I have a Siglent SDS 1052DL+ scope, a Dosy meter, a Victor VC165 counter and a Koolertron 40mhz signal generator..the scope seems to be the most challenging.
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Cobra 29 ltd classic not transmit
- Thread starter SteveM
- Start date
I was using the Cobra service manual I found online, I started from the beginning which was the PLL.
Sonoma, I looked at that one but I wasnt sure if that was the right model so I googled the cobra 29 ltd classic alignment procedure
Greetings!
I wanted to share this...
When it comes to time base reference - the Scope only sweeps at known reference rates in it's own time base...
So for to help you ...
T = 1 / F
T = the time it took to complete the count
1 = the count length of time - in seconds
F = Frequency of interest or Cycles Per Second
So solving...
T = 1 / 27,000,000
T = 0.00000003703 for 1 cycle to occur for 27 MHz
or
37 nS - Nano seconds...
To prove you're correct lets see how far the "wave" or 1 cycle travels at the speed of light... or 186,000 miles per second...
L = 0.000000037 X 186,000
L (distance in miles)
L = 0.00689
Ok, now, for miles it's 5280 feet - lets convert that to feet
0.00689 X 5280 feet
or
36.3 feet.
Does that sound like 11 meter?
Well, we know 1/4 wave is about 108" or 9. feet
Also, 1/2 wave is over 18 feet
9 feet X 4 (4 1/4 times) or (2 X 1/2)
or 36 feet for 1 full wave...
Looks about right due to propagation of wave in free space versus mediums like steel that affect length...
Ok, so you know now, for your O'scope to see the 27MHz waveform - you'll need to remember your sweep rates...
Hope the above will whet your appetite for more as you gain experience...
Welcome to CB and enjoy your tools or toys - your preference...
Regards!
:+> Andy <+:
I wanted to share this...
When it comes to time base reference - the Scope only sweeps at known reference rates in it's own time base...
So for to help you ...
T = 1 / F
T = the time it took to complete the count
1 = the count length of time - in seconds
F = Frequency of interest or Cycles Per Second
So solving...
T = 1 / 27,000,000
T = 0.00000003703 for 1 cycle to occur for 27 MHz
or
37 nS - Nano seconds...
To prove you're correct lets see how far the "wave" or 1 cycle travels at the speed of light... or 186,000 miles per second...
L = 0.000000037 X 186,000
L (distance in miles)
L = 0.00689
Ok, now, for miles it's 5280 feet - lets convert that to feet
0.00689 X 5280 feet
or
36.3 feet.
Does that sound like 11 meter?
Well, we know 1/4 wave is about 108" or 9. feet
Also, 1/2 wave is over 18 feet
9 feet X 4 (4 1/4 times) or (2 X 1/2)
or 36 feet for 1 full wave...
Looks about right due to propagation of wave in free space versus mediums like steel that affect length...
Ok, so you know now, for your O'scope to see the 27MHz waveform - you'll need to remember your sweep rates...
Hope the above will whet your appetite for more as you gain experience...
Welcome to CB and enjoy your tools or toys - your preference...
Regards!
:+> Andy <+:
Well I followed all your advice and on the Dosey it seems everything is working but there is no modulation when I put the dosey on modulation and key up the needle raises a little and falls, but the s-meter moves on TX and RX, I'm going to recap it on Monday and see if that helps..
If you get a carrier that's good we just have to check for audio...
Does the thing PA ok?
Does the radios' TX when you key it up and a receiver close by, hears it, the carrier - that is.
I was under the presumption that the radio just lost tuning - meaning it keyed up and TX'd just fine and you had audio - e.g.- it worked and only you wanted (needed?) to tune it up - this may be a whole different ballgame...
Does the thing PA ok?
Does the radios' TX when you key it up and a receiver close by, hears it, the carrier - that is.
I was under the presumption that the radio just lost tuning - meaning it keyed up and TX'd just fine and you had audio - e.g.- it worked and only you wanted (needed?) to tune it up - this may be a whole different ballgame...
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It did lose TX and had nothing on the S-meter after I messed it up but after I followed your advice I was able to get all that back but all I get is static on the test radio when I key up, the meter on the test radio swings and I get static but no audio, same thing when I let the test radio, the need on the 29 swings and I get static but no audio.
Greetings!
Don't worry, I'm not panicking...
(checking Avatar...Whew)
There was a reason for me to ask if it PA's function even works, many a bad CB/PA switch "killed" a radio due to dirt. Helps with verifying Volume and Mic gains' conditions... But - I'd rather not go there...
Just ensure you have all the front panel contacts in good condition and set them right for it.
Because you can use PA to verify Audio and Mic Gain path is ok, but doesn't help the RF side much...
Ok, sounds like we have a heartbeat, make sure your L19 TP R88 on Channel 40 is the 3.2 Volts we talked about earlier - this proves the PLL is in lock.
The one you check the Varactor for voltage on channel 40 - and remember I told you to "detune L23 to the middle on it's core if you were not sure?
Now it just may be a little off center slot- but to prove that, there's a condition ...ONLY if it does TX a carrier into the monitor radio - don't forget the dummy loads on both radios for this...you can use the frequency counter to help as you "tweak" L23 to center it's carrier into the receiving radios channel.
So "scratchy" it just may mean it's TX'ing just you are not on "center slot" (An audio artifact like bleedover) - L23 will need to be tuned for best results in getting the dummy loaded donor radio and your patient to be on center slot.
To Reduce Confusion - Voice Lock trim is L24...not L23 - L23 uses output from the 10.240 and the Voice Lock Trim circuit for fine tuning of RX signal and provides the PLL and IC3 TX mixer a signal that is "pre set" thru the use of L24 as the tuning part of the 10.240.
If I had you playing with L24 you may not even be able to know if you're even TXing a signal - you may see something on the radios RF meter but not on a monitor radio - because you can accidentally tune and set L24 to a middle of a channel and the radio is TX'ing on dimes instead of nickels.
D17 is RX Power Steering Diode and D18 is TX Power Steering Diode - these two diodes form a routing pair to send L24 power but thru separate circuits - with RX using VOICE LOCK Trim pot you control from the outside. TX uses a pre-set 4.7K resistor-R99 for it's own "pre-set" un-moveable "center-slot".
As I worked on these photos' - I realized this error and I do apologize...
Pay attention to this post for I will be expanding I'm making up a few photo's to help provide a guide. For as I went thru all those "Service Manuals" - I found several issues around the LABELING and Identification - so I'm using a "donor" radio that will serve as a "Template" and you can use it to locate the parts for they are in the corrct spot just may not be labeled properly for the boards - each board has different "silkscreen" .
So now would be a good time to tell me your boards number KEPC - XXXX version to see if the labeling can work or if it's one of the later ones and similar to the photos I have from my own donor...
So return to this post as I update...
Regards!
:+> Andy <+:
Don't worry, I'm not panicking...
(checking Avatar...Whew)
There was a reason for me to ask if it PA's function even works, many a bad CB/PA switch "killed" a radio due to dirt. Helps with verifying Volume and Mic gains' conditions... But - I'd rather not go there...
Just ensure you have all the front panel contacts in good condition and set them right for it.
Because you can use PA to verify Audio and Mic Gain path is ok, but doesn't help the RF side much...
Ok, sounds like we have a heartbeat, make sure your L19 TP R88 on Channel 40 is the 3.2 Volts we talked about earlier - this proves the PLL is in lock.
Now it just may be a little off center slot- but to prove that, there's a condition ...ONLY if it does TX a carrier into the monitor radio - don't forget the dummy loads on both radios for this...you can use the frequency counter to help as you "tweak" L23 to center it's carrier into the receiving radios channel.
So "scratchy" it just may mean it's TX'ing just you are not on "center slot" (An audio artifact like bleedover) - L23 will need to be tuned for best results in getting the dummy loaded donor radio and your patient to be on center slot.
This may be a little confusing but bear with me...
That "scratchy" may indicate another problem.
If you already "peaked RX" you'd have to adjust L23 to peak out RX.
Meaning you would already have L23 pretty much lined up even for the TX side.
The reason why I asked you to keep it near the middle is to help you in finding the RX side to even get a receive.
L23 - when you adjust RX peak and tune, L23 provides the 2nd IF so to peak out the meter L23 would also have to be tweaked to max out receiver signal on the S-meter as you tune...
Meaning that L23 should have not needed any further tweaking - but if you set it to the middle and got RX to peak out - great! But L23 needs peaking to send a strong signal to IC3 the TX mixer - it's the other half of the ballgame to make 27.MHz CB Band
So if it doesn't "TX" right - adjust L23 on TX then ALSO realign RX with the L23 at this new setting and see if the performance is any better.
I find that getting the radio (29's or even 25's) do better for Tuneups when RX is peaked out first and I usually find that the "hit" on TX of output power and bandwidth is marginal and not worth the hassle of tweaking TX to peak only to lose RX performance because of it.
That "scratchy" may indicate another problem.
If you already "peaked RX" you'd have to adjust L23 to peak out RX.
Meaning you would already have L23 pretty much lined up even for the TX side.
The reason why I asked you to keep it near the middle is to help you in finding the RX side to even get a receive.
L23 - when you adjust RX peak and tune, L23 provides the 2nd IF so to peak out the meter L23 would also have to be tweaked to max out receiver signal on the S-meter as you tune...
Meaning that L23 should have not needed any further tweaking - but if you set it to the middle and got RX to peak out - great! But L23 needs peaking to send a strong signal to IC3 the TX mixer - it's the other half of the ballgame to make 27.MHz CB Band
So if it doesn't "TX" right - adjust L23 on TX then ALSO realign RX with the L23 at this new setting and see if the performance is any better.
I find that getting the radio (29's or even 25's) do better for Tuneups when RX is peaked out first and I usually find that the "hit" on TX of output power and bandwidth is marginal and not worth the hassle of tweaking TX to peak only to lose RX performance because of it.
To Reduce Confusion - Voice Lock trim is L24...not L23 - L23 uses output from the 10.240 and the Voice Lock Trim circuit for fine tuning of RX signal and provides the PLL and IC3 TX mixer a signal that is "pre set" thru the use of L24 as the tuning part of the 10.240.
If I had you playing with L24 you may not even be able to know if you're even TXing a signal - you may see something on the radios RF meter but not on a monitor radio - because you can accidentally tune and set L24 to a middle of a channel and the radio is TX'ing on dimes instead of nickels.
D17 is RX Power Steering Diode and D18 is TX Power Steering Diode - these two diodes form a routing pair to send L24 power but thru separate circuits - with RX using VOICE LOCK Trim pot you control from the outside. TX uses a pre-set 4.7K resistor-R99 for it's own "pre-set" un-moveable "center-slot".
As I worked on these photos' - I realized this error and I do apologize...
Pay attention to this post for I will be expanding I'm making up a few photo's to help provide a guide. For as I went thru all those "Service Manuals" - I found several issues around the LABELING and Identification - so I'm using a "donor" radio that will serve as a "Template" and you can use it to locate the parts for they are in the corrct spot just may not be labeled properly for the boards - each board has different "silkscreen" .
So now would be a good time to tell me your boards number KEPC - XXXX version to see if the labeling can work or if it's one of the later ones and similar to the photos I have from my own donor...
So return to this post as I update...
Regards!
:+> Andy <+:
Last edited:
Ok, the board is a KEPC 060 K..
So when I'm tuning the RX and im.listening for the static or hiss, do I stop when it starts to crackle or stop when the crackling is at its loudest?
Greetings!
I thought you had a monitor radio set up to TX on a channel so you could tune the radio on that channel?
But, the Hiss at the loudest is best. S/RF meter helps too...what are you using for Tuning tool in the slug cores?
To continue, send a RF dead carrier on the Monitor radio so you can set and tune for RX on your radio. Do this on channel 1 and 40 - both ends of the band and you may notice that L22 needs a little turn to make the hiss return on one or other ends of the band - this is normal and I find it more effective if you use a monitor radio to help you hear the monitor radio in your patients receive. L22 "peaks" near channel 30~40 and usually broadbanded enough so you can pretty much tune to channel 40 at maximum and leave it alone
I don't know if you have one but people tend to use an archaic SINAD meter to listen for noise carrier or tone and this tester is placed on the speaker terminals and you tune for loudest or largest forward swing on the meter. You can do the same with a AC side of the DVM but your success will be limited.
Again the above is used with either a signal generator set for 27MHz or your Monitor radio set up in TX mode to produce a carrier into a dummy load.
Then once you're done with that, you'll need to locate D15 - by D19 voltage regulator. D15 lays flat against the board and banded end it going towards the right if you are looking down into the radio, with the front panel knobs towards you, component side up - it's by the PLL between it and L22 / C94 section.
D15 is the TX LOW/RX HIGH switch function.
It tells the PLL to shift frequency 455kHz when in TX mode.
Remember the conversation earlier about the three RX Frequencies the radio uses? 16MHz~17MHz for 1st IF "image" (10.69MHz) and 10.240 (thereabouts per VOICELOCK) to make 455kHz 2nd IF? 16~17MHz, 10.240 - these two frequencies make the three "images" or Intermediate Frequencies, the radio uses for RF to IF to AF detector diode (a process of decoding).
Part Location D15
D15 is the last key to this, for is D15 doesn't work, you will "hear" on the nickels but TX on the dimes - because you're 455kHz off of frequency (The PLL can't tell the difference unless D15 goes Low on TX and stays high on RX) The PLL TX's using the 16~17MHz signal SHIFTED 455kHz off else it uses RX programming via the channel selector and pin programming (ROM) so it uses the 1stIF Intermediate Frequency instead of the 455kHz offset TX frequency for the IC3 mixer which includes L23.
So Check D15, banded end should go low when you TX (into a dummy load please) and go high on RX (above 5 volts more like 10VDC) - the Unbanded end should follow LOW on TX and float at about 4.2~4.9 volts on RX - means the Diode is working and your PLL should be shifting 455kHz when you toggle between modes.
Then that means the radio is healthy and all you have to do is, undo what you did to make the booboo that got you here gone - and redo the right "do" to make the radio do what you want it to do...
Hope this helps!
:+> Andy <+:
I thought you had a monitor radio set up to TX on a channel so you could tune the radio on that channel?
But, the Hiss at the loudest is best. S/RF meter helps too...what are you using for Tuning tool in the slug cores?
To continue, send a RF dead carrier on the Monitor radio so you can set and tune for RX on your radio. Do this on channel 1 and 40 - both ends of the band and you may notice that L22 needs a little turn to make the hiss return on one or other ends of the band - this is normal and I find it more effective if you use a monitor radio to help you hear the monitor radio in your patients receive. L22 "peaks" near channel 30~40 and usually broadbanded enough so you can pretty much tune to channel 40 at maximum and leave it alone
I don't know if you have one but people tend to use an archaic SINAD meter to listen for noise carrier or tone and this tester is placed on the speaker terminals and you tune for loudest or largest forward swing on the meter. You can do the same with a AC side of the DVM but your success will be limited.
Again the above is used with either a signal generator set for 27MHz or your Monitor radio set up in TX mode to produce a carrier into a dummy load.
Then once you're done with that, you'll need to locate D15 - by D19 voltage regulator. D15 lays flat against the board and banded end it going towards the right if you are looking down into the radio, with the front panel knobs towards you, component side up - it's by the PLL between it and L22 / C94 section.
D15 is the TX LOW/RX HIGH switch function.
It tells the PLL to shift frequency 455kHz when in TX mode.
Remember the conversation earlier about the three RX Frequencies the radio uses? 16MHz~17MHz for 1st IF "image" (10.69MHz) and 10.240 (thereabouts per VOICELOCK) to make 455kHz 2nd IF? 16~17MHz, 10.240 - these two frequencies make the three "images" or Intermediate Frequencies, the radio uses for RF to IF to AF detector diode (a process of decoding).
Part Location D15
D15 is the last key to this, for is D15 doesn't work, you will "hear" on the nickels but TX on the dimes - because you're 455kHz off of frequency (The PLL can't tell the difference unless D15 goes Low on TX and stays high on RX) The PLL TX's using the 16~17MHz signal SHIFTED 455kHz off else it uses RX programming via the channel selector and pin programming (ROM) so it uses the 1stIF Intermediate Frequency instead of the 455kHz offset TX frequency for the IC3 mixer which includes L23.
So Check D15, banded end should go low when you TX (into a dummy load please) and go high on RX (above 5 volts more like 10VDC) - the Unbanded end should follow LOW on TX and float at about 4.2~4.9 volts on RX - means the Diode is working and your PLL should be shifting 455kHz when you toggle between modes.
Then that means the radio is healthy and all you have to do is, undo what you did to make the booboo that got you here gone - and redo the right "do" to make the radio do what you want it to do...
Hope this helps!
:+> Andy <+:
- No one is chatting at the moment.