There is a Cobra Service Bulletin for that 'warble' sound on SSB transmit; dunno why I didn't see that one before.
Gonna try it tomorrow . . .
Anyway, here it is:
Gonna try it tomorrow . . .
Anyway, here it is:
-
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Help needed for Cobra 148 NW ST
- Thread starter CTStallion
- Start date
hope it works for you. I have tried it on a couple and it did not work for me. if can find it there is another mod to repair this that I tried one time and it worked. have got yo see if can find it again. will post it if I can.
here is a warble repair mod I did one time and it worked. just hunted it down for you guy's to read.
COBRA 148 MALAYSIA WARBLE MODS.
The nominal Uniden design has the power supplied to the audio section of the radio run through a large
choke to filter stray RF out of the audio section. The power that supplies the MB3756 comes directly
from the DC jack in the back of the radio.
The Malaysian 148s have the totality of the 13.8v input run through that big choke on the board (that
looks like a transformer by VR10). This works fine in a stock radio, but when you open it up, the
additional current draw through that choke can cause a voltage drop. This, in turn, plays havok with
the PLL and is what leads to the FM condition (the warble) across the PLL.
The modification involves returning the design to the Uniden design, thankfully this is easy to
accomplish. First, you will notice that there is a marking (with holes drilled) for a J44, but there is no
shunt soldered across it. Second, you will notice that there is a continuous trace between pin 1 on the
TA7222P and pin 2 on the MB3756 (pin 2 on this is the +13.8v supply line). The jist is that the
MB3756 gets its power from pin 1 of the TA7222P, which draws power through that choke.
You will see a spot along this trace that is thin (and on a Uniden radio, there is no copper at all there).
Scrape that copper off, you don't need to get it perfect, just ensure there is an interruption of continuity
there. Now, solder a wire onto the two holes for J44- this will supply line power to the MB3756
(replacing what you took away by scraping the trace off).
Now, the only thing running through that choke is the power for the audio section - the rest of the radio
now gets its power from the jack, just as Uniden designed.
Check C181 and C172 and replace with ones with a higher a working voltage.
they're the same part number on the Malaysia units as the older ones, D35 if i remember right, however
on the Malaysia units when you look where D35 should be all you find is empty holes, thats because
they used a junky surface mount varactor on the trace side, just remove it, and put a good varactor from
a junker in the holes on the parts side where it belongs,
always use an ECG612 A.K.A. NTE612 varactor diode instead of those junky ones (bought in mass
and on the cheap). These diodes are frickin' tough, and worth the wait....over the mail and ect. I very
strongly suggest you order this diode instead of doing a "parts pull".
The factory part numbers are 1S2687D, 1S2687 or 1S2686.
COBRA 148 MALAYSIA WARBLE MODS.
The nominal Uniden design has the power supplied to the audio section of the radio run through a large
choke to filter stray RF out of the audio section. The power that supplies the MB3756 comes directly
from the DC jack in the back of the radio.
The Malaysian 148s have the totality of the 13.8v input run through that big choke on the board (that
looks like a transformer by VR10). This works fine in a stock radio, but when you open it up, the
additional current draw through that choke can cause a voltage drop. This, in turn, plays havok with
the PLL and is what leads to the FM condition (the warble) across the PLL.
The modification involves returning the design to the Uniden design, thankfully this is easy to
accomplish. First, you will notice that there is a marking (with holes drilled) for a J44, but there is no
shunt soldered across it. Second, you will notice that there is a continuous trace between pin 1 on the
TA7222P and pin 2 on the MB3756 (pin 2 on this is the +13.8v supply line). The jist is that the
MB3756 gets its power from pin 1 of the TA7222P, which draws power through that choke.
You will see a spot along this trace that is thin (and on a Uniden radio, there is no copper at all there).
Scrape that copper off, you don't need to get it perfect, just ensure there is an interruption of continuity
there. Now, solder a wire onto the two holes for J44- this will supply line power to the MB3756
(replacing what you took away by scraping the trace off).
Now, the only thing running through that choke is the power for the audio section - the rest of the radio
now gets its power from the jack, just as Uniden designed.
Check C181 and C172 and replace with ones with a higher a working voltage.
they're the same part number on the Malaysia units as the older ones, D35 if i remember right, however
on the Malaysia units when you look where D35 should be all you find is empty holes, thats because
they used a junky surface mount varactor on the trace side, just remove it, and put a good varactor from
a junker in the holes on the parts side where it belongs,
always use an ECG612 A.K.A. NTE612 varactor diode instead of those junky ones (bought in mass
and on the cheap). These diodes are frickin' tough, and worth the wait....over the mail and ect. I very
strongly suggest you order this diode instead of doing a "parts pull".
The factory part numbers are 1S2687D, 1S2687 or 1S2686.
Need to find mods for this NW/ST version. One would think that by now this radio would have a greater list of fixes. The 2200uf cap didn't do squat to fix the TX warble in SSB mode. Something else is causing it to become unstable there.
Last edited:
Robb,
On mine, I noticed that IF/WHEN I turn the ALC VR pot ALL THE WAY UP, I begin to get 'the warble', no doubt putting more strain on it than the power supply circuitry was designed to accommodate, even THOUGH the 2166 driver and 1969 final are more than adequate for the task at hand.
However, if/when I back off ever so slightly on the ALC VR POT, then, voila, the warble is gone.
At first, that seemed a wee bit counterintuitive to me, as I was actually considering bypassing the ALC completely in order to take full advantage of all the 2166/1969 combo had to offer. I was getting over 18W SSB PEP with the ALC all the way up, but alas, WITH the dreaded warble. However, once I backed it off to about 15W SSB PEP, no more warble.
Suffice it to say, at this point, I like the RX of this rig much better than the TX. And of course, Night Watch feature, no workie.
On mine, I noticed that IF/WHEN I turn the ALC VR pot ALL THE WAY UP, I begin to get 'the warble', no doubt putting more strain on it than the power supply circuitry was designed to accommodate, even THOUGH the 2166 driver and 1969 final are more than adequate for the task at hand.
However, if/when I back off ever so slightly on the ALC VR POT, then, voila, the warble is gone.
At first, that seemed a wee bit counterintuitive to me, as I was actually considering bypassing the ALC completely in order to take full advantage of all the 2166/1969 combo had to offer. I was getting over 18W SSB PEP with the ALC all the way up, but alas, WITH the dreaded warble. However, once I backed it off to about 15W SSB PEP, no more warble.
Suffice it to say, at this point, I like the RX of this rig much better than the TX. And of course, Night Watch feature, no workie.
Update #2:
Radio is now working GREAT, (thanks to YOU GUYS!), and have performed the following on it thus far:
1. Replaced meter and wheat bulb
2. Replaced channel selector
3. Replaced MMB8179 with MB8719
4. Performed TSB 10002 (R130 replacement).... which (again) I wouldn't have been able to do without YOU
5. Performed TSB 10015 2200uF cap at C172
6. Performed TSB 10031 which basically changes components to allow the MB8719 to perform optimally
7. Unlocked the clarifier: obtained -16 KHz / +6 KHz
What's left to do for me on this one is:
- Receive mods
- Channel mods
- AGC fast/slow switch for SSB use
Finally, if possible, I'd like to get the NW functioning, but I fear that I'll need a new EL lamp faceplate in order to make it work. And (unfortunately) even IF I get it functioning properly, I believe it's just a matter of time before this (weak part) fails again. So, not so sure I'll even bother with it.
Will keep you posted.
Radio is now working GREAT, (thanks to YOU GUYS!), and have performed the following on it thus far:
1. Replaced meter and wheat bulb
2. Replaced channel selector
3. Replaced MMB8179 with MB8719
4. Performed TSB 10002 (R130 replacement).... which (again) I wouldn't have been able to do without YOU
5. Performed TSB 10015 2200uF cap at C172
6. Performed TSB 10031 which basically changes components to allow the MB8719 to perform optimally
7. Unlocked the clarifier: obtained -16 KHz / +6 KHz
What's left to do for me on this one is:
- Receive mods
- Channel mods
- AGC fast/slow switch for SSB use
Finally, if possible, I'd like to get the NW functioning, but I fear that I'll need a new EL lamp faceplate in order to make it work. And (unfortunately) even IF I get it functioning properly, I believe it's just a matter of time before this (weak part) fails again. So, not so sure I'll even bother with it.
Will keep you posted.
At last....
Radio is FINISHED.
RX mod was a little tricky, as the TR14 it uses is a 2SC3194 instead of a 2SC1674. Couldn't find specs on a 2SC3194, but it defaulted to a 2SC3229. Either way, the NTE substitute for BOTH of those AND for the 1674 is an NTE107, so I used a genuine 2SC2999.
As for the TR19, it was a 2SC1815 instead of a 2SC945, but the schematic shows a 2SC3198. Again, no matter because the NTE sub for all 3 was the NTE85. However, when it comes to being LOW NOISE, the 2SC1815 is actually BETTER than the 2SC945 or the 2SC1674, so I kept it in place.
Schottky diodes (all 1N5711's) were used for the AM Product Detectors and Noise Blankers to replace the germanium 1N60's with success.
As far as the AGC for SSB (C23), on MOST Cobra 148's I simply remove the 0.47 uF electrolytic and substitute a SWITCHABLE 10 uF device; on AM the capacitor is switched OUT of the circuit, and on SSB the cap is switched IN, in order to slow the AGC on SSB to prevent receiver (and meter) 'PUMPING'. While it worked GREAT on SSB, on AM I noticed with the cap in OR out, AM was distorted, PARTICULARLY with the Sound Tracker engaged. (BTW, I found an extra VR pot which sets the 'gain' of the Sound Tracker compander circuit that, like DSP, if you don't set it all the way, rather set it until the ST 'just engages', it will work MARVELOUSLY on BOTH AM & SSB.
So, to fix the AGC problem I COULD'VE used a 0.47 uF capacitor for AM and a 10 uF for SSB however, sooner or later (like ALL of my radios) this thing will wind up in someone else's hands. Therefore, LESS is MORE; I left it STOCK with the 0.47 uF cap in place, works GOOD (but not like a Ham rig) on SSB.
Channels: Actually, the problems I had with the rig not locking on freq being a defective PLL, along with TSB 10002 and 10031 & the subsequent MB8719 swap wound up being a blessing in disguise. NOW I was able to channelize the radio, just like ANY 148 GTL, and have coverage from 26.815 to 28.045 via two (2) SPDT switches. Now, the two (ugly) side-by-side holes in the bottom case of the radio that the previous owner drilled (no doubt a FAILED attempt at a channel mod) are occupied with a FUNCTIONAL channel mod. Radio is also pretty broadbanded, no significant power drop off on the higher freqs. Also, PLL locks up EVERY time; didn't even have to adjust the VCO.
Assuming this radio is (essentially) the SAME as a front mic 148 GTL, with the obvious additions of the Sound Tracker board AND the NW feature, (with I never did get working, nor do I care to), when compared against my Grant XL (with similar mods) I have to note the following differences:
- Grant XL (and 5 pin side mic 148 GTL equivalent) is an obviously more ROBUST platform; has higher wattage resistors, a better regulator, a more 'tube like' receiver sound quality, and easier to mod.
- 148 NW ST, offers Night Watch feature (if/when it actually WORKS, which is a rarity, and extremely costly to fix), continuously variable dimmer (whereas the XL & old 148's are bright/dim ONLY), 3 position tone switch: Hi/Normal/Low which is actually quite sweet, and the Sound Tracker which, before THIS radio (on Cobra 29's I've owned) never seemed to function properly BUT on THIS radio, actually works quite WELL, once adjusted. There is actually nothing I can hear on the Grant XL that I can't hear on the 148 NW ST. However, there ARE very weak stations (on BOTH AM and SSB) that I can actually pull up out of the noise on the NW ST with the Sound Tracker engaged. When TRANSMITTING with Sound Tracker (on a stock mic), on AM, the difference (to a NON-ST equipped radio) is un-noticeable. On SSB however, it seems that the ST actually narrows the audio response of the transmitted audio to peak somewhat HIGHER in freq response, albeit with less fidelity (think Heil HC5 element = ST OFF, and HC4 element with ST ON, although NOT as pronounced an effect.) For receive purposes, on either AM or SSB, the ST does a GREAT job of eliminating background his, heterodynes, noise and static, thereby making for a quite pleasant receive to have on in the shack.
Receiver gain: for some odd reason, the OLD 148's and Grant XL's have a TON of gain in their receivers; very easy to adjust for S9. However, THIS particular NW ST, I had to do all I could to get an S9 on the receiver during alignment. Likewise, BOTH the SSB and AM rx S meter pots on the NW are almost turned up all the way; just the OPPOSITE effects on the Grants & older 148's.
Power requirements: I don't have a digital power supply to display amperage draws on TX/RX, rather instead, purely analog metering, but I did notice that when powered up, the NW ST actually draws slightly MORE CURRENT on RX (roughly 0.25 amps) than any of my 148's, XL, or SS-3900 (different chassis); I am assuming that this is due to the transformer for the Night Watch feature... but not sure. On TX, it draws the same as the its older counterparts; depending upon the output power you wish to run.
SSB TX performance - no doubt due to the cheapened regulator, the NW ST is slightly less stable than the XL, HOWEVER, once I did the cap mod (that Robb had no success with) and adjusted the ALC down to 18 watts PEP, the rig sounds as good on the air and on my monitor as any 148 or Grant. However, try to crank up the SSB output much more than THAT and it begins to go unstable and warble a bit. This may ALSO occur during low voltage or low amperage power supplies. At 13.8 VDC, with a good power supply, after precise USB/LSB alignment, the ONLY way I could get the NW to 'warble' was to run the ALC at FULL BORE, with the dynamike wide open and a power mic. Again, back off on the ALC and no problem.
AM Audio - interestingly, on MOST (if not ALL) old school Cobra 148's, Grant XL's, or Texas Ranger TR-296's I find the need to replace either (or BOTH) R124 (typically 5.5K) and/or R126 (typically 10K) to a value somewhere between 2.2 and 4.7K for increased modulation. BUT, on the 148 NW ST, the FACTORY values of these resistors are R126 = 3.9K and R124 = 1.8K, so Cobra must've realized this and ALREADY made the change. With the AMC VR POT turned all the way up, and the limiters STILL INTACT, the dynamike turned all the way up, 100% modulation is achievable, even WITH a good stock mike. Radio responded well to tuning and alignment (once I got the bugs worked out), and sounds quite good on AM (and SSB).
NPC-RC, Swing Mods, or Volting the Final for AM use - I didn't even try, nor would I recommend it on the NW ST by virtue of it's degraded and low tolerance components. I think it would "work", but not for long. Transistors, wiring, driver and finals COULD handle it, but I'm not so sure the RESISTORS would respond too well, and certainly, neither would the regulator. I say leave well enough alone; LESS is MORE.
Last thing: the mounting bracket screws for the NW ST are LARGER in MM than the older 148, 286's or XL's, so even though the bracket and casings are the same dimensions, you will need different mounting bracket screws for the NW ST.
That's all I got for now.
Thank YOU so much guys for helping me troubleshoot and REPAIR this rig. Special shout outs and gratitude to: Exit 13, Sonoma, and Robb WHO, without YOU GUYS, I would've used this rig for 'parts'. Now, thanks to you fellas, it WORKS, and BETTER THAN NEW!
Radio is FINISHED.
RX mod was a little tricky, as the TR14 it uses is a 2SC3194 instead of a 2SC1674. Couldn't find specs on a 2SC3194, but it defaulted to a 2SC3229. Either way, the NTE substitute for BOTH of those AND for the 1674 is an NTE107, so I used a genuine 2SC2999.
As for the TR19, it was a 2SC1815 instead of a 2SC945, but the schematic shows a 2SC3198. Again, no matter because the NTE sub for all 3 was the NTE85. However, when it comes to being LOW NOISE, the 2SC1815 is actually BETTER than the 2SC945 or the 2SC1674, so I kept it in place.
Schottky diodes (all 1N5711's) were used for the AM Product Detectors and Noise Blankers to replace the germanium 1N60's with success.
As far as the AGC for SSB (C23), on MOST Cobra 148's I simply remove the 0.47 uF electrolytic and substitute a SWITCHABLE 10 uF device; on AM the capacitor is switched OUT of the circuit, and on SSB the cap is switched IN, in order to slow the AGC on SSB to prevent receiver (and meter) 'PUMPING'. While it worked GREAT on SSB, on AM I noticed with the cap in OR out, AM was distorted, PARTICULARLY with the Sound Tracker engaged. (BTW, I found an extra VR pot which sets the 'gain' of the Sound Tracker compander circuit that, like DSP, if you don't set it all the way, rather set it until the ST 'just engages', it will work MARVELOUSLY on BOTH AM & SSB.
So, to fix the AGC problem I COULD'VE used a 0.47 uF capacitor for AM and a 10 uF for SSB however, sooner or later (like ALL of my radios) this thing will wind up in someone else's hands. Therefore, LESS is MORE; I left it STOCK with the 0.47 uF cap in place, works GOOD (but not like a Ham rig) on SSB.
Channels: Actually, the problems I had with the rig not locking on freq being a defective PLL, along with TSB 10002 and 10031 & the subsequent MB8719 swap wound up being a blessing in disguise. NOW I was able to channelize the radio, just like ANY 148 GTL, and have coverage from 26.815 to 28.045 via two (2) SPDT switches. Now, the two (ugly) side-by-side holes in the bottom case of the radio that the previous owner drilled (no doubt a FAILED attempt at a channel mod) are occupied with a FUNCTIONAL channel mod. Radio is also pretty broadbanded, no significant power drop off on the higher freqs. Also, PLL locks up EVERY time; didn't even have to adjust the VCO.
Assuming this radio is (essentially) the SAME as a front mic 148 GTL, with the obvious additions of the Sound Tracker board AND the NW feature, (with I never did get working, nor do I care to), when compared against my Grant XL (with similar mods) I have to note the following differences:
- Grant XL (and 5 pin side mic 148 GTL equivalent) is an obviously more ROBUST platform; has higher wattage resistors, a better regulator, a more 'tube like' receiver sound quality, and easier to mod.
- 148 NW ST, offers Night Watch feature (if/when it actually WORKS, which is a rarity, and extremely costly to fix), continuously variable dimmer (whereas the XL & old 148's are bright/dim ONLY), 3 position tone switch: Hi/Normal/Low which is actually quite sweet, and the Sound Tracker which, before THIS radio (on Cobra 29's I've owned) never seemed to function properly BUT on THIS radio, actually works quite WELL, once adjusted. There is actually nothing I can hear on the Grant XL that I can't hear on the 148 NW ST. However, there ARE very weak stations (on BOTH AM and SSB) that I can actually pull up out of the noise on the NW ST with the Sound Tracker engaged. When TRANSMITTING with Sound Tracker (on a stock mic), on AM, the difference (to a NON-ST equipped radio) is un-noticeable. On SSB however, it seems that the ST actually narrows the audio response of the transmitted audio to peak somewhat HIGHER in freq response, albeit with less fidelity (think Heil HC5 element = ST OFF, and HC4 element with ST ON, although NOT as pronounced an effect.) For receive purposes, on either AM or SSB, the ST does a GREAT job of eliminating background his, heterodynes, noise and static, thereby making for a quite pleasant receive to have on in the shack.
Receiver gain: for some odd reason, the OLD 148's and Grant XL's have a TON of gain in their receivers; very easy to adjust for S9. However, THIS particular NW ST, I had to do all I could to get an S9 on the receiver during alignment. Likewise, BOTH the SSB and AM rx S meter pots on the NW are almost turned up all the way; just the OPPOSITE effects on the Grants & older 148's.
Power requirements: I don't have a digital power supply to display amperage draws on TX/RX, rather instead, purely analog metering, but I did notice that when powered up, the NW ST actually draws slightly MORE CURRENT on RX (roughly 0.25 amps) than any of my 148's, XL, or SS-3900 (different chassis); I am assuming that this is due to the transformer for the Night Watch feature... but not sure. On TX, it draws the same as the its older counterparts; depending upon the output power you wish to run.
SSB TX performance - no doubt due to the cheapened regulator, the NW ST is slightly less stable than the XL, HOWEVER, once I did the cap mod (that Robb had no success with) and adjusted the ALC down to 18 watts PEP, the rig sounds as good on the air and on my monitor as any 148 or Grant. However, try to crank up the SSB output much more than THAT and it begins to go unstable and warble a bit. This may ALSO occur during low voltage or low amperage power supplies. At 13.8 VDC, with a good power supply, after precise USB/LSB alignment, the ONLY way I could get the NW to 'warble' was to run the ALC at FULL BORE, with the dynamike wide open and a power mic. Again, back off on the ALC and no problem.
AM Audio - interestingly, on MOST (if not ALL) old school Cobra 148's, Grant XL's, or Texas Ranger TR-296's I find the need to replace either (or BOTH) R124 (typically 5.5K) and/or R126 (typically 10K) to a value somewhere between 2.2 and 4.7K for increased modulation. BUT, on the 148 NW ST, the FACTORY values of these resistors are R126 = 3.9K and R124 = 1.8K, so Cobra must've realized this and ALREADY made the change. With the AMC VR POT turned all the way up, and the limiters STILL INTACT, the dynamike turned all the way up, 100% modulation is achievable, even WITH a good stock mike. Radio responded well to tuning and alignment (once I got the bugs worked out), and sounds quite good on AM (and SSB).
NPC-RC, Swing Mods, or Volting the Final for AM use - I didn't even try, nor would I recommend it on the NW ST by virtue of it's degraded and low tolerance components. I think it would "work", but not for long. Transistors, wiring, driver and finals COULD handle it, but I'm not so sure the RESISTORS would respond too well, and certainly, neither would the regulator. I say leave well enough alone; LESS is MORE.
Last thing: the mounting bracket screws for the NW ST are LARGER in MM than the older 148, 286's or XL's, so even though the bracket and casings are the same dimensions, you will need different mounting bracket screws for the NW ST.
That's all I got for now.
Thank YOU so much guys for helping me troubleshoot and REPAIR this rig. Special shout outs and gratitude to: Exit 13, Sonoma, and Robb WHO, without YOU GUYS, I would've used this rig for 'parts'. Now, thanks to you fellas, it WORKS, and BETTER THAN NEW!
Near the back, not far from the TX MTR VR pot. It is a VR pot that if you engage your Sound Tracker, you can set the level of it with that particular VR. I couldn't find a label number on the PCB.
YES, that's THE one, HOWEVER, I believe it adjusts RX GAIN in BOTH ST on and ST OFF, so be careful WHERE you set it.
For example, I actually had to turn it down enough that I couldn't get an 'S9' on my RX on AM (but I do get it on SSB) w/o turning the AM S-Meter pot all the way up. Then, I backed off on the VR pot above (with the ST ON and OFF) until the ST no longer distorted strong signals.
RX sensitivity is probably just a little bit down, HOWEVER, the 2SC2999 (and diode) mods more than make up for it. BUT, best of all, Sound Tracker is actually quite USABLE when adjusted.
Does YOUR VR pot (in the above picture) have a 'label' on it on the PC board? Mine doesn't.
Also, MOST of the VR pots on the NW ST are NOT labeled the same as a 148-GTL 5 pin, so beware.
Glad you got it all working! 
Got a question tho, are you planning on using this mobile or for a base? If you're using it on the base, the large transformer/choke (I think it can/might be labeled as T1) can be jumped and bypassed and that will help with the warble on SSB. Also, on some of the 148 NW/ST's, the C1969 final in some of the later production radios are NOT genuine, they are fakes, and will also cause a warble issue.
If the radio is used mobile, you can still bypass this transformer/choke, but you might get a little bit of whine or noise, you can build your own choke with heavier wire and retrofit that inline with your positive power wire on the power cord.
The diodes for the RX mod is a great idea, but for TR14, it's best to sub in a C1730L instead of a C2999. Then use a C1674 for TR19, that combination yields the bests results that I have seen so far in testing.
~Cheers~
Got a question tho, are you planning on using this mobile or for a base? If you're using it on the base, the large transformer/choke (I think it can/might be labeled as T1) can be jumped and bypassed and that will help with the warble on SSB. Also, on some of the 148 NW/ST's, the C1969 final in some of the later production radios are NOT genuine, they are fakes, and will also cause a warble issue.
If the radio is used mobile, you can still bypass this transformer/choke, but you might get a little bit of whine or noise, you can build your own choke with heavier wire and retrofit that inline with your positive power wire on the power cord.
The diodes for the RX mod is a great idea, but for TR14, it's best to sub in a C1730L instead of a C2999. Then use a C1674 for TR19, that combination yields the bests results that I have seen so far in testing.
~Cheers~