Cobra 148gtl No Modulation and...

[Gunner357]

Im tinkering with this radio and with the external Dosy swr/watt meter is show dead key 4 watts key up no swing no modulation no movement same goes for the radios meter, its a new cobra power mic and yes i tried another mic too Can someone lead me in the right direction please?

 

[sp5it]

Does it work on ssb?

 

[Gunner357]

no it does not thanks for the reply Im going to find a speaker see if it works on pa on Monday

 

[nomadradio]

First thing to try when this happens is another mike, preferably one that you know works on another radio. Mikes break more often than radios. Trying this mike on another (working) radio would accomplish the same thing.

73

 

[Handy Andy]

One of the first things I'd look at is the MIC - being wired properly - 5 pin or 4 pin?

Next would be the Dynamike control - key up mic and rotate the control - monitoring this on another radio - any scratch or "crunch" sound? It would be better to hear some static wiper arm schrunch sound from movement than blank dead nothing carrier.

If it's blank dead nothing, you have an open circuit for even the Dynamike for older radios it should make some type of static noise - for if none, the absence of noise indicates a dead amp circuit internally.

CB PA switch can do this - so work the switch - put it in PA mode use an external speaker - see if audio works, then half the job is done. At least you have PA.

So we'll keep it simple - try the "Scratchy sounds effort" first...

 

[nomadradio]

Whups, he said he tried another mike. Somehow missed that first read.

One of the problems with a model that was made and sold for 40 years is to narrow down WHICH 148GTL this is. Big differences between the 1979 radio and the 2019 version. Big differences.

73

 

[Gunner357]

Whups, he said he tried another mike. Somehow missed that first read.

One of the problems with a model that was made and sold for 40 years is to narrow down WHICH 148GTL this is. Big differences between the 1979 radio and the 2019 version. Big differences.

73

I have a std mic and a new cobra power mike the reason I got the power mic was people were saying I sounded far away from mic also its 1996 vintage sorry.... I have a bunch of stuff coming in Tuesday to test this thing at least will be able to eliminate some stuff and it is a 5 pin mic I took both ends apart to see if any wires were broken all looked good

 

[Gunner357]

ok just to recap.....key mic up meter shows dead key of 4 watts talk into mic nothing

i got a speaker, turned on pa dx audio comes threw when i key it up ya here it crackle then static talk nothing.....

im going to get another mic just to be safe this mic has 5 pins i don't have another one available right now

oh btw the wattage shows on my swr ext meter it does not show on the radios meter



Update:
I went ahead and tested the finals and the c1969 does not come up with the right values so I went ahead and got me 2 of each coming they have the original look so hopefully not fakes but gotta wait till the 21st so.... ant thoughts guys?

Also these schematics was looking for a D44 diode for 3 hours last night,,, one schematic shows it right next to D43 that is next to IC4 well theres no D44 next to D43 so I got another PC board top view and bottom view schematic It shows NO D44 any where on that board yet everyone is saying "Its your D44 Diode" What the ***** ......anyway I hope the finals are it in a way.....

 

[Gunner357]

possible bad solder joint?

 

[Handy Andy]

If careful, you can always reheat the soldering joints and see if this recovers.

148's are an older chassis and although work well, they are prone to age-related problems like drying out of capacitors that supply RF and audio power to another stage, but block DC - when they dry out due to age and lack of use - they make the radio operate erratically.

So any help from you to check the front panel/side panel (148's had both - earlier were Side-Mount - the latter years were FRONT panel mounts) for broken wiring or loose or poor soldering - which shows above....

Can help go a long way in solving this strange problem...

 

[Gunner357]

got it working..... After posting that picture above I re-soldered that joint... and others that looked abnormal in the audio area.... I also re-soldered the finals .....I didn't like the looks of those either.... plugged it in and with the exception of the S meter barely moving on TX AM it works now, I had a friend talk in the Cobra and I went out to truck and it pegged my S meter so that's it Ill take it to the CB shop so they can Align and verify....... there's a 25 y/o radio back in action .....


UPDATE:

I wasn't real happy with the modulation with a regular mic so i adjusted VR5 and nothing so i did something I really didn't want to do and clipped R131 and no major change, so I guess unless when I go get it aligned he cant boost it , ill just keep the power mic on it...


Thanks for the replies all and Handy Andy Yeah its an oldie 1996 vintage

 

[NZ8N]

When the TX audio is too low for the AMC or ALC circuits to react to, adjusting or otherwise "cutting" a part out of the AMC/ALC circuit will have no effect. AMC and ALC only react to higher level audio.

 

[Gunner357]

When the TX audio is too low for the AMC or ALC circuits to react to, adjusting or otherwise "cutting" a part out of the AMC/ALC circuit will have no effect. AMC and ALC only react to higher level audio.

so which means my only answer would be this power mic.... when using a reg mic modulation unless im whistling into mic is only at 25% whistling into it barely gets to 100%

 

[NZ8N]

D44 is (should be) a glass 18.2v zener diode in front of T1. This "pops" the fuse (and dies) if it is over volted beyond 18.2v.

 

[Gunner357]

View attachment 38842

D44 is (should be) a glass 18.2v zener diode in front of T1. This "pops" the fuse (and dies) if it is over volted beyond 18.2v.

OMFG Thank You Now if it is bad how to check it and will this effect my modulation issue? as stated above I re soldered a few bad points now radio will TX but people say the power is there but voice sound like im 10 feet away from mic