148gtl (4pin)

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222
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148gtl (4pin)

Post by 222 »

Radio Make and Model: cobra 148gtl
PCB Number: "newer" 4 pin "made in china". (KEPC-332-E6)
PLL Number: MMB8719
Test gear available: basic meters, dummy, freq cntr, dmm & anolog
Symptoms: no apparent rx signal getting in. (no pickup at all, from an adjacent radio) no reading on meter (meter is good). can't hear anything (rx audio circuit seems to be ok)

What is working: tx side (signal & audio) is fine.

What has been tried: checked pcb for any obvious faults & checked for intermittent (none found). wiring looks ok.

Did it suddenly happen ?: came to me this cond. seems like rx front end is bad? i need help getting pointed in right direction.
Anything else that may be of help?: basic ch. conv. done. otherwise looks "clean". (on back plate: 148 GTL D)

thanks,
jeff
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148gtl (4pin)

Post by timrim »

First check all voltage regulators, if they all are good, then I would take a close look at the mixers or possibly first rf amp and/or protection diodes.
CBTricks has the service manual.
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148gtl (4pin)

Post by 222 »

timrim, are you saying that some of regulators are common to rx signal only? i don't know of that. (tx is good). also, what protection diodes are you referencing? and i may have misread something but it looks to me that the service manual you mentioned is not for model i have. at cbtricks, the 148gtl is the old 5 pin model & the other, newer, 148 (cb) models listed (148gtl/st/nwst) are probably older models, from what i have. they are not the same. it is a model 148 GTL D. it doesn't use an mb3756 regulator, it uses instead a 7808 (3 pin ic?). i don't know "how different" they are, but certainly different. i'm thinking i'd like to know locales of am detector and rx rf amp? not sure. wondering about possibilities of D21, TR14.

i'm pretty lost.

rick, help, please?

thanks for any/all help.
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148gtl (4pin)

Post by rockhavenradio »

Is the no rx symtom on ssb and am?
If no am only rx, 7.345 LO?
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148gtl (4pin)

Post by 222 »

rockhavenradio,

thanks 4 reply. i'm quite sure that there's no rx on am & ssb. but i'm going to go right now & do a triplecheck, to make absolute certain, since i know it's a little easier to miss on ssb, than on am, the rx that is.

i don't understand your wording on your 2nd line: "if no am only rx, 7.345 LO?".....

sounds like you might be asking me "if there's rx on am only, check a local occillator (7.345)??.....i'm not sure what you're saying there, but if that question is based on no am rx only, then i guess it's a moot point, beings no rx at all, but i still have to go recheck on that.

i'll be back to report on that issue, shortly.
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148gtl (4pin)

Post by 222 »

ok, just came back from rechecking. there is indeed, no rx signal, am or ssb, but tx is fine, am/ssb/moduation. i have no true signal injector or generator handy, but i can say that an adjacent radio is putting no sound & no needle indication on it whatsoever, with respect to rx. the meter is good & the rx audio circuit is intact.
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148gtl (4pin)

Post by 222 »

ok, i think you are talking about 2nd mixer (rx local oscillator outputs). as i was saying earlier, i don't know if this radios' circuitry/pcb matches with what is available on cbtricks, or i should say, a good enough match, because i'm pretty sure none of what 's on cbtricks is this board. if it is (if anybody can tell me), then do i need to be checking voltages of TR2, TR15, and/or FET4? also. or, are you telling me i need to check for the 7.345 mhz? if so, i gotta find the test point for that, plus figure a way to measure. somewhere i've got a handheld freq cntr. i'm guessing that i could probably run a wire from the center pin of its bnc connector, to the test point, in order to read & guessing that the voltage present at that point, would be low enough for the counter, to not hurt it. but again, no rx, am or ssb, so is any of above necessary?
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148gtl (4pin)

Post by rockhavenradio »

I recently worked on the same radio, "148 D"
I have the schematic of that radio.
Cobra sent it to me a few months ago.
If you need a copy, let me know how I can get it to you.
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148gtl (4pin)

Post by 222 »

rockhaven radio, yes, that would be great, as i don't know, but i think i'm right about it being different than the rest of 148's. it would be appeciated.

i don't know my way around this forum or it uses/how to's, etc., but as far as i can tell, can't you just "add an attachment"? when you post, that option is there, at the bottom. i would assume the file size wouldn't be any problem, nor am i aware of any forum reason/rules, why you shouldn't send. if there is some reason why not, please inform me so i know too. plus, i don't know, but that would make it available to others as well, no?

also, i'm extremely limited in knowledge & ability & equip., as to the areas we're into, but i'm not clueless either, so i can try and/or do the testings, with some help & guidance. i asked you & other poster some questions about the specific things you posted to me, but i see no answers at all to any of them..? did i ask the wrong questions?...i thought they made sense. for ex: i asked for clarification on your "7.345 LO" comment, (although after i looked into that further, i'm sure i got you right about rx local osc. output freq., being what you're talking about). you obviously know much more than i, so i was looking for confirmation.

but more importantly, what about the original thing you said (that i also asked you to clarify) about 'if only on am, or not'?....where i confirmed for you that issue was not am only....so i was asking you if that meant that checking that 7.345 LO (or whatever the # is) was not necessary...as that's what you seemed to be saying in that original question, but you haven't answered about that, so i'm left swingin on that point...

and...if indeed you still want me to check that LO, i'm asking in advance, do i find that at the gate of an f.e.t. it is fed to? (i read that in the manual for the (older i believe) 148. that was the reference i made earlier, to f.e.t. 4.

please advise on the attachment question & the rest...

thanks alot for your help/info/patience w/me
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148gtl (4pin)

Post by rockhavenradio »

Sorry, about that...I posted too soon...so disreguard. I was thinking about a different radio. Some radios use a seperate local oscilator for am. I'll look into getting that file to you.
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148gtl (4pin)

Post by rockhavenradio »

148 D Schematic...It's labled 148 F on the top. They said the 148 E is exported to Mexico, and the 148 D is the Domestic version...Don't know what the 148 F model is. This is What Cobra sent me when I asked them for the 148 D, china radio.
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148GTl-E SCH.pdf
148 D
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148gtl (4pin)

Post by 222 »

thanks loads for getting the schematic to me. & it's a high quality file also!

i don't know what's going on for sure w/the numbers/letters & such... it's obviously not a 148gtl-f, as it says (in 2 seperate places), cuz a 148gtl-f is an old, 5 pin radio, although maybe that was sans the "gtl", can't remember for certain. still, this schem. isn't marked at all like the radio is, as a "D" model, nor does the schem. have any pcb #. in any event, it does at least appear to be the right schem. it's the only one i've seen that new (2006), plus only one i've seen with the 7809 reg, intead of 3756. it lists the pll as [mb8719], while radio actually has an [mmb8719]..don't know how much that matters, but i do know that the various 8719 chips are not all exactly the same.

getting back to the earlier posts, alot of my questions didn't get answered & i was trying to say how lost i was with this whole issue in general, but i'm trying to "deal"...

your last before last post, said to "disregard", but the wording used overall, regarding the 7.345 LO, left me confused as to the bottom line of that issue.

what i wound up doing, and/or observing, trying to make some sense out of it all, and hopefully garner some more opinions, is this:

1st; after identifying the general area of the rx I.F. on the schem., then looking at the pcb, i thought i found the problem, when i spotted 1 of the center conductors of 1 of the several coaxial jumpers, used in this radio, being unconnected, right in that area in question & i thought "eureka"!...but as it turned out, regardless of when that wire came loose, it was not the fix.

2nd; determined that cb/pa switch is functional.

3rd; notice that there is significant "audio pop", heard when switching positions, on the mode switch.

4; noticed there is a very "lo level", radio generated, white noise present in speaker, but it is not affected by the squelch cntrl., and it is affected by the tone control.

5; bypassed rf gain control. to "rule it out".

6; replaced X1, 7.345 xtal

7; replaced TR14 (rf amp)

8; replaced TR9 (can't remember what that is or why i did, but it "looked guilty")

in trying to figure out the circuitry, i was wondering also about d12,13,14,18 & 19, and f.e.t.'s 1,2 & 3, but did not replace those.

radio doesn't care what i think. it still has 0 rx sig/aud.

thanks again, for schematic

help...
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148gtl (4pin)

Post by rockhavenradio »

Good luck with the radio.
Maybe there are some others out here with some better input.
Later.
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148gtl (4pin)

Post by 222 »

ok, thanks alot.


how bout it rick, or somebody...to please help me w/this?
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148gtl (4pin)

Post by 222 »

something to add, after yesterdays 'go at it'...

while examining radio, i felt heat on chassis around area of pll, which led me to an overheated pll chip, very hot, obviously a short somewhere. the strange part about this, is that i had not done any work at all, in that area of the radio that would explain 1, why i hadn't noticed this level of heat before (spent a lg. amt. of time handling radio, since trying to fix) and 2, what i found as the obvious cause of the heat. i found a glob of solder, shorting from a pin of pll, (where there is an electrolytic (solderside)), to at least 1 point (or possibly more) nearby. it was pin#11, not positive on what it was shorting to, but it definately was the cause of the overheated pll & it cooled down after cleaning out that solder. the radio continued to work just fine, on tx, even in the shorted, overheated cond., and after cleaning it up, but still have no rx.
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148gtl (4pin)

Post by 222 »

please need more help.
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148gtl (4pin)

Post by 222 »

changed out fet2 & 3. was gonna change fet1 also, but broke it so didn't. still no rx. checked the 7809 reg. (it"s good) & the 1 on the back of the radio looks good.
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148gtl (4pin)

Post by 222 »

at this point, my question is this: can some kind soul please tell me what i have said wrong, in my postings so far, that might explain why i am not getting any more responses from anybody?

i honestly don't understand why this is so...

thankyou for any replies.
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148gtl (4pin)

Post by 222 »

could the person who runs this site please tell me if this is working or not?

i don't get what the problem is or what i'm doing wrong.
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148gtl (4pin)

Post by rockhavenradio »

Voltage on collector of TR44 should be 8.3 volts during receive. This is a receive only voltage.
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Silkscreen.pdf
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148gtl (4pin)

Post by 222 »

thanks 4 replying. tr44 is marked as an [S8550]. i could find no cross 4 that (to help me determine which is the collector). in the schematic, it's labeled as an [LM8550], but i can find no such listing for that either. in using my very quirky transistor tester (my good one was stolen), it appears to be pin 3 (and device tests as good). not certain as to the accuracy of voltmeter, but dialing in as close as i can to 13.8v supply, i read 8.15v on collector (assuming i'm correct on pin 3). since you say it's a regulator, i'm concerned about the readings on the 3 pins, so i'll report that i showed 8.19v, 7.40v, 8.15v, e,b,c, respectively. does that sound right to you?

also, i have a parts radio & removed & tested this device from it, as good also, and decided to change them out for the heck of it, and got the exact same voltage readings, as well as the same symtom of no rx. now what?

thanks so much for your time. (and the layout drawing, which is very good)

jeff
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148gtl (4pin)

Post by 222 »

i've done everything i could figure, plus what i've been advised to do. i posted all i did. could someone please help me further along with this process? hate the dead end...
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148gtl (4pin)

Post by 222 »

i've now replaced fet 1, as well. still no rx.

trying to look at schematic & make some associations w/rx. replaced d65 & tr19. no change...

also i should report that in my 1-7-09 post, i said that i detected a very lo-level "white noise" being generated from the radio & that it was not affected by the squelch & was affected by tone control..well..i could've been mistaken, the 1st time, as to the squelch (since it's so hard to hear the volume level of the white noise), but i'm pretty sure i checked that very carefully...anyway, it definately is affected by the squelch now.

rockhaven radio: could you confirm that i did what you've said so far...and that the readings i reported are ok.....and based on such, what you would look for next?

jeff
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148gtl (4pin)

Post by rockhavenradio »

your green rx led is lit?
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148gtl (4pin)

Post by rockhavenradio »

The service manual is on cbtricks.com...Look for :cobra_148_gtl_sm_pg01_pg15.doc
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148gtl (4pin)

Post by 222 »

yes, green rx light is lit.
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148gtl (4pin)

Post by 222 »

rockhavenradio, back on 1-3-09, i posted (in response to someone telling me service man. was on cbtricks) that as far as i could tell, what they had listed there, was not the same as my radio, i.e., it showed ic4 as the old mb3756 switch/regulator, whereas my radio uses a 3pin 7809 reg. i went back to check your exact url/link & it is indeed the same manual i was referencing in that post (the mb3756 one). i'm not saying that it's not very similar, i'm just saying that i noticed it wasn't the same pcb.

the schematic you sent me, as i reported to you, is the correct one, as far as i can tell (even though it is labled as an "f" model, yet supposed to be an "e" model, and mine being a "d" model), and not the same as what is on cb tricks.

i'm pretty sure that the same is true for the layout you sent me. it is not only a much higher quality than the one on cb tricks, but it is the correct pcb, where the one on cbtricks is not.

so i'm confused as to why you would send me high quality files of the correct schematic & pcb layout, (much appreciated, because i could not find them anywhere) yet refer me to that manual on cbtricks, which is different. if i'm mistaken or missing something regarding this, i apologize in advance, but that's how i see it....and i also previously said that i'm not a tech., when it comes to rf design and/or circuitry.

also i did the test you suggested (the best i could & reported what i came up with) and asked you to confirm what i reported, as ok or not, but no response, so sometimes i'm wondering if my posts are getting seen. i see them, but i know i don't know my use of forum real well.

appreciate your help so far & hope you can/will continue to help me to figure out (and learn) the solution to this problem.
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148gtl (4pin)

Post by 222 »

happy to report that i finally have figured out & fixed the problem, although how much of it i figured out is debatable.....

i also repaired another similar boarded 148 (the modern ones are not all the same), which coincidently appeared (at first) to have the same problem, but as it turned out, the cause/effect was a completely different issue. this one came in after the 1st one & i was very surprised & dismayed to have another one come in w/apparently same problem. but as i say, it turned out to be a same symptom, w/a totally different reason. ironically, it was the 2nd radio i fixed 1st, (with an unusual cure)....and getting that accomplished is what made me think it was possibly the same fix for the 1st (the one i've been posting about) and actually led me to the fix, which was completely different.

i'm reporting my findings & fixes, although from the relative lack of response, to my questions & postings, as well as actual help, i'm not sure if anyone cares. that's not to take away from the several responses i did get, mainly from 1 person, but even there, nearly every question i had (good ones) went unanswered.

the 2nd 148, which had the same exact symptom (no rx, tx fine) turned out to have a bad pll chip. the history, to my knowledge, was that it was purchased new, worked a couple times, then 'apparently' lost rx. when i got it, it keyed up & tx'd on the channel i was testing on (a major coincidence, as it turns out), but seemed to have 0 rx. i thought it had been tx'ing on other channels as well, at 1st, but now i can't say for certain that that is the case, reason being: in a later test, i found it "skipping certain channels", in tx. a later test, found it to not lock down properly, i.e., it was tx'ing on 27mhz, but i had to "fine tune" the pll/vco section to get it on freq for that channel, but it would not change channels at all, plus still no rx was apparent. that is because of a result of a bad pll, and i guess the way that particular pll functions internally, it was able to tx on a correct freq (originally it went there by itself, but later i had to "tune it in" to that tx freq), but it was either rx'ing on a far removed freq (too far away, for me to notice), or not rx'ing at all (rx freq mute?).

it now looks like most likely it worked ok for a day or 2, when purchased, then, after i got it, pll failed, but in 2-3 stages, which was confusing to me. incidently, nothing was overheated, incl. pll chip.

not sure what brought me to conclude to change out pll chip, but doing so fixed. (incidently, i inadvertantly wound up using an mb8734, (must've had from an old 2000) so if it comes back to me where they want "extras", i'll have to change the chip again)

so now, spurred on by that fix, i started thinking about how in the 1st 148 (no rx), i have nearly replaced all the components in the rx section, yet still had no rx, and wondering if some "connection", or similarity, to the fix on the 2nd one. but what was confusing is that i had perfect tx fuction (including all channels), so while that made me doubt another bad pll, still, i noted that it was possible to be able to tx properly, yet not rx, due to pll/vco issues, so i was going to attempt another pll chip changeout, but 1st i decided to check again what i have already done before, which was relative alignment of rx stages, plus vco align. in the past, doing that had resuted in 0 rx signal, that i could see or hear....but....this time i decided that it could be possible to "be in pll/vco lock", with tx working fine, yet still find "a different spot" within that "in lock condition", on the vco adjusting can, that might with some very fine tuning, show me a touch of rx signal...that, with the assumption that there may be no actual component breakdown, just tuning. so with everything cranked up as much as possible on radio, i super fine tuned on that vco can, within its wide range of "still being in lock"...and bingo, i finally picked up a very, very slight audio, in the rx, upon injecting some signal. from that, i surmised that it was possible that the rx was so far out of tune, plus taking into account the relative instability of these "china made peices of junk", that i could've missed finding any rx signal before, even though i tuned for it, because it was too unstable, therfore the rx could be hiding. so further fine tuning upon that small signal i found, got me to the realm of ballpark decent rx, but still not where it should be & that is because 1 of the cans in tx strip (at least what i thought was the tx only strip) is common to tx/rx & was out so far, that it provided the rest of the missing rx. i'm not by the radio or the drawings at the moment, so i can't say which can that is, but when i look it up, i'm sure to find that it's some kind of mixer, common to tx & rx, but i thought it was only part of tx & i thought it was the last one in tx strip. guess not.

strangely, this one (this radio & pll) is the one that i earlier found that short, which was burning up the pll chip, but had nothing to do with this problem, plus it continued to tx perfectly, even when it was burning up from that short, and pll still is fine. yet the other radio was brand new, pll was not shorted or getting hot, yet that is what was bad, the pll chip.

finally, i don't know if any of this will help anyone or not, but i'm thinking that it just might spur on some, with even less knowledge than me, to figure out or fix something. tenfoe?

p.s. 4got 2 mention that all those parts i replaced at 1st, may have fixed "something else" that i never identified, thereby now allowing me to find that "tiny bit of rx signal"...because in my initial post, i reported that the squelch had no effect on the tiny bit of internal white noise present in the speaker, but as i reported a day or 2 ago, in a subsequent test, after replacing all those parts, i now found that the squelch was indeed working uopn that noise.
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148gtl (4pin)

Post by rockhavenradio »

This alignment procedure fits the 148 D china radio
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148gtl (4pin)

Post by rockhavenradio »

The 148 GTL series are all similar. 3 pin regulator or otherwise. I also had 2 in. One had a bad 8.3 volt rx switch, TR44 and a shorted audio amp TR41. No rx and poor am tx audio.

the other was no rx...cold solder at c175. This one also suffered form poor quality construction requiring reinstallation of all heat sinked components. Installed a variable am power and swing cap mod in this one also.

I did clarifier unlocks on both...required installing a dedicated 9 volt regulator for tx/rx to track. A 5 volt reg would be better...9 volts gives too much range for the control. I prefer +/- 1 kHz. I think +/-5 is way too much for the one turn crapy pot.

once these radios are aligned, they're not too bad...get the bugs out.
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148gtl (4pin)

Post by 222 »

at my last post, i thought the case was closed on my two 148's, and indeed the 2nd one (the one that needed a new pll chip) is ok, but as my luck would have it, i guess i spoke too soon on the other one...the 1st one...the one i've been trying to get help w/here. i guess i got so excited, after all the time & testing & questions & parts changes, then rechecking & finding that little bit of rx signal, which in turn i was able to tune up to where it appeared it had good rx signal now...but i was using too strong of a signal source & not paying close enough attention...so that when i went to finish it up, i realized it only has approx 40-50% rx, which compared to 0, seemed like everything....but it's not good. went thru tuning over & over, but it's not there...

very frustrated w/it & ready to chuck.....

yes, it occurred to me that the the various 148's are similar & that alignment procedure you reference is usable, but it is a different board & not being a tech, i thought it logical to not assume that the schematics & align. proced. would be the same for different pcb's. i will take your word for it, but there must be some differences...?

in any event, i'm not a circuitry tech & don't have alot of equiptment. i'll take a look at the rx align procedure & see if i can glean something from it, but i need some further pointers, or source voltages to check, or which rx amps and/or supporting components, etc., to look for, other than what i have posted as to what i have replaced so far....and keeping in mind that i now have roughly 40-50% rx. i will check that solder joint you mentioned for that cap, and check tr44.

thanks for continuing info & support!
222
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Re: 148gtl (4pin)

Post by Johnno »

Hey 222, I enjoyed your posts immensely, keep up the good work.

Cheers and 73's
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Re: 148gtl (4pin)

Post by 222 »

thanks johnno. appreciate that. been a few months, so was surprised. and to be honest, i come back & reread my posts & see i'm way too longwinded with them. i keep trying to work on that.

i think i may have in the meantime seen another one that needed a new pll, so keep in mind.

and also, another unusual fix to report: although this was on an older 148.....was missing a few channels....drove me crazy....found a cracked tuner (channel switch). crack was up around where the knob stem comes out from body of tuner. in fact, someone apparently had found this previously & tried to fix cracked tuner, because it had a wire wrapped & twisted around the crack, which didn't hold. easy change out, even tho 30 pins. 1st time i recall seeing that.

nowurtalkin
Johnno
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Re: 148gtl (4pin)

Post by Johnno »

Hi 222, good to see you are still here, and TY for the reply, you seem like you know a bit about radios, could you give my other 2 posts a look? You seem to know what you are talking about, and I am following a similar path to you, I reckon, although far behind you :)

Cheers and 73's
222
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Re: 148gtl (4pin)

Post by 222 »

johnno,

i'd be more than happy to at least try to help, but you'll need to direct me to the 2 posts you refer to? this thread is 2 pages & i don't see your posts here. i'm not good at all on using the forum, so maybe you could post a link to, or repost a copy of...well wait..that method might not be right unless the topics of the posts you refer to, are similar to this thread...so if not, post a link to them, or "something"...cuz i don't know where to find them.

deuce on the loose
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Re: 148gtl (4pin)

Post by Johnno »

Johnno
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Re: 148gtl (4pin)

Post by Johnno »

Johnno
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Re: 148gtl (4pin)

Post by Johnno »

Hi 222, did they work?

Cheers
John
222
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Re: 148gtl (4pin)

Post by 222 »

yes they did, thankyou. i posted my input.
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Re: 148gtl (4pin)

Post by Challenger »

222,
I,ve read this entire thread with great intrest. I say you have done very well! Keep it up. I learn quite a bit by reading informative threads like this one. If i can add anything positive i will. Just wanted you to know your posts are getting read.......alot. My cobra 25 quit receiving, and i found the mic plug to be at fault. I changed the plug and it straightened right up. Wouldn,t receive without the mic hooked up. Sometimes i find its the simple solutions i overlook.
Challenger
222
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Re: 148gtl (4pin)

Post by 222 »

hey challenger...i appreciate that alot! really. when i am doing all that longwinded "covering of the bases", so to speak, i'm not only doing it for my own benefit, but esp. in a case like this 148 problems thread, and with all the posts, i'm realizing at the same time, that others "should" be reading and "should" be getting valuable knowledge from it. your post exemplifies that, as fact!

more proof of this...is that when i clicked on the link in the "notification" email from the forum, to see what was posted, it brings you not to that directly, but to the beginning of the thread, and since i'd forgotten much of it, i reread the whole thing again, to get to your post. well, since those other 148's, i've had a "spate" of them recently, w/ similar (but varying) problems. in fact 3 or 4 very recently, in a row, that i've not been able to straighten out. i've tried most of the things that've already been mentioned in this thread, but i did forget some of those things & now have been reminded of them, by my own posts & some things that were mentioned to me as well, so "energized" to go out to shop & "get back to them!

i should say that it definitely appears that the china made (newer) 148's have "issues" in the pll/vco section. and i'd be interested to hear from anyone else who has identified one of them as having a bad pll chip, or confirm any "other issues" in the pll/vco.

222out
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Re: 148gtl (4pin)

Post by COBRAMAN148 »

oh man get an old 148 i have modded a couple of the new 148-d and they lasted for 1 yr. i am sorry but they are crap. it is a parts radio. that is where mine ended up. :banghead:
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Re: 148gtl (4pin)

Post by travieso »

rockhavenradio wrote:I recently worked on the same radio, "148 D"
I have the schematic of that radio.
Cobra sent it to me a few months ago.
If you need a copy, let me know how I can get it to you.
hello my friend, my name is Jorge and i'm looking for a service manual for a 148 GTL D .... if you have it, please send a copy to my e-mail address montavaio@hotmail.com and if I have to pay anything let me know.... i would realy appreciate it thank you
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