was looking again at your first post, and it looks as though i missed a few clues on my first read. sorry.
look at your schematic as you read through this.
first, the voltage on pin 5 of your VCO chip is wrong as you mentioned.
you will notice that pin 5 of the VCO connects to pin 1 of the PLL chip, and the voltage you noted for the pin 5 of the VCO is the same as the voltage on pin 1 of the PLL.
so, the VCO is not getting enough of a signal to do its thing, which is why your clarifier coils arent responding to tuning and you cant change the voltage on L19.
the voltages you note on pins 1-9 of the PLL are all off, some by quite a bit.
to me, this says bad PLL chip, but before we assume thats the problem, lets check the lock detector circuit to make sure that the problem doesnt lie in that circuit. it could be that your PLL is out of lock which is causing the weird voltages and lack of output.
on the MB8734 PLL, pin 6 is your lock detector pin. (im sure you already checked that the PLL was oriented correctly with the notch toward the 11.3258 crystal.)
pin 1 is connected to R95, just so we are both looking at things the same.
count across from pin 1 to pin 6.
when the pll is locked, pin 6 should have 8 volts on it. you indicated 5.44 volts here telling me that the PLL is out of lock.
to determine whether the PLL itself or the lock detector circuit is the problem, unsolder and lift the anode (UNbanded end) of D31 from the PC board.
now jump a wire from pin 1 of the MB3756 voltage reg to the anode of D31 that you just lifted from the board.
recheck the voltage on pin 1 of the PLL, it should be about 3.4 volts.
you should also notice some change in the radio's operation, like you can now change the voltage on L19, and the clarifier coils should now respond to tuning.
if this worked, and operation is restored, the problem is in your lock detector circuit. trace the line on your schematic going from pin 6 of the PLL to the circuit its connected to. i think its TR47 and TR53. check these transistors with your DMM using the diode function test to determine if they are bad.
also check the components around them, especially the caps and diodes.
if this did not work (my guess is that it wont) then most likely your PLL chip is bad. first thing to do is remove that jumper you just added so it doesnt mess you up later, and resolder D31.
the previous owner probably messed it up when they were trying to mod it for more channels.
you need to check for solder bridges made by him (or you!) around the PLL area. also check for cut traces and things like that.
one thing that i noticed right away was that you said you had 0 volts on pin 10.
in this chassis, pin 10 is high (8v) and is not connected to anything.
with the MB8734 chip, it is 8v all the time and is not controllable like it is with the MB8719. the 0 volts you read sends up HUGE red flags that someone tried to control pin 10 by grounding it. ive never tried it on an 8734 chip, but i would guess that its not good. LOL
the fact that you read 0 volts says that either the PLL is messed up internally or that pin 10 is somehow shorted to ground outside the chip.
using your continuity tester, put one lead on pin 10 and the other lead to PC board ground.
do you have continuity? you should not. you should read infinite resistance.
if you find that pin 10 does seem to be shorted to ground (continuity) then remove the PLL chip from its socket and check again.
if the short goes away, and the socket itself isnt messed up, then you can be pretty damn sure the PLL chip itself is bad.
if when checking pin 10, with the PLL chip in its socket, you find that there is no short to ground, but you are still reading 0 volts, then the chip is bad and its time to replace it.
do these checks in the order i placed them.
if you find that everything outside the PLL chip seems to be in order, but its still not working, then go ahead and replace the chip.
after you replace the chip, re-tune the VCO like i posted earlier, and you should have at least somewhat normal operation restored.
the clarifier coils are going to need to be re-tuned, and you'll be glad you bought that freq counter!
(if your counter didnt come with a probe, just get a piece of coax with a BNC on one end and fashion some test leads to the other end. it will work just fine for you.)
ok thats it! like i said, i really think the last owner messed up the chip while trying to get extra channels and that may be one of the reasons pin 10 read 0 volts when it should have read 8volts.
good luck and be sure to let us all know how it works out, (so i can either gloat or hang my head in shame LOL)
LC
look at your schematic as you read through this.
first, the voltage on pin 5 of your VCO chip is wrong as you mentioned.
you will notice that pin 5 of the VCO connects to pin 1 of the PLL chip, and the voltage you noted for the pin 5 of the VCO is the same as the voltage on pin 1 of the PLL.
so, the VCO is not getting enough of a signal to do its thing, which is why your clarifier coils arent responding to tuning and you cant change the voltage on L19.
the voltages you note on pins 1-9 of the PLL are all off, some by quite a bit.
to me, this says bad PLL chip, but before we assume thats the problem, lets check the lock detector circuit to make sure that the problem doesnt lie in that circuit. it could be that your PLL is out of lock which is causing the weird voltages and lack of output.
on the MB8734 PLL, pin 6 is your lock detector pin. (im sure you already checked that the PLL was oriented correctly with the notch toward the 11.3258 crystal.)
pin 1 is connected to R95, just so we are both looking at things the same.
count across from pin 1 to pin 6.
when the pll is locked, pin 6 should have 8 volts on it. you indicated 5.44 volts here telling me that the PLL is out of lock.
to determine whether the PLL itself or the lock detector circuit is the problem, unsolder and lift the anode (UNbanded end) of D31 from the PC board.
now jump a wire from pin 1 of the MB3756 voltage reg to the anode of D31 that you just lifted from the board.
recheck the voltage on pin 1 of the PLL, it should be about 3.4 volts.
you should also notice some change in the radio's operation, like you can now change the voltage on L19, and the clarifier coils should now respond to tuning.
if this worked, and operation is restored, the problem is in your lock detector circuit. trace the line on your schematic going from pin 6 of the PLL to the circuit its connected to. i think its TR47 and TR53. check these transistors with your DMM using the diode function test to determine if they are bad.
also check the components around them, especially the caps and diodes.
if this did not work (my guess is that it wont) then most likely your PLL chip is bad. first thing to do is remove that jumper you just added so it doesnt mess you up later, and resolder D31.
the previous owner probably messed it up when they were trying to mod it for more channels.
you need to check for solder bridges made by him (or you!) around the PLL area. also check for cut traces and things like that.
one thing that i noticed right away was that you said you had 0 volts on pin 10.
in this chassis, pin 10 is high (8v) and is not connected to anything.
with the MB8734 chip, it is 8v all the time and is not controllable like it is with the MB8719. the 0 volts you read sends up HUGE red flags that someone tried to control pin 10 by grounding it. ive never tried it on an 8734 chip, but i would guess that its not good. LOL
the fact that you read 0 volts says that either the PLL is messed up internally or that pin 10 is somehow shorted to ground outside the chip.
using your continuity tester, put one lead on pin 10 and the other lead to PC board ground.
do you have continuity? you should not. you should read infinite resistance.
if you find that pin 10 does seem to be shorted to ground (continuity) then remove the PLL chip from its socket and check again.
if the short goes away, and the socket itself isnt messed up, then you can be pretty damn sure the PLL chip itself is bad.
if when checking pin 10, with the PLL chip in its socket, you find that there is no short to ground, but you are still reading 0 volts, then the chip is bad and its time to replace it.
do these checks in the order i placed them.
if you find that everything outside the PLL chip seems to be in order, but its still not working, then go ahead and replace the chip.
after you replace the chip, re-tune the VCO like i posted earlier, and you should have at least somewhat normal operation restored.
the clarifier coils are going to need to be re-tuned, and you'll be glad you bought that freq counter!
(if your counter didnt come with a probe, just get a piece of coax with a BNC on one end and fashion some test leads to the other end. it will work just fine for you.)
ok thats it! like i said, i really think the last owner messed up the chip while trying to get extra channels and that may be one of the reasons pin 10 read 0 volts when it should have read 8volts.
good luck and be sure to let us all know how it works out, (so i can either gloat or hang my head in shame LOL)
LC
Last edited:
