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Wow, this cobra 2000 brings back memories...

Thank you, that makes things easier...
I have the internal freq counter completely disconnected for now. Using my oscope and 2 external freq counters to troubleshoot. One is a B&K precision and the other is a fluke.
 
Before I go through the channel board, I decided to replace both the 10.240 and 11.325 crystals again with known working ones. Same result.

Next I adjusted the VCO while keyed up. Got it keying up on 27.205mhz now, but as earlier it will only work on 1 frequency which is now 27.205.

Although the channel indicator says, 1, 2, 15, 36, 40, or whatever, it's really only receiving and xmiting on 1 frequency, which is now 27.205 since I adjusted the VCO while keyed. Before I adjusted vco, it was only on 25.765, but now it's only on 27.205.
 
Something else I need to check is C80 (tantalum) and C81 (tantalum). I'll check those in morning when I check channel board.

It acts as though the VCO isn't wide enough to cover all the frequencies, but also as if the PLL isn't doing its job.

I did notice when I key up, the meter lights, mode light, channel indicator dim slightly, and when modulated on SSB they dim more.

As stated by someone else, could be something on channel board dragging down the voltage.

Seriously guys, this one is old, one of the first productions and it's been in storage for years. I'm not gonna give up though guys, it has a lot of sentimental value.
 
I think I found where my issue is coming from. Haven't nailed it down yet, but I did go back over the PLL pic Handy Andy posted and rechecked the voltages on pins 1, 2, 3, 4 and 5.

Pin 1 = 8.03v
Pin 2 = 8.01v
Pin 3 = 7.99v
Pin 4 = 7.28v
Pin 5 = 7.28v
Pin 6 = 7.77v
Pin 7 = 7.35v
Pin 8 = 7.55v
Pin 9 = 8.03v

Pin 10 = 0.48v
Pin 11 = 0.47v
Pin 12 = 0.01v
Pin 13 = 0.01v
Pin 14 = 7.88v
Pin 15 = 0.01v
Pin 16 = 7.89v
Pin 17 = 4.16v
Pin 18 = 0.00v

Pin 6 does not change when rotating channel knob.

Pin 14 does change when rotating channel knob. It changes from 7.88v to 0.01v when rotated from ch20 to 21.

Pin 15 does change when rotating channel knob. It changes from 0.01v to 7.89v when rotated from ch20 to 21.

Pin 16 does change when rotating channel knob. It changes from 7.89v to 0.01v when rotated from ch20 to 21.

That's bout as far as I got, I'm tired.

This does not correspond with the voltages listed on that pic.

According to the pic:

Pin 1 = 3.4v
Pin 2 = 4.28v
Pin 3 = 5.15v
Pin 4 = 3.44v
Pin 5 = 3.44v
 
Just checked TR20:
B = 3.97V
C = 8.09V
E = 0.00V
This one is missing the Emitter voltage.
This could be why L21 has no effect when adjusting.

TR29:
B = 1.42v
C = 7.92v
E = 1.17v
This one seems close.

TR30:
B = 3.76v
C = 7.70v
E = 3.19v
This one seems close.
 
Removed TR20 and tested in tester:
4 times tested, 3 tested as bad or unknown part, 1 tested as 2 diodes.

Checked voltage on board at TR20 with transistor removed:
All pads test the same with or without transistor in place. Shouldn't there be around 3.35v coming off of the emitter with thmransistor in place? Or does the pin from L21 supply that 3.35v to the emitter? According to the schematic, the 3.36v flows from that L21 pin to the emitter pin of TR20, or am I misunderstanding that? If that's the case, then why isn't L21 producing that 3.36v?

Checked input voltage (3 pin side) of L21:
Both connected pins show around 8v, the unconnected pin also shows around 8v.

Any ideas?
 
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When you put a working transistor into that position - the power flows into and thru it to make L21 "ring" and is why you get DC bias readings.

Ok, to keep this a little more on track...
1685148569420.png

L21 is used as the "tripler output" TR 20 as an amp - as in - the loop - look at how it is oriented, the UHIC chip sends it a "sample" and this is where the PLL looks for errors.

This circuit is PART OF the tripler that goes to the UHIC007 vertical chip - which contains the VARACTOR that looks at the PLL for something in which to tune it's resonant circuit using - that tiny variant of voltage changes the channels - and the PLL takes care of the rest - in a convoluted way.

The UHIC007 chip takes the tripler, and the IF mixes in with it, the PLL sees this and corrects any drifting errors by adjusting the trickle voltage it sends to that VARACTOR. The RF output is from the tripler, MINUS the IF and any errors are corrected then feed back into the Varactor voltage as a trimming effect.

That is the purpose of TR20 - that is the one part that amplifies the errant signals to even let them be detected - that is the other portion of the PLL uses - a LOW-Pass filter is used and the output is sent to the PLL to be checked and run against as a divisor result. Pin 17 is run against the internal pin select from the Channel Selector - so it's "clocked" and Pin 1-2, and 3-4 on the other side - provide a set of "windows" that Pin 5 (result) works against.

When the Gate result is a "miss" the varactor voltage sent to UHIC007 chip is lower - so that changes the capacitance to a lesser value - increasing the ringing speed of the free running oscillator the UHIC chip works.

When the Gate result is a "Hit" the varactor voltage is higher so the UHIC varactor sees more (summed) voltage - increasing this capacitive effect. This SLOWS down that free running Oscillator - simply due to the change in capacitance in this free running Oscillator (as controlled by the UHIC chip can)

1685149352922.png
This means that when the Pin 5 pulse is gated and no hit or misses - the result stays the same - so the next count then occurs and it's sampled again and Pin 5 shoots a pulse and compared thru Gates at 1-2 and 3-4 with that result sent to the varactor in the UHIC chip.

Pin 6 acts as a "back stop" when the oscillator gets out of range - Pin 5 annd Pin 17 are key to how Pin 6 operates and by how you control Pin 6 - either by external or an internal failure or control - tells the system to stop what it's doing or continue.

  • That is why I mentioned that #248 line - that line is from the channel selectors "off detent" detect switch - tells the system the pins are not set right and await to stabilize and then re-load and run.
  • This function is important to know for if the system can't change the channel - you may change it - the PLL still thinks it needs to latch onto that last known good input on the selector pins. If the detent switch is not used, the system may not understand your new pin assignment and the frequency you're selecting.

So since the L21 Value - side stays pretty much the SAME, the tiny change in Varactor capacitance by changing its voltage - changes the resonate frequency L21 presents to the UHIC chip and TR20 - and makes this work the way it does.

To touch on that transistor - that 3.36 volts in the emitter output is for the bias being stable and there is power flowing into and thru that L21 coil into TR20 - so when you measure - that 3.36 V is the expected DC bias - it will also have an AC component too - that being the RF the tripler will need to send to that UHIC007 chip.

There are several threads out here explaining the effect.
 
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Thank you for explaining that setup in more detail. Makes more sense now.

What I did today:

Replaced TR20 with new one. The emitter pin now shows 3.51v. I can now adjust L21 properly. But still same end result.

Yanked out C80 and C81, tested both.
The uf's were about right, and the esr on C80 was 0.49 ohms, whereas C81 esr was 53.0 ohms (too high). Replaced both with new ones of same uf and v. Same end result.

Checked voltage on 7222ap chip, all correspond to schematic voltages.

Checked voltage on 3756 voltage regulator chip, all correspond to schematic voltages.

Yanked out the UHIC vertical chip between VCO adjustment and PLL. Gonna put it back in later with fresh solder. Reason for pulling out was to see if any legs were eaten up by that glue, and because if I pushed it slightly toward with my finger then it would slide off frequency. Not sure if this is the problem, but gonna try it anyway.

It acts as though the VCO is too narrow to cover any more than 1 frequency, very touchy on adjusting VCO, move it just a hair and it drops. Or the PLL isn't functioning properly.

I'm stumped. Off to reading more into what Andy just shared. Thanks again Andy.
 
Well, I think the PLL is bad. I looked over the radio (148 gtl ST) that I pulled the PLL (MMB8719) out of and compared the jumpers and components surrounding the PLL. The 148 the MMB8719 came out of has a jumper under the PLL that goes in between L21 and L23, and stretches all the way across the PLL silkscreen to the 1000uf cap at the end of the PLL. Whereas the MB8734 in this 2000 has a jumper from one PLL pin across to the 1000uf cap at the edge of the PLL.

I also read somewhere else that the MMB8719 is not a direct drop in replacement for these radios. I read that the MMB8719 is a 5v PLL whereas the MB8734, MB8719 and RCI8719 are all 8v PLLs. I read over the swap instructions and it calls for replacing 3 caps in the synthesizer section and jumping L501.

Solution:
I've got an old Stalker with an MB8719. Gonna yank it out and replace the MMB8719 with it then go from there.
 
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And we have success!!!

Replaced the MMB8719 with a known working MB8719.
Realigned VCO, VCO Buffers, and oscillators.

All frequencies are now back where they should be.

Transmit is superb for this old chassis with the 1306/1307 combo. Keys about 5w (gonna drop that down some) and swings 14w peak, and swings up to 6w in average. Side bands do exactly 15w peak each.

Now gotta figure out the receive audio issue: no receive audio at all, not even a peep on all 3 modes (am, usb, lsb). Checked all voltages on the 7222AP audio IC, all correspond to schematic. Checked volume control, it works as it should, checked squelch control and VR, both work as they should. Meter needle does jumped way up when I key a nearby radio on same channel, but then meter sticks so I have to tap meter to get it to drop back down (got new meters, just haven't installed them yet).

I did replace C186 per the 10v blues suggestion on 2000s. But instead of using the factory value of 220uf, I used 330uf, that may be my problem with receive audio.

Get back with y'all later on an update.
 
Replaced C186 with a new 220uf 25v. Same result, no receive audio.

Removed and tested C180, C181, C232, C179, C230, C175. Most were out of tolerance and 2 had high esr values, so I just replaced them all with new ones of factory specs.

Any other caps in the receive audio chain I should be looking in to?

After replacing just those above caps, I turned it on and I could hear a little static, so I turned up the volume some and the audio started coming up and then the audio went out again. Speaker connection good, speaker switch is working properly. CB/PA switch seems to work fine.

Checked audio IC voltages, still all good according to schematic. Hooked speaker into PA jack, flipped switch to PA mode and heard receive audio just fine, volume would bring the audio up and down, squelch would drop audio out when turned up as it should while in PA mode. But no receive audio when in CB mode.

Any suggestions?
 
Headphone jack in the front bad contacts? Or Speaker 1 speaker 2 speaker 1+2 switch? The fact that you have RX audio in PA mode, eliminated my first though that TR19 was bad.
 
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Thank you NZ8N, turned out to be the large headphone jack in the front. Squirted some CRC in all the jacks, worked some plugs in and out of them, once I did the large headphone jack then the receive audio kicked back in.

Another old classic brought back to life, better than I expected.

Almost done, now I just got to go back over all my solder work, put in all new lights, clean it up and get it back to him.

You guys are great. Thank you all for your help, I greatly appreciate y'alls time and knowledge.
 

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