cobra 148 unlock PLL

[Handy Andy]

Ok, so channels don't seem to get you anywhere..

Ok, put a scope on TP10 - set your scopes' 20MHz Bandwidth to ON.

Now I need you to look at TP 10 output on your scope - not the 35MHz part, the other the one that Pin 17 of the PLL is trying to catch. Less than 2MHz...

You adjust L19 to obtain a lower speed signal - fewer MHz - like around 1MHz at the most - this signal is your DIVISOR for the PLL ROM and Channel input TO COMPARE and DIVIDE down and obtain something it can use for Pin 1-2 and 3-4 of the PLL free-running INVERTERS...

The TP 10 is in a 35MHz TRAP filter - it supplies power, but does not let HF get thru - it is a LOW PASS filter you see a SLOWER waveform on TP 10, to do that you check that on junction of C87 and C88 - not TR20. You need to see any signal filtered out of the mess of 35MHz out of UHIC chip. NOT what the UHIC is dealing with.

Adjusting L19 let's you SYNC the output of the VCO to TR20 - TR20 then sends/reflects a signal back to Pin 17 thru a low-pass filter. So don't look for 35MHz look for the SLOWER signal - Pin 17 is trying to locate something it can track or BFO to - and compare it to the INVERTERS used as COMPARATORS on Pin 1-2 (Fast) and Pin 3-4 (Slow) and the Pin 5 injects as needed to start correcting the loop signal it got back from TR 20 thru pin 17...

 

[Robalo]

Let me work on it. I have tu put back all the components been removed.

 

[Handy Andy]

That's ok, it's not a "Space Race" - take your time - you may find something that is wrong and when you correct it - it may work just fine work our alright after all!

“Mad Matter: "Have I gone mad?"
Alice: "I'm afraid so. You're entirely bonkers. But I'll tell you a secret. All the best people are.”
​

That makes two of us...

 

[Robalo]

any more ideas?

 

[Handy Andy]

What does Pin 17, in the low MHz range (use the 20MHz bandwidth filter on your scope) show ? any low-frequency MHz stuff? or? is it all 35 MHz? - then suspect TR20 and UHIC (tune L19 while you check)

 

[Robalo]

SOMETHING WRONG NO READING NOW HUMM ! NOTHING NADA

 

[Robalo]

Test prove is on test point 1 and L19,L20 BEST PEAK TP-10 NOT READING AT ALL

 

[Handy Andy]

IF you checked TR20 and it's still good, and you have voltage to it - but it's passing just 35MHz - recheck the Low-pass filter trap - you should not have a big large signal of only 35MHz in there...

The UHIC chip or L19 - one or the other is not working...

​

 

[Robalo]

still out, been working on the radio all day long and nothing lift cathode of d31 and tx good. but no 8v on pin 6 off pll.

 

[Handy Andy]

Do you still see 35MHz or did you have a chance to look at the lower 2MHz or so - frequency - off of TP 10?

Was wondering what you see - because of PLL's Pin 17 can't or isn't designed for 35MHz - it needs the lower 2MHz or less frequency presence.

 

[Handy Andy]

Don't worry I see the scope but do you have the 20MHz bandwidth filter on?

The 3.14V P-2-P result - shows the VCO is not oscillating...

L19 - can you swap it out for a spare LA-216?

 

[Robalo]

This is what I did, I pulled up a few parts again trying to insulate IC 1. remove r95,r207,r96 ,c79 and found out the vco (so vco is good)pin 5 from 6.9v goes to o volts and ic1 pin 1,4,5 has 8 volts and pin 6 0 volts. pulled pll and tested it with 8 volts on pin 9 and pin 18 ground and all pin tested right and voltage on the socket pin 6 and 8 has 8 volts pin 17 3.4 volts so there is something changed in the program on mb8719 what is the part or voltage to make microship full. Also L19 been replace for 3rd time and no difference. I know this seems funny with what I'm doing but I'm trying to learn and fix the problem.

 

[Handy Andy]

Then going back to an LC post from earlier - those PLL's unless you have an encoder (that channel selector) that is done up right as a matching encoder for that particular PLL - the pin for pin encoding of the P0-P6 channel programming pins won't work for the channel selector you have.

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

Have you tried just running a DIP 8-pin on-off programming pin switch in place of the channel selector to see if you can develop a way to see if and what the PLL needs to get Pin 6 "high" (lock) and actually works?

 

[Robb]

This is what I did, I pulled up a few parts again trying to insulate IC 1. remove r95,r207,r96 ,c79 and found out the vco (so vco is good)pin 5 from 6.9v goes to o volts and ic1 pin 1,4,5 has 8 volts and pin 6 0 volts. pulled pll and tested it with 8 volts on pin 9 and pin 18 ground and all pin tested right and voltage on the socket pin 6 and 8 has 8 volts pin 17 3.4 volts so there is something changed in the program on mb8719 what is the part or voltage to make microship full. Also L19 been replace for 3rd time and no difference. I know this seems funny with what I'm doing but I'm trying to learn and fix the problem.

If L19 has fried for the 3rd time; then there is a still a short in there that took it out.
Solve the short.

Sooo, 8v power is coming into pin 5 of the VCO and then is being shunted by L19 with the VCO's varactor, which has to be shorted to allow L19 to get full power, fry, and go open.
Right?

So, is pin 5 getting 8v?

 

[Robalo]

So, is pin 5 getting 8v?

right now its 0 volts its not connected to pin 1 of ic 1 if i put the components back it will be 6.7 volts . there is something that changed the microchip to go 8 volts on pin 1,4,5 i have 7 new mb8719 and all of them does the same thing, i know the microchips are good because i have another radio with socket and test them and they work just fine, and also i just replaced channel selector with no luck.