Fixed and aligned a Cobra 148F GTL once before.
Used the same schematic as a regular 148 for the most part.
Used the same schematic as a regular 148 for the most part.
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Ranger built cobra 148F GTL
- Thread starter skybolt58
- Start date
well I am slowly checking components and all seem to be checking out ok? I do have the correct schematic for the EPT014811Z.
I am working on 2 radios for the same guy, the other one is a Washington PC 385 board, MB8719. that one is going to be trash I think, no matter what you adjust it stays only on one frequency when you turn the channel selector. I am working on getting another MB8719 Pll. Maybe that is the problem???
I am working on 2 radios for the same guy, the other one is a Washington PC 385 board, MB8719. that one is going to be trash I think, no matter what you adjust it stays only on one frequency when you turn the channel selector. I am working on getting another MB8719 Pll. Maybe that is the problem???
148FGTL is just a 148 with a freq counter as far as i know.
since its a ranger built chassis instead of a uniden built chassis, there may be small differences.
i will look at the schematic for the TR-296 which should be the same as what you have minus the counter.
just checked: use R182 instead of R187. use C505 instead of C156. the other parts i mentioned are the same.
here is a link to the service manual and other info for the TR-296, which should be the same radio you have but without the counter.
http://www.cbtricks.com/radios/rci/tr_296gk_dx/index.htm
you might find some valuable troubleshooting info in the service manual so be sure to give it a good read through.
you probably already know this, but you need to remove the transistor from the circuit board before testing them. just wanted to put that out there.
on that washington, do not put another MB8719 chip in there until you are sure that the problem was the old chip.
if not, there's a chance that the new one will blow as soon as you turn the radio on.
you need to try and do a PLL alignment on the washington first.
since you say that no matter what you adjust, it just stays on the same freq.
that means that you need to be able to measure the output of a single tuning can with a scope or an RF voltmeter.
you need to make sure that you have signal going in and coming out of each can.
this will help you determine where the problem is.
my first guess on this one is VCO, or tripler can (L18). the tripler can is very touchy and gets messed with when people try to put channels in these radios.
LC
since its a ranger built chassis instead of a uniden built chassis, there may be small differences.
i will look at the schematic for the TR-296 which should be the same as what you have minus the counter.
just checked: use R182 instead of R187. use C505 instead of C156. the other parts i mentioned are the same.
here is a link to the service manual and other info for the TR-296, which should be the same radio you have but without the counter.
http://www.cbtricks.com/radios/rci/tr_296gk_dx/index.htm
you might find some valuable troubleshooting info in the service manual so be sure to give it a good read through.
you probably already know this, but you need to remove the transistor from the circuit board before testing them. just wanted to put that out there.
on that washington, do not put another MB8719 chip in there until you are sure that the problem was the old chip.
if not, there's a chance that the new one will blow as soon as you turn the radio on.
you need to try and do a PLL alignment on the washington first.
since you say that no matter what you adjust, it just stays on the same freq.
that means that you need to be able to measure the output of a single tuning can with a scope or an RF voltmeter.
you need to make sure that you have signal going in and coming out of each can.
this will help you determine where the problem is.
my first guess on this one is VCO, or tripler can (L18). the tripler can is very touchy and gets messed with when people try to put channels in these radios.
LC
Last edited:
Hi Again,
When I got the Washington he said it had no receive, So I hooked it all up with my freq counter Antenna connected at the time, it was reading 27.9856. no matter what channel I put the selector on? so I have the service manual, and did the vco tune up. and set it up on channel 19 on selector, and 27.1850 on TP1
Attach frequency counter to TP1 (R157)which is located directly next to L27.
Double check that the clarifier is centered before starting ! Also did this!
Mode Adjust Frequency Comments
AM L20 34.985
LSB L19 34.9825
USB CT3 34.9875
TX frequency VR3 34.985 Check during TX on AM
then when I changed the channel selector it stayed right on channel 19?
The radio did have channel mod in it, but I took it all out back to stock to make sure that wasn't the problem.
Where do I test the voltage on what you are asking, this is what I have on the VCO, But I cant do it, because the radio will not go down to ch 1 and I do not have a scope?
VCO alignment:
Connect DC Volt meter / oscilloscope to TP9 (top leg of R72, just below the VCO card)
Set radio to channel 1 on the lowest band, adjust L13 so that the base voltage just lifts (should be set to around 2.0 VDC).
Check that the radio will transmit at the set voltage, if not adjust L13 to increase it a little until it does transmit.
Go to channel 40 on the highest band (or the highest channel available), check that the voltage is below 6.8 VDC and that the radio will transmit.
VCO output alignment:
Connect oscilloscope to TP1 (left leg of R184, just above L13, L14)
Adjust L14 for maximum reading.
Loop mixer alignment:
Connect oscilloscope to TP10 (left leg of R193, just above L20)
Adjust L18 for maximum reading.
Connect DC Voltmeter to TP9 (top leg of R72, just in front of the VCO card)
Set radio to channel 1 adjust L13 be set to around 2.0 VDC.
(You don't need a O-scope for this adjustment, just a common Digital Volt Meter set on DC volts)
The Defpom PC-385 Alignment Page
http://www.worldwidedx.com/general-cb-services-discussion/35748-call-cobra-142gtl-service-info.html
*EDIT*
Check ALL solder joints around the PLL.
If the VCO voltage is at/above +5v and does not change by adjusting L13 and the voltage remains the same on every channel you select, then there may be an open circuit between the VCO and the PLL. This will cause the 'one channel syndrome' you are seeing. Been there; done that.
Fixed a couple of Cobra 142 GTL's that had this problem, which uses the same PC-385 chassis that the Pres. Washington has. The cause/fix was poor workmanship on channel hacking this radio, poor solder joints, and/or bad switches.
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The lowest voltage I got at TP9 is 3.69 Volts with the selector on ch 1, shouldn't it be 5 volts on ch 40?
It should.
Something is open or out of adjustment.
The owner of it, said while he was talking on it , it just lost receive, well I don't think it lost Receive, it is just up on 27.90 something and no one there!
just wish I had a scope. make things a little easier
if you can key the radio on ch. 19 (the TX light turns red) and get a reading of 27.185 on your freq counter connected to the antenna connector, then the PLL is locking, and working. at least somewhat.
if this is not the case, please explain which part of what i posted isnt happening.
now for some clarification. are you saying that when you turn the channel selector, the channel display doesnt change?
or are you saying that you can change channels, and the channel display follows the turning of the selector, but the frequency doesnt change on your counter?
with your counter connected to TP1, radio on channel 19 AM mode, are you getting a reading of 34.985mhz?
now, when you change channels, is that 34mhz number going up when you turn the selector up, and down when you turn the selector down?
LC
if this is not the case, please explain which part of what i posted isnt happening.
now for some clarification. are you saying that when you turn the channel selector, the channel display doesnt change?
or are you saying that you can change channels, and the channel display follows the turning of the selector, but the frequency doesnt change on your counter?
with your counter connected to TP1, radio on channel 19 AM mode, are you getting a reading of 34.985mhz?
now, when you change channels, is that 34mhz number going up when you turn the selector up, and down when you turn the selector down?
LC
OK, ON TP1 I can't get 34.985. closest I get is 34.9306. no change on L18, But L13 makes it change very little, and when I change the channel selector the display changes but the frequency don't, But Now I notice it don't key up no more it use to. red light came on before, But it don't now?
I am just going to give up on this one, and just tell the guy it is un repairable by me! , I would love to get it fixed for him, he is 63 years old never could work most of his life because he has epilepsy, so I tried to do what I know, so he didn't have to pay me. I guess he is going to have to get a power supply and try to use this 148F, I got it max doing 11 watts now. on both ssb and Am.
We do not want t put a a lot of money in that Washington. guess he will just have it as a shelf ornament. thanks all for all info given to me!
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ok, the PLL is out of lock and thats why you cant transmit. it actually makes more sense that you cant TX, because if you could, it would mean that the VCO and pll are locking.
only L13 should be adjusted using a DC voltmeter on TP9.
L18 is a different beast and you need to use an Rf voltmeter or an o-scope to peak it.
after you peak it, you need to give it a tiny crank off the peak so that it wont double peak.
try this, key the mic and slowly turn L18 to see if the red light will pop on.
mark its original spot so you can put it back there.
you shouldnt have to make a full turn in either direction if this is going to work.
if that doesnt work, put L18 back where it was, and try the same thing with L13.
if it doesnt work, re-set L13 to the proper voltage.
im thinking that the radio is just out of lock, and if there was channels in it, and you removed them, i would check over your work.
LC
only L13 should be adjusted using a DC voltmeter on TP9.
L18 is a different beast and you need to use an Rf voltmeter or an o-scope to peak it.
after you peak it, you need to give it a tiny crank off the peak so that it wont double peak.
try this, key the mic and slowly turn L18 to see if the red light will pop on.
mark its original spot so you can put it back there.
you shouldnt have to make a full turn in either direction if this is going to work.
if that doesnt work, put L18 back where it was, and try the same thing with L13.
if it doesnt work, re-set L13 to the proper voltage.
im thinking that the radio is just out of lock, and if there was channels in it, and you removed them, i would check over your work.
LC
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