142 GTL Frequency Synth Error

[LeapFrog]

Cobra 142 GTL failure to TX / RX

Loop Osc counter @ TP1
USB reads 34.75726
LSB reads 34.75724
AM. reads 34.75725

Counter @ TP3:
USB reads 7.80248 MHz
LSB reads null
AM reads null

VCO voltage measured @ TP9
1.7 VDC, adjustment of L13 changes nothing.

Any insight is greatly appreciated
73

 

[LeapFrog]

Update/Correction


TP1 Tripler

USB: 32.61215
AM: 32.61235
LSB: 32.61225

TP9 VCO Voltage
1.7 Volts (doesn't change w/channel)

TP3

USB: 7.80250
AM: 0
LSB: 0

Clarifier Pot:
1 leg 8.3 Volts constant
Wiper to VCO
Last leg directly to GND!!
(Here I placed 8K Ohm)

@ TP1 max rf with L14 slug almost all the way flush with the top of the can.

Time for me to look at the schematic, critical thinking, perhaps a little simple math!

 

[kaos513]

I would recap that section 1st then check it again

 

[kaos513]

Also look for any bad solder joints as well in that section

 

[Handy Andy]

On top of that, you have several other issues with Age I think you'll need to be addressing...

The UHIC chip - note the discoloration at the base leads going into the board - looks like something is going on reactively to the leads. See Pink Arrow.

You're getting lower MHz readings - you supposed to get a free running 35MHz + so you 3MHz low usually this means other problems too...

Note the Orange arrow pointing the your L19 - may mean an internal cap failed - so you know. Nothing serious but for best "Hints from HELLEN" - turn the slug in two turns and try a retune around it. IF the Frequency won't raise up or even get close to 35MHz free running (you may or may not get lock on this) - if you don't get anywhere - remove the coil and check the slug problem - you may have one in that one too.

Yellow Circle and Arrow, you have the "glue" that others have discussed as possibly reactive - which if you try to unsolder this - be gentle - it's not welded on there - those components are soldered to a simple board internally.

 

[nomadradio]

Did you check the 10.24 MHz crystal? If it shuts down, so does the MB8719 PLL chip.

73

 

[kaos513]

Agree with Andy on that glue it’s notorious for eating up leads

 

[nomadradio]

1977 date code on the MB8719 PLL chip.

Might be more than just one issue in a 1978 or 1979 radio.

Never learned to troubleshoot a PLL without a 'scope.

Seeing 10.24 MHz on pins 7 and 8 is where I would start, followed by pin 17.

Those are the two input signals the chip depends upon.

73

 

[kopcicle]