this radio is old, anl blocks alot of noise, but far away stations just don't come in. any ideas to better the recive on this radio other than a amplifier preamp or trashing it ? thaanks Robinhood
hogwld1964@tds.net
hogwld1964@tds.net
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cobra 89 gtl weak receive
- Thread starter robinhood1964
- Start date
the first thing you want to do is give the receiver a tune up.
yes, you should have a signal generator and other test equipment to do this, but it can be done without that stuff if need be.
if this were a brand new radio i would advise you to find a shop, but seeing as its an old radio and you just want to see if you can get it to receive a bit better; here's what you can do.
first, take off the top cover of your radio. look at the PC board and study the metal cans with the screwdriver slots in the tops of them.
they are all over the board and all serve different purposes.
DO NOT TURN ANY OF THEM YET!!! and only turn the ones i will list for you.
turning other ones can render your radio useless in a matter of seconds.
you will see that they have different size screwdriver slots in them. some very small and almost square, some bigger like a flathead screwdriver will fit.
well, you need some tools to "peak" the cans for maximum receiver sensitivity.
you can buy them if there is an electronic supply store around. Some radio shacks used to sell a set but most wont have them anymore. call them first.
if you dont want to buy them, you can make your own that will work ok for you. they might not last forever, but they will get the job done.
good materials for these tools are popsicle sticks, toothpicks, and chopsticks.
also, plastic eating utensils work very well.
you need a file and some sandpaper so you can sand them down so the tip will fit snuggly in the tuning cans' slot.
make sure you do a good job and get a tight fit. these "slugs" inside the tuning cans are made of ferrite and are VERY easily cracked if forced or torqued too much.
DO NOT EVER use jewelers screwdrivers!!!!!!
first, they are metal and will throw your tuning off. second, they LOVE to break ferrite slugs.
A WORD OF CAUTION before you decide to attempt this.
if you do break a small piece off of one of the tuning cans; you will have to unsolder the whole can from the circuit board, unscrew the slug from the bottom of the can, turn it over and re-install it in the can from the top.
if you are careful and turn slowly, you should be fine. better to be prepared though.
first step is to get a sharpie and draw a line on top of the tuning cans you are about to turn. this line should indicate the direction of the slot in the slug.
these are very important because sometimes you get a slug that doesnt show any change when you turn it. when you get one of those, just turn the slug back to your mark and move on.
this is also why you should always turn slowly and never turn more than a 1/2 turn at any one time.
ok, the tuning cans you are looking for are as follows: L1, L2, L3, L5, L6, L7, and L8. (now is the time to draw your lines!!!)
set the radio like this:
channel 20 (you can use 19 or 21 if you have to in order to find an empty channel)
RF gain set at maximum.
ANL off.
squelch all the way down.
delta tune at center.
volume at a comfortable level. (you will be using your ears as well as your eyes to judge peaks)
the best thing to do is have a friend transmit a deadkey for you from their location, but you can do it with just receiver static too.
the thing with the friend situation is, they must hit you with an S-3 or less, and you cant lower your RF gain to achieve this.
you cannot tune your receiver to a signal of S-5 or more, or it will not work as well because of AGC action.
start at L1 and work in ascending order.
while watching your S meter closely, slowly turn L1 and watch for the needle to rise a bit. if it gets lower, turn the other way.
you will see a defiinite "peak" and "dip" in the signal strength.
tune L1 for maximum S meter reading.
again, if you dont see or hear any change while turning the can, and you have made almost a full turn already, its time to stop and turn it back the other way until you get back to your mark. then just leave it alone and move on.
by the time you get to L8 its not uncommon to have gained over 2 S units in some cases!
now go back and do the same procedure over again, but this time, you will only be making minute tweaks to the cans as they are already mostly peaked.
now go to channels 1 and 40 and make sure the sensitivity is relatively equal from one to the other.
if one channel seems dead and one doesnt, you can slowly turn L1 while checking both 1 and 40 to get the gain balanced out. only use L1 to do this.
you are done!
you have just aligned your receiver for maximum sensitivity!
now, there is another adjustment inside the radio that will increase the receiver gain, but it can also increase the internal noise the radio creates, so messing with it is a matter of your personal tastes.
it is VR1 and is located near TR5 and L7.
turn it and you will see the receive go up or down. turn it until you dont like the noise its creating and leave it there.
these techniques should have given you a nice little increase in your receive.
and you have also learned how to do a receiver alignment!
have fun!
LC
yes, you should have a signal generator and other test equipment to do this, but it can be done without that stuff if need be.
if this were a brand new radio i would advise you to find a shop, but seeing as its an old radio and you just want to see if you can get it to receive a bit better; here's what you can do.
first, take off the top cover of your radio. look at the PC board and study the metal cans with the screwdriver slots in the tops of them.
they are all over the board and all serve different purposes.
DO NOT TURN ANY OF THEM YET!!! and only turn the ones i will list for you.
turning other ones can render your radio useless in a matter of seconds.
you will see that they have different size screwdriver slots in them. some very small and almost square, some bigger like a flathead screwdriver will fit.
well, you need some tools to "peak" the cans for maximum receiver sensitivity.
you can buy them if there is an electronic supply store around. Some radio shacks used to sell a set but most wont have them anymore. call them first.
if you dont want to buy them, you can make your own that will work ok for you. they might not last forever, but they will get the job done.
good materials for these tools are popsicle sticks, toothpicks, and chopsticks.
also, plastic eating utensils work very well.
you need a file and some sandpaper so you can sand them down so the tip will fit snuggly in the tuning cans' slot.
make sure you do a good job and get a tight fit. these "slugs" inside the tuning cans are made of ferrite and are VERY easily cracked if forced or torqued too much.
DO NOT EVER use jewelers screwdrivers!!!!!!
first, they are metal and will throw your tuning off. second, they LOVE to break ferrite slugs.
A WORD OF CAUTION before you decide to attempt this.
if you do break a small piece off of one of the tuning cans; you will have to unsolder the whole can from the circuit board, unscrew the slug from the bottom of the can, turn it over and re-install it in the can from the top.
if you are careful and turn slowly, you should be fine. better to be prepared though.
first step is to get a sharpie and draw a line on top of the tuning cans you are about to turn. this line should indicate the direction of the slot in the slug.
these are very important because sometimes you get a slug that doesnt show any change when you turn it. when you get one of those, just turn the slug back to your mark and move on.
this is also why you should always turn slowly and never turn more than a 1/2 turn at any one time.
ok, the tuning cans you are looking for are as follows: L1, L2, L3, L5, L6, L7, and L8. (now is the time to draw your lines!!!)
set the radio like this:
channel 20 (you can use 19 or 21 if you have to in order to find an empty channel)
RF gain set at maximum.
ANL off.
squelch all the way down.
delta tune at center.
volume at a comfortable level. (you will be using your ears as well as your eyes to judge peaks)
the best thing to do is have a friend transmit a deadkey for you from their location, but you can do it with just receiver static too.
the thing with the friend situation is, they must hit you with an S-3 or less, and you cant lower your RF gain to achieve this.
you cannot tune your receiver to a signal of S-5 or more, or it will not work as well because of AGC action.
start at L1 and work in ascending order.
while watching your S meter closely, slowly turn L1 and watch for the needle to rise a bit. if it gets lower, turn the other way.
you will see a defiinite "peak" and "dip" in the signal strength.
tune L1 for maximum S meter reading.
again, if you dont see or hear any change while turning the can, and you have made almost a full turn already, its time to stop and turn it back the other way until you get back to your mark. then just leave it alone and move on.
by the time you get to L8 its not uncommon to have gained over 2 S units in some cases!
now go back and do the same procedure over again, but this time, you will only be making minute tweaks to the cans as they are already mostly peaked.
now go to channels 1 and 40 and make sure the sensitivity is relatively equal from one to the other.
if one channel seems dead and one doesnt, you can slowly turn L1 while checking both 1 and 40 to get the gain balanced out. only use L1 to do this.
you are done!
you have just aligned your receiver for maximum sensitivity!
now, there is another adjustment inside the radio that will increase the receiver gain, but it can also increase the internal noise the radio creates, so messing with it is a matter of your personal tastes.
it is VR1 and is located near TR5 and L7.
turn it and you will see the receive go up or down. turn it until you dont like the noise its creating and leave it there.
these techniques should have given you a nice little increase in your receive.
and you have also learned how to do a receiver alignment!
have fun!
LC
LC:
Can't you also use a digital volt meter across the speaker output to guage smaller changes in increases/decreases in voltage while tuning up the receive?
Just wondering...
Can't you also use a digital volt meter across the speaker output to guage smaller changes in increases/decreases in voltage while tuning up the receive?
Just wondering...
yes, you can set a voltmeter to AC volts on a small scale and read the increases and decreases.
its nice to be able to say that the coils are absolutely maximum peaked, but if you have to balance the receive gain because the radio has more than 40 channels all that perfection goes right out the window.
the procedure i lined out here will work for his situation, but there is more to tuning a receiver than going for maximum noise from the speaker.
in some radios, if you are just using background noise, you may have increased the resting S-meter reading and the noise from the speaker, but you have actually desensitized the receiver and a signal that shows up will read lower.
you can also put a microammeter in series with the positive lead of the radio's S-meter and judge the changes that way.
i have used these methods in the past, but i think you will find that your ear is your best, and quickest responding tool.
LC
its nice to be able to say that the coils are absolutely maximum peaked, but if you have to balance the receive gain because the radio has more than 40 channels all that perfection goes right out the window.
the procedure i lined out here will work for his situation, but there is more to tuning a receiver than going for maximum noise from the speaker.
in some radios, if you are just using background noise, you may have increased the resting S-meter reading and the noise from the speaker, but you have actually desensitized the receiver and a signal that shows up will read lower.
you can also put a microammeter in series with the positive lead of the radio's S-meter and judge the changes that way.
i have used these methods in the past, but i think you will find that your ear is your best, and quickest responding tool.
LC