it will need to have that event happen and use a tester with hook on the leads so you can probe a transistor leg and as you key up and test - when it quits - we need to see if the transistor as part of the chain - goes dead. If it does - its a power supply or toggle issue, if it still holds power but doesn't send - you move onto the next part up the chain...
Considering the problem that Robalos' TRC-453 had - if the IF of one section quits while you're in transmit - this can cause the "quit" syndrome. Doesn't mean you'll lose the RED TX light - just that it failed while you talked and the radios' PLL thinks it's ok, but it's not sending.
Now, we also removed the LIMITER transistor before and it still did this , so I'm not leaning on TR28 as being even part of the problem - just wanted to make sure it was not pulling power. IT would affect the Limiter - even before TR28 - but TP5 is still connected to that portion of the IF - your Audio side of it - if you have a Frequency counter - put it on there (TP5) - a scope is better but frequency counter would show if that section is still working when it drops out ... nothing on the counter or it would shift to 17MHz because it lost the lock on the 10.6~10.7 MHz audio side meaning that section is not locking.
The same could be said for the PLL's IF section - if that were to lose lock - the PLL would switch to OOL (Out OF Lock) but it CANNOT change the TX LIGHT - that is locked in by your Mic PTT - it can only keep the light off, when the PLL thinks it's out of lock in the RX MODE - but when it does go out of lock - IN TRANSMIT - it can't change the status, but it can keep the radio from transmitting - by not providing any IF to the TX Mixer so it just sits there...
That issue is part of the PLL's and can be a voltage problem.
So when you have time to spend on it, it will need to have that event happen and use a tester with hook on the leads so you can probe a transistor leg and as you key up and test - when it quits - we need to see if the transistor as part of the chain - goes dead. If it does - its a power supply or toggle issue, if it still holds power but doesn't send - you move onto the next part up the chain...
Considering the problem that Robalos' TRC-453 had - if the IF of one section quits while you're in transmit - this can cause the "quit" syndrome. Doesn't mean you'll lose the RED TX light - just that it failed while you talked and the radios' PLL thinks it's ok, but it's not sending.
Now, we also removed the LIMITER transistor before and it still did this , so I'm not leaning on TR28 as being even part of the problem - just wanted to make sure it was not pulling power. IT would affect the Limiter - even before TR28 - but TP5 is still connected to that portion of the IF - your Audio side of it - if you have a Frequency counter - put it on there (TP5) - a scope is better but frequency counter would show if that section is still working when it drops out ... nothing on the counter or it would shift to 17MHz because it lost the lock on the 10.6~10.7 MHz audio side meaning that section is not locking.
The same could be said for the PLL's IF section - if that were to lose lock - the PLL would switch to OOL (Out OF Lock) but it CANNOT change the TX LIGHT - that is locked in by your Mic PTT - it can only keep the light off, when the PLL thinks it's out of lock in the RX MODE - but when it does go out of lock - IN TRANSMIT - it can't change the status, but it can keep the radio from transmitting - by not providing any IF to the TX Mixer so it just sits there...
That issue is part of the PLL's and can be a voltage problem.
Andy Having a local transformer problem on one of the poles outside so ham radio, Cb radio and oscilloscope are pretty useless lately. On TP5 I am getting a 10.6 to 10.7 MHz floating around so that tests good.
Then I would now focus on the Mic amp - because you've ruled out PA, Mic starts, but fails - else the AM Regulator - but in either case, if SSB audio quits, or AM audio quits, then the Mic amp - it's the only thing that is left.
PA is a separate AMP in itself - and only to the Audio Amp chip - which when you look at the way things route, the Mic amp and PA's own Mic amp - share the Audio Pin 2 line going into - then splits off. And their outputs are sent to two different locations.
So, you'll need to backprobe, just clip a test lead onto that 100 ohm resistor by C109 - make sure the voltage doesn't' fall to zero as you "Audio" into the mic, for if it does, then you found the section that's causing the problem.
AND if the PA amp is still on, this can cause that same symptom, you'll hear it thru the EXT speaker, not the PA speaker (Bad talkback mod)
Near the front of your main board - the PA amp sits, but there is a resistor R157, that goes to a trace that goes and is dependent on the Front Panel switch that controls CB/PA - if R157 is still getting power, meaning that the PA switch is still "on" - doesn't mean the speaker has to be - just power - it will supply power to the PA section.
BUT - and pay attention to this - if the PA switch isn't right - Power is applied to both sections Mic and PA - and PA will "steal" the audio from the Mic amp. Takes more power to use it than the Mic amp behind it. PA takes all the power, Mic amp quits working.
Another condition (I've seen this in smashed panels) is where PA amp is wired wrong, and it receives power during RX, but when you key up - the LINE that C109 and that 100 ohm resistor feed - HAS A RESIDUAL CHARGE FROM C115 - Charged in RX mode by R157 but DISCHARGES thru R194 into the power line that has C109 in it - allows the Mic amp to work "part time" until you unkey the mike - so re-check your work in the front panel - you might have a similar scenario unfolding.
The graphic I'm giving you is part of another I've been working on to help you - but this shows the areas I'm discussing in this post. So I hope it helps...attached
PA is a separate AMP in itself - and only to the Audio Amp chip - which when you look at the way things route, the Mic amp and PA's own Mic amp - share the Audio Pin 2 line going into - then splits off. And their outputs are sent to two different locations.
So, you'll need to backprobe, just clip a test lead onto that 100 ohm resistor by C109 - make sure the voltage doesn't' fall to zero as you "Audio" into the mic, for if it does, then you found the section that's causing the problem.
AND if the PA amp is still on, this can cause that same symptom, you'll hear it thru the EXT speaker, not the PA speaker (Bad talkback mod)
Near the front of your main board - the PA amp sits, but there is a resistor R157, that goes to a trace that goes and is dependent on the Front Panel switch that controls CB/PA - if R157 is still getting power, meaning that the PA switch is still "on" - doesn't mean the speaker has to be - just power - it will supply power to the PA section.
BUT - and pay attention to this - if the PA switch isn't right - Power is applied to both sections Mic and PA - and PA will "steal" the audio from the Mic amp. Takes more power to use it than the Mic amp behind it. PA takes all the power, Mic amp quits working.
Another condition (I've seen this in smashed panels) is where PA amp is wired wrong, and it receives power during RX, but when you key up - the LINE that C109 and that 100 ohm resistor feed - HAS A RESIDUAL CHARGE FROM C115 - Charged in RX mode by R157 but DISCHARGES thru R194 into the power line that has C109 in it - allows the Mic amp to work "part time" until you unkey the mike - so re-check your work in the front panel - you might have a similar scenario unfolding.
The graphic I'm giving you is part of another I've been working on to help you - but this shows the areas I'm discussing in this post. So I hope it helps...attached
Attachments
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Good Possibly we are getting there. Oh yes 7.8MHz on AM when talking 10.7 on SSB when talking. By the way nothing on a dead key. I forgot to mention that. So the frequency shows up only when talking.
100 ohm resistor staying at just under 8V while talking. I will be looking at a few of the other things you mentioned next
This is why I mentioned the Remove TR19 part, a floating voltage indicates the transistor may be bad - and the line is active all the time.
But too, the VR4 Carrier Balance also feeds a error correction offset voltage to Pin 1 - and TR19 is supposed to keep that voltage at ZERO when in AM - your offset is a different beast in AM mode so VR4 is not functional in AM mode - only in SSB modes.
It (TR or Q 19) turns off the AN612 Audio side (Pin 1) in AM mode - so the AN612 Pin 1 should (it needs to) go to 0. Base of TR19 goes active and shunts all the audio off of AN612 so the AM regulator and its' amp can pick it up.
But, in SSB mode, TR 19 goes off AND only in the SSB modes. That means you should see a voltage on that pin 1. (Remember Carrier Balance VR4) But in AM? AN612 - Pin 1 should be 0 volts. TR19 stays on - all the time, TX or RX, it's pulled high by the MODE switch - you just said the voltage goes up - did you replace the cap feeding Pin 1? C104 a 563 disc cap, by Pin 1 of the AN612. look back at my previous post from here, that long leg lead labeled Audio in the photo - look that up - see if audio can return. (long shot don't expect much)
It doesn't mean the AM regulator is perfect either, you need to look for any voltage on the line that goes back to the AM regulator. There is a cap back there, C166 - it is by the 8 Volt regulator in the photo I posted - that green mylar Chiclet cap 0.022uf or 223. One side will have the mic amp power, the other side has the AM power trimpot - that feeds R185 back at the AM regulator.
- VR10 the AM PWR pot, can fail - part of the substrate can go open - especially if the user wanted to get more power out of it and tried changing it out and trying a different part. When that happens 8 volts is sent all the time - only it's impedance is affected - good luck eh? So you can lose audio because the AM PWR and MIC sides are even more unequal. Too much voltage from one side swaps out the signal the other side of the cap is trying to send.
- C166 will have a LOWER voltage on the MIC amp side than on the AM PWR side - and the AM PWR side would rise and fall as you adjusted the AM POWER pot.
If it's not sending a voltage up to that cap (like it's Zero) or negative, that is different than the Mic amp side, then we have a bad foil lead somewhere - AM mode has an 8 Volt line - it has to turn off the AN612 in AM mode else you be sending out FM too. May have a broken foil trace in there.
This may mean TR 19 might be bad, or the line that feeds TR19 - is not working (its' open no voltage from AM mode gets to it)
Only way to know this is to remove TR19 - if audio returns check/replace TR 19 - Test that transistor, see if the transistor has shorted out - it's possible that the base voltage is flowing backwards into the Collector - when this happens the line to the AN612 gets saturated and there's nowhere to "dump" the voltage so it just sits on the line, pinches off audio - this also affects the Carrier Balance control VR4.
OR the AM mode switch is also damaged by that impact, and you'll need to go back to the Front panel and look at those switches all over again. TR 19 needs to be ON in AM mode, but OFF in SSB mode - so it'll need a voltage from the MODE switch AM on the front panel.
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7.8MHz? Ouch...ok, something is wrong then...
You shouldn't get any that low - you're out of the Crystals passband...
It could be one of many reasons why you don't have any audio ... out of the filter range the AN612 is set up for.
Remember the VR4 Carrier Balance - its' an offset voltage ADDED to the audio - as a bias to push up or down the zero-beat frequency of the audio wave. The Zero point. It works as if you "center" the audio in SSB, but the AN612 and the way it works with AM - you don't send audio into the AN612 when you're in AM mode, just what is on Pin 3 from the PLL / IF and that's flat out 10.690MHz the center of the channels' frequency. Audio applied to Pin 1 adds or subtracts from that frequency (your positive non-inverted and negative inverted) depending on your SSB mode select USB or LSB - AM is center.
So TP5 should have shown you 10.69MHz (If any - D9 acts as an RF switch to turn off this section so it doesn't interfere with the AM detector - again powered by AM mode that powers TR19) - the purpose of TP 5 is because that is where the CONVERSION occurs for SSB, but when you use AM mode, you don't need the conversion - you have the detector - and that is where TR19 comes into play - grounds out audio in and sets up the detector in AM mode. But TP5 is from the CLARIFIER side (audio side) of the IF that mixes with the 17MHz from the PLL You'd see that, at the AN7310 the TX mixer on the side rail by the Pre-Driver . So that would only be BETWEEN USB (10.7) and LSB (10.65) - TP 9 is AM - that should be your 10.69 - that test point is D14, between FT1 (that BIG metal Filter can) and the AN612 - close to the PLL and VR 4.
TR19 needs to keep Pin 1 low so it doesn't drift off into neverland - so look back and locate that JV jumper for TR19 - the one that turns it on - and make sure it works and pull and check/test TR19 - good/bad - act accordingly.
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Thanks Andy
I have to read the previous two posts again also. You probably will not hear back from me on these tests until Fri or Sat.
Thank You
I have to read the previous two posts again also. You probably will not hear back from me on these tests until Fri or Sat.
Thank You
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