cobra 148 unlock PLL

[Robb]

I agree with HA. I've seen this issue only a few times. But what I usually found was a clarifier circuit that was improperly modified. Mostly from acquired radios from flea markets, Craigslist, and garage sales - and such that were hacked and done wrong and then dumped for cheap because they didn't know how to restore what they broke.

Not saying that is the problem Robalo. You know radios far better than I do. But that is what I'd seen - is all.

 

[Handy Andy]

the radio is running on 35.xxx mhz or tr29

That's good news!

We have a heartbeat...

(I reworded the below with emphasis on Clarifier)

Ok, if the clarifier was completely disabled, you would NOT get 35 MHz from the tripler.

You have 35MHz so that tells me the radio would/should still have a receiver and transmitter - just no slide - it wouldn't hear much either - it's inbetween channels - so it looks like one section of what the PLL still needs, is dead...

Try this...there's a Voltage chart to go along with it - Euro Radio - Via CB-Tricks


You can get the whole Kaboodle - here...

http://www.cbtricks.com/radios/cobra/148gtl/index.htm

So if the OUTPUT of the clarifier is not there no PLL activity no signal - it isn't there - one of the two - the radio does need three to work --- Tripler...PLL and IF - 7.8MHz - BUT as far as the PLL is concerned - only two, it's 10.240 and the 34~35MHz...

What would kill it?

Well, you have 35MHz already on TP10 - did you check Pin 17 with a scope to see if the Frequency even gets back to the PLL?

TP9 - seems still a bit high to me, what is the status of the R95 resistor? Not shorted is it? Is it even there?

It's a 1.5K resistor supplanted with your 2.7K resistor R207

The TP 9 gets a ramp voltage (why I asked you to scope it) when the PLL is out of lock - it tries to send a signal "ramp" to make the VCO fire when the Varactor finally responds.

There is a low-pass filter - or smoothing filter - to help the Varactor find common ground. That is C90 (Polarized) and C91 (Disc) and R104 1K.
If C91 is leaky - it can goof up the results - DC ok but RF wise - it sux...right to ground...
​

The UHIC-007 chip, uses supply voltage on two of it's pins, isolated from each other internally. Pin 8 feeds the VCO side, while Pin 1 is the IF / RX side...

​

The power feeds are different - Pin 1 of the UHIC chip IS PROVIDING POWER TO 1/2 OF THE CHIP on the chip and it uses that diode thingy in the middle of the schematic as a polarity protection device as well as a power divider (yes divider - best word) because it keeps the POWER used for the IF for RX - (that) uses Pin 1 -from mixing with Pin 8 POWER FEED as that pin is the VCO side and is your TP 9 and TP 10 (You don't want a filter from one type of signal - interfering with the signal level of the other). Some IC's use a power rail to share loading internally - the diode paring keeps the supply feed separate from the output of Pin 1 IF/RX and the VCO Mix loop frequencies using Pin 8 to supply that side.

Might be a good idea to see if the chip has a blown power feed trace to it - you can unsolder it and check pin 1 to pin 8 for short (none) or Pin 1 to Pin 7, 4, 3 or 2 - to ground - pin 1 to 7 should show diode style continuity - one-way only no short or even an open - if it can't show continuity for one way - the power line trace internal to it is popped open - like a fuse - DO THE SAME for checking Pin 8. <<< There! If this test fails it may be the issue on VCO side of things...

So PLL IC sends a signal thru TP 9 to Pin 5, Pin 10 "counts" and "Beats" to the oscillation started by Pin 5 "trimmer for Varactor" voltage and what it does - sends it to TR20 out thru Pin 10 - and the output of TR20 to Pin 17 of the PLL. Pin 1 is a reflection of what occurs on Pin 5 and Pin 10 - only on Pin 1 - which takes in power and sends out 34~35MHz IF thru it into L20 and TP1...

*WHEW***

So, that means TP10 is running VERY HIGH - and check TR20...you should only have 1MHz or less signal to loop back - its' the correction signal used to determine the Loops (high and low speed ones)

A Truth Table I look for...
Courtesy of CB - Tricks...


I updated it - to show RELEVANT information...

And please accept my apologies for updating and editing this post so many times, it's due to some family issues not unlike BOLTR's experience in the vids he posts (THE CAGED COIL RINGS - thread) only I have it with an 86 year old.

 

[Handy Andy]

So to add even more to this mess....

Wanted to share this to help...


An a side note to help you understand what I'm trying to do here.

Every radio I've repaired, I've treated as if I was building the radio for the customer, not just installing a mod - I am creating a unique product specific to the needs of the person. So when you look at the above "mods" take into consideration that I don't want this customer bringing it back because something broke The steps and the parts replaced, or kept and relocated are for the reasons the customer came to me in the first place. All these mods are specific to a trait I wanted in MY radio - so I do this work as if it was my radio.

 

[loosecannon]

every time i've seen 6.xx volts on that VCO and testing seems to lead nowhere, it has been a faulty MB8719 chip.

you have to do some testing either way though, just to ensure you're not going to blow a new chip.

sounds to me like you are at that point.

if you have another PLL, put in a socket and try it. do use the socket, as the traces in this area are thin and won't take too many heating and cooling cycles before they lift.

I don't want to de-rail the awesome treatise on PLL troubleshooting that Andy is posting, but if you end up nowhere after all the testing, replace the PLL.

i have had three of these just this year.
LC

 

[Handy Andy]

Here's another way of looking at this Robalo...

It's how TR20 works...

The PLL sends Varactor a signal voltage - to oscillate at a frequency.

But when it gets to TR20 - what happens - the effect is similar to what goes on down at the TX mixer - where two signals are mixed.



Only in this case - the diagram above uses the 7.8MHz signal to "clock" and "gate" so many cycles of the 34~35MHz signal arriving from the other half of that IF.

So there is a mixing - and by the way the transistor is used, it's used to "subtract" part of the 34~35MHz signal at a 7.8MHz rate - so we use subtraction to take away some of the 34~35MHz signal - as if we we're letting some waves pass - but blocking others - so we get something at 27MHz instead...

So in light of the 34-35MHz signal - The Tripler actually supplies the "oscillation ring" needed in the VCO side - only the VCO side uses the Capacitive effect of the Varactor - to SLOW - DOWN or SPEED UP the IF that the tripler is being used for, at a rate that gives the PLL a chance to check accuracy thru PHASE - using the two "clocks" on the PLL on Pins 1-2 and 3 - 4 - one is high speed and the other is a low-speed - with Pin 5 - the "push" or piston that is triggered when the twin clocks and the FIN (PIN 17) when it's divided down - don't match - the PLL's Pin 5 Fires a pulse to the two free running inverters set up to run as astable clocks (Multivibrators) raising the signals power level - the outputs are tapped - only the signal is a frequency made between the two or appears as a ramp to TP9 - which the output of those clocks are smoothed out further and sent in to Pin 5 of the UHIC chip ...

So in a way, as long as TR20 can send a signal back to Pin 17 - the PLL will attempt to "track" by letting the astable clocks send something to Pin 5 of the UHIC chip (VCO side) - but the BASE of TR20 - uses the UHIC chips TR601 to start the oscillation ringing effect - which is passed onto TR602 thru C607 and C602 - and rapidly accelerates from there...

So what it is telling me, with your 35MHz signal - its' too fast for the PLL's FIN - Pin 17 - it needs something far lower in frequency, like less than 2MHz in bandwidth - and as the truth table shows, even closer to 1MHz and 44kHz around it - above and below it to find the divider and process the FIN and compare it.

We're doing what is happening at the TX mixer, only we are using two 35MHz or thereabouts as signals to accomplish this. The UHIC chips own frequency is on the Base of TR20 using the Varactor side of the UHIC chip, while L20 uses TR29 - the other missing piece, uses the 11MHz signal and it's associated (and varactros too) but it's tripled, and so the errors in the multiplication of that frequency are even greater - but more closely meets an EXPECTED oscillation frequency range (34~35MHz) - so Gate times at TR20 are different - closer in spacing - so we can only let fewer waves pass - like a Beat Frequency Oscillation - a heterodyne if you will - to let the PLL know we're alive and kicking...those fewer cycles show up on Pin 17 and the PLL takes it from there.

So that means Pin 5 of the UHIC chip needs a ramp voltage to begin the process - if you put straight DC in, the oscillation may not take over - because of the sheer level of current present at that voltage - it'll provide too much capacity and wash off the effects of the ringing you need to begin and maintain the oscillation.

In a way, it needs a jitter or kick start to begin the oscillation, but dampens the effect of that, once the system can track it and is underway - for the very same reason.

 

[Robalo]

I've been working on it, I'll be making a review on it soon. This is a hobby for me, Thanks for the help and information. 73's

 

[Handy Andy]

If you have one, use the scopes' 20MHz bandwidth filter when looking at Pin 17 - so you don't see a garbled, jumbled mess of shifting frequencies as the PLL "clocks" and compares results from Pin 17 - thru Divider input and the output of the two inverters running at two different speeds, generates a "window" that tells the PLL's pin 5 to fire or to stay low (pull down) so it will always be "working" but don't look at the 35MHz - look for approximately 1MHz signal instead. That's what the PLL is looking for off of TR20.

Another note:

Was working on a project a long time ago dealing with 556 timers - and keeping their outputs from mixing within the chip itself - found the power "divider / splitter" reference to keep both halves of the single chip from suddenly wanting to oscillate their outputs asynchronously - meaning they'd free run on their own f they were left open - the technique of the twin diode splitter keeps the stages internally separate and isolated - if not done, they'd use the power rail and you'd never be able to use the chip for anything but a self-starting oscillator...

In the case of the UHIC chip - the VCO's correction error signal shifts can interfere with IF so some of the chip oscillates - on it's own - but it's tunable (L19 and the Pin 5 Varactor) so the multivibrator portion uses just enough to make the system "cycle" but not so much as to make it untenable and un-tunable...it's why they used the values they do...

In some other radios - the UHIC is done up in discretes...and often referenced as "Exciter"...

 

[loosecannon]

Andy,

Don't want to hijack the thread, but i think Robalo may need a few days to dive into the radio, so i thought i'd ask you a question related to this specific VCO (UHIC070).

Have you ever considered putting a capacitor, say, 47pf, across pins 5 and 6 in order to broadband it? I have this written down in some very old notes and just saw it last night, but don't remember ever trying it, or where the idea came from for that matter.
LC

 

[Handy Andy]

Andy,

Don't want to hijack the thread, but i think Robalo may need a few days to dive into the radio, so i thought i'd ask you a question related to this specific VCO (UHIC070).

Have you ever considered putting a capacitor, say, 47pf, across pins 5 and 6 in order to broadband it? I have this written down in some very old notes and just saw it last night, but don't remember ever trying it, or where the idea came from for that matter.
LC

Hey, haven't heard from you in a while - they got you busy too eh?

You can try it in there - a cap won't hurt it - but also remember the input of Pin 5 on the UHIC chip is "buffered" from the outside - limits current and in some instances, the ability to go broadbanded. That resistor is the biggest stumbling block there is, when it comes to really opening this radio up.

Now I know that there has to be precautions - so that is one of them.

Here's a completely insane, stupid, crazy (why did I even mention it) idea...What if the SIPP chip is installed backwards? Well - look at what could happen in pin count - Pin 5 becomes 6 - 8 becomes Pin 2 (grounded) could be a disaster but shouldn't hurt the chip (I hope)...Just kills things...

Maybe that is his whole problem - reverse the chip...I've had THE MOST SIMPLE stuff throw me for a loop making it look like I work for the CSM (Cigarette Smoking Man) and Mulder in X-Files like a paper pusher - so it would not surprise me. But, here's the problem - the weekend is close at hand, should I go tell him and get my head bit off about this? Or, do you want to tell him?


https://i.giphy.com/media/3o7abu6MhmCNxqUT4I/giphy.mp4

 

[Robalo]

 

[loosecannon]

Did it work Robalo?
LC

 

[Robalo]

No, not yet. I only get to work on my hobby from around 11:00pm to 2am or 3am, getting only a few hours of sleep. With having to commute to work 150 miles round trip everyday I'm not left with much time.

 

[Handy Andy]

Woof...

 

[Robalo]

from Lou Franklin and HA, to my understanding:
IC 4 (MB3756) voltage regulator is a combination 8 volts regulator and T/R switching The input is 13.xx volt DC on pin 2. There are three regulated outputs. Pin 1 is a constant 8.0 VDC. Pin 5 is a T/R control: when pin 5 is high, pin 6 will be high and when pin 5 is low, pin 8 will be high.
Pin 6 is the receive-only voltage 8 volts, and pin 8 is the transmit-only voltage. Pin 5 is in turn controlled by TR35 (2SC945). A small base bias is provided by R164 (560). R162 (3300) pulls up its collector, which means pin 5 is normally high so the receive-only voltage is present on pin 6. The mike PTT button is also connected to source #33 to mike pin 5. When the mike is keyed, source #33 grounds the emitter of TR 35, turning on its collector goes low, causing pin 5 to switch the IC 4 (MB3756) output from pin 6 to the transmit-only voltage at pin 8 instead. The base of TR35 is also connected to the PLL lock detector. (pin 6 of the MB8719 PLL chip) this lock detector pin is normally about 8 VDC. If the loop unlocks PLL pin 6 goes low, which we'll also keep TR35 from switching to the transmit-only voltage. this prevents off-frequency transmission. TR20 (2SC1625) Loop Mixer please continue description of the functions, that's when I get lost. If IC 4 Pin 8 is not switching TR20, TR29, TR30, plus the voice lock not working. Am I getting this right? On my case R187 (2700) goes low

 

[Handy Andy]

Wonderful!

Yes!

The basis for the all the outside or non-PLL work was to make sure the MB3756 would work. You found that out when you short Pin 5 to ground - only for a moment.

There was one small miss...

When PLL pin 6 is low, meaning, the PLL can't find or lock in a frequency - it also MUTES the audio chip.

It is done this way to let you know something's wrong.

If you don't hear a hiss - and even though you have a "Green Light" showing you the RX is on - TR47 and TR53 - TR47 is PNP while TR53 is NPN - it acts like an inverter - staying low when the PLL is working and in lock - but when the PLL is NOT in lock - it turns on one and in doing so - allows the other to turn on and "mutes the useless RX noise" or diverts off the audio by sucking all the audio out of the line going into and thru TA7222AP Pin 4.

Now I hope you're not mad at us for trying to find and make sure that whatever was on Pin 6 - any device connected to that line - was not causing a short or pulling too much power into it, keeping that PLL Lock line low and NOT allowing the PLL to apply POWER to the line (Go high) preventing power from the resistor that is R163, the 22K back by the PLL - that pulls up power (applies the same 8 volts that powers the PLL) to keep the line from killing receiver and yet preventing the radio from TX'ing when it's not ready to.

By having you momentarily ground Pin 5 of the MB3756 - you proved the radio could TX - just we don't need to keep doing that - we've proven it is not that part.

However - would CB PA switch cause it? Perhaps - because even though we may hear the audio from the speaker - that means the audio was routed to the correct speaker jack then onto the internal speaker.

But WAIT - that CB/PA switch has more than one function - it also has to tell the system NOT to transmit when you busy yelling at a motorist thru the PA horn wired to the radios' PA jack back in the truck. Turning that switch back over to CB - allows the connection you see at TR35 to - engage the TX and RX side - while the other half of the CB/PA switch took care of the speaker wiring by sending it to the right spot on the main board.

We also needed to make sure the DIODE was not causing problems - when we have the radio power up and someone is working on aligning the radio they can inadvertently short out a line and possibly put a lot of current thru one of the diodes. It is a NOS radio - which really means little except we still have the box and original packaging - we can't tell or ask about why it sat there for so long and don't have a lot of history or knowledge about that radio otherwise - so we could be in for a wild ride of problems as we uncover why the radio never got sold in the first place.

Thank you for letting me, us, work thru this - together.