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Cobra 148gtl No Modulation and...
- Thread starter Gunner357
- Start date
Couple of things . . .
First, you said your radio is the original or 'classic' 148GTL. If so, it will have an eight pin Toshiba MB3756 voltage regulator. It will have a five pin mic plug on the SIDE of the radio, and not a four pin mic plug on the FRONT of the radio. The four pin radio is a real 'piece of work' that will have MANY issues that will need addressing all on its own. The number of five pin mic radio problems? Not so much.
It is important to know this because you may need to look at the right schematic to deal with components that may well be the problem you are facing. Actually, this is the reason I bothered to comment at all. Using a schematic - not mentioned in any of your posts - is essential for staying on top of this radio - rather than 'being behind the eight ball' both now and in the future. Just a sound practice.
You didn't mention that you are using an O'scope to evaluate modulation. If you are using a watt meter, you cannot really gauge what is going on with the audio issues or output. Watt meters are the worst possible gauge there is. Other than someone telling you 'yeah, it sounds good' when it could be distorting and throwing off a really nasty signal. The scope doesn't lie, but liars don't use scopes to do 'their best work'.
Quite often when getting a used radio, I find it is most likely hacked, and hacked by people that do not have a clue of what they are doing. Or worse, it has been hacked by a shop that allegedly knows what they are doing. It is best to check out the radio and be sure that the radio is back to stock and therefore responds properly when given an alignment. The best way to do this, is to go to the 'Defpom' page, and undo all of the mods you might find in your radio and put it back to stock.
I take it that you turned up the modulation pot before you clipped the resistor. Be sure that TR24 is in the radio too, as removing it will give more - not better - AM modulation; but at the cost of outright ruining SSB transmit audio, as well as making for a horrendously distorted audio signal on the scope. It is NOT a real mod; but a poorly and often widely accepted hack. Not a bargain; far more is lost than gained.
First, you said your radio is the original or 'classic' 148GTL. If so, it will have an eight pin Toshiba MB3756 voltage regulator. It will have a five pin mic plug on the SIDE of the radio, and not a four pin mic plug on the FRONT of the radio. The four pin radio is a real 'piece of work' that will have MANY issues that will need addressing all on its own. The number of five pin mic radio problems? Not so much.
It is important to know this because you may need to look at the right schematic to deal with components that may well be the problem you are facing. Actually, this is the reason I bothered to comment at all. Using a schematic - not mentioned in any of your posts - is essential for staying on top of this radio - rather than 'being behind the eight ball' both now and in the future. Just a sound practice.
You didn't mention that you are using an O'scope to evaluate modulation. If you are using a watt meter, you cannot really gauge what is going on with the audio issues or output. Watt meters are the worst possible gauge there is. Other than someone telling you 'yeah, it sounds good' when it could be distorting and throwing off a really nasty signal. The scope doesn't lie, but liars don't use scopes to do 'their best work'.
Quite often when getting a used radio, I find it is most likely hacked, and hacked by people that do not have a clue of what they are doing. Or worse, it has been hacked by a shop that allegedly knows what they are doing. It is best to check out the radio and be sure that the radio is back to stock and therefore responds properly when given an alignment. The best way to do this, is to go to the 'Defpom' page, and undo all of the mods you might find in your radio and put it back to stock.
I take it that you turned up the modulation pot before you clipped the resistor. Be sure that TR24 is in the radio too, as removing it will give more - not better - AM modulation; but at the cost of outright ruining SSB transmit audio, as well as making for a horrendously distorted audio signal on the scope. It is NOT a real mod; but a poorly and often widely accepted hack. Not a bargain; far more is lost than gained.
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I do not have a scope and yes its a 5 pin and yes the schematics haven't been matching my board. Its a 1996. I inspected the D44 its good.....Im using a Dosy Meter and it DK 5 watts I haven't done any mods except the pot and clipping which made no improvements . I since replace the R131 because I hate hacking something so radio is back to stock.... Ive had radio checks and people are saying that they hear the power but it sounds like the mic is far away......thats using a regular mic a power mic on level 2 sound good and shows 100 percent on the Dosy but with out the Power mic Its averaging 25-50 percent on the modulation scale..... Now the Finals I purchased were from a electronics store with the exact markings on them so hopefully they aren't copies at least the price would indicate they weren't..... Can it be the finals?..... I checked the regulator on the side per a service manual and it shows all the correct voltages... OH by the way the pot for the modulation is all the way to max if i whistle when turning it it didnt seem to have and change..... please help and thank you
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Well first of all, the mic is in question.
Can you try that mic on another radio - and know that it is working properly and gets the right amount of modulation?
If it tests fine on another radio; then you will need to trace down the mic amp circuit from the schematic in the radio and then test those components.
Bad cap, transistor, or solder joint; or a combination of all of the above.
Once the audio is getting the right level (hard to know if you don't have a scope); then it is off to the mixing IC to be slapped onto the carrier.
BTW - you said absolutely NOTHING about how it works in SSB mode . . . Does your Dosy swing in SSB mode when talking into the mic?
. . . Not likely that the final is bad if you are seeing 5w dead key and more modulated watts of any kind - on top of that . . .
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No, D44 will not have any effect on modulation, it is only there for voltage protection.
Yeah I Stated above I have 2 mics a power mic and a regular mic they both work fine on another radio in fact I tried the reg on another Cobra, Im confident that both mics are working
and if I switch to ssb, and speak AAAAUDIOOOO it swings to 15watts and modulation seems fine I have to speak up a little louder than other radios but it is working
For years, it seemed like every third yard sale had a 3-knob CB radio on a table for five bucks or so. If you were lucky, there would be a power supply for not much more. Having a second radio so you can hear your own signal can be incredibly valuable, especially if you don't already have any 'tools of the trade' like a 'scope and wattmeter.
Much better to rely on what your own ears can hear from your radio.
Relying on someone else to evaluate the audio quality of your modulation is iffy at best, a waste of time at worst.
A yard-sale CB is the cheapest diagnostic tool out there any time you have a question about your transmitted signal.
73
Much better to rely on what your own ears can hear from your radio.
Relying on someone else to evaluate the audio quality of your modulation is iffy at best, a waste of time at worst.
A yard-sale CB is the cheapest diagnostic tool out there any time you have a question about your transmitted signal.
73
I ended up
I have the cb in the house I ended up going outside to my truck and had my step son key it up say audio test 1234 etc...... and sure enough my s meter in the truck would peg but it sounded like he was 10 feet away from the mic so I put the power mic on told hmm to set it on level 2 he spoke it sound crystal clear had him put it on 10 and wow no distortion but very loud however i still want to know why the std mic sounds so low??
Maybe it's an old mike that's just gone bad inside?
Or maybe the radio's mike amplifier is weak, and the power mike just makes up for it by driving harder than a stock mike.
Either one sounds plausible.
Trying the straight mike on another radio, or another stock mike on your radio might narrow it down.
73
Or maybe the radio's mike amplifier is weak, and the power mike just makes up for it by driving harder than a stock mike.
Either one sounds plausible.
Trying the straight mike on another radio, or another stock mike on your radio might narrow it down.
73
All the mics I used worked on all the other radios I have so its not the mic. I have 4 mics. 2 -Astatic 636L, 1- Ranger SRA-108, and a Cobra CA76 power mic
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More than likely they did a mod to the mic circuit.
Might have to go in there and look around for the Mic connector and look for "fresh work" - like you found with soldering.
So the Dynamike and Mic plug need to be investigated. Look for taped up work in the wiring harness. You may find they put some kind of resistor in line to attenuate the drive level of the power mic and ran the radio that way.
It's when you try to put a stock mic back on it, that resistor or attenuator - kills so much of the signal you can barely hear them.
Gunnar:
It occurred to me that you are using the 636 mic!
You cannot use a 636 like a regular mic. Did you notice that there is a rubber lip guard on the mic? You MUST keep you mouth close to those types of mics that have a rubber lip guard. You cannot be a foot away from that mic and expect it to have full output; it only gets full output when it is held close to your mouth. It is a 'noise cancelling' type of mic and rejects sound - even from a foot away.
Seems to me that there is nothing really wrong with your radio since your SSB test results. Use a power mic if you are using the radio as a base station; but use the 636 if using it in a vehicle and talk close to it. I think you should send it off to a shop and get the radio inspected and aligned.
It occurred to me that you are using the 636 mic!
You cannot use a 636 like a regular mic. Did you notice that there is a rubber lip guard on the mic? You MUST keep you mouth close to those types of mics that have a rubber lip guard. You cannot be a foot away from that mic and expect it to have full output; it only gets full output when it is held close to your mouth. It is a 'noise cancelling' type of mic and rejects sound - even from a foot away.
Seems to me that there is nothing really wrong with your radio since your SSB test results. Use a power mic if you are using the radio as a base station; but use the 636 if using it in a vehicle and talk close to it. I think you should send it off to a shop and get the radio inspected and aligned.
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I bought it new nobody other than me has been in there.... However I look some more for solder issues......Thanks
all those mics are use for as test i know how to use a 636 and a Ranger no this cb isnt a base it one im just fixing the other mics go with my other cb's..... Im going to just start over in the Mic amp circuirt and go from there thx all
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