It transmit normal carrier and RX is fine. But no modulation at all in AM or SSB or PA . Yes, tried different mikes and mike gain is on. Any ideas? I did a full electrolytic cap replacement several years ago.
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Cobra 2000 no modulation.
- Thread starter Brian Gosper
- Start date
First place I usually look is C18, a 330uf 10-Volt cap that filters the mike amplifier's supply voltage. But if it's a new part, this seems less likely. When it shorts, no power to the mike amp.
TR24 is the 2SC945 transistor that serves as the mike-circuit attenuator. Voltage fed into its base terminal causes it to reduce the mike gain in proportion. This one routinely gets clipped or removed to increase the wattmeter-giggle quotient of the radio.
Find the foil pad for the center lead of TR24, if it's present. Lay one finger alongside the shaft of a small screwdriver and touch the tip of the blade to that foil trace. If you get a loud hum and/or squeal while transmitting, your problem is upstream from this spot.
And if you get nothing on AM, SSB or PA, the problem is downstream from that point.
Oh, and the finger on the screwdriver shaft trick? We call that the "Manual Digital Analog Signal Injector".
73
TR24 is the 2SC945 transistor that serves as the mike-circuit attenuator. Voltage fed into its base terminal causes it to reduce the mike gain in proportion. This one routinely gets clipped or removed to increase the wattmeter-giggle quotient of the radio.
Find the foil pad for the center lead of TR24, if it's present. Lay one finger alongside the shaft of a small screwdriver and touch the tip of the blade to that foil trace. If you get a loud hum and/or squeal while transmitting, your problem is upstream from this spot.
And if you get nothing on AM, SSB or PA, the problem is downstream from that point.
Oh, and the finger on the screwdriver shaft trick? We call that the "Manual Digital Analog Signal Injector".
73
Also I noticed C18 in mine is a 680uf cap. Weird because yes it looks like originally it was a 330 like you said. But the 680 has been in there since the recap was done and no issues (until now). That ok like that? It’s a 35 volt one so I doubt it shorted.
We use a 1000 uf part for C18 any time there is a hint of feedback squeal on AM transmit. Usually only happens when the carrier is turned way down. Making that part larger than original has no down side that I have found.
73
73
So a small update. When I put the radio into PA mode and key the mike, it’s like it wants to work. If I tap on the mike while it is keyed up, I can “hear” the tapping over the speaker but yet there is no voice when intel into it. And again nothing in the way of modulation when switched to on the air.
. I’m still convinced the issues is around the mic amp area or the audio chip. When is listen on anotheraudio, I just hear a hum on the other radio speaker. Could it be the audio chip?
Well, when you hear the hum, yes, that carrier, but is the Audio amp producing the humm
If that is the case, when you TX you should - by just touching the board by the audio chip - make it hum louder, even squeal thru the other radios speaker, that's RF using your fingers to loop back into the audio path, if you can do that, then you're in really good shape.
Then it would be close to the Mic, or the Mic jack, onto the MIC (Dynamike) control pot being open. circuit
If that is the case, when you TX you should - by just touching the board by the audio chip - make it hum louder, even squeal thru the other radios speaker, that's RF using your fingers to loop back into the audio path, if you can do that, then you're in really good shape.
Then it would be close to the Mic, or the Mic jack, onto the MIC (Dynamike) control pot being open. circuit
Why would the tapping on the mike be heard across the speaker but not the actual modulation audio? The audio chip is that TA7222 chip correct? If so, yes, when I put my finger to the board around that chip, the hum definitely gets louder. I can also create squeal and loud hum by touching some of the components in the mic amp strip area.
Well, if the Mic amp has a bad resistor or cap - you'd hear a tick, from distorted audio - not a true "thump thump" from a working microphone audio amp.
The issue was to determine if the Audio amp, and its' associated amplifier used to operate a pre-amp buffer input is bad, or you have just a bad audio path to the mic amp thru the Mic Dynamike rheostat (Potentiometer)
Good to know that you get that effect, your problem looks to be in the audio path from Mic jack thru Dynamike into the Mic amp.
It's the process of elimination, the steps were doing here, that seems to sound goofy, but I'm trying to determine from the lack of details; of the radio not being in front of me - to having your input on what is working, thru this Forum as the medium.
IT's not as easy as it looks...
Time to do some simple ohmic checks - you'll need to verify the path from Mic Audio Pin 1 and Pin 2 are good.
Locate your MIC Pin 1 on the Radio side - that pin is right next to the NOTCH.
Locate Dynamike - rotate the knob thru it's full rotation (that wiper - use it rub the contacts of the substrate under it) a few times - then leave it fully Clockwise (no attenuation full on)
Pin 2 is Audio GROUND Return - should be Foil Ground ohmic reading.
Note, VR405 SUBSTRATE - all pins are connected. So you can do your first check from the comfort of the front Pin 1 and 2 on the Radio...
Set up DVM for 10K -
IF these steps pass, the INPUT from the MIC seems to be ok. not out of the woods yet though...
I'll wait for your response before we go further - if any of the above doesn't pass, this will show up in the extremely attenuated/distorted mic audio...
The 7222 chip, it may be set up for talkback - so brace yourself and wear earplugs as you work thru, remember hearing that audio thru the speaker, it may not be getting thru to the AM Regulator AF Amp, that is where it sends it to the Final and Driver what they need, so that is why I had you check the CB/PA switch to see if Audio routing is correct - since you hear squeal - the Radio in TX mode should MUTE thru it's own speaker, but you will hear it pipe thru into and be heard thru the other radio close by.
The issue was to determine if the Audio amp, and its' associated amplifier used to operate a pre-amp buffer input is bad, or you have just a bad audio path to the mic amp thru the Mic Dynamike rheostat (Potentiometer)
Good to know that you get that effect, your problem looks to be in the audio path from Mic jack thru Dynamike into the Mic amp.
It's the process of elimination, the steps were doing here, that seems to sound goofy, but I'm trying to determine from the lack of details; of the radio not being in front of me - to having your input on what is working, thru this Forum as the medium.
IT's not as easy as it looks...
Time to do some simple ohmic checks - you'll need to verify the path from Mic Audio Pin 1 and Pin 2 are good.
Locate your MIC Pin 1 on the Radio side - that pin is right next to the NOTCH.
Locate Dynamike - rotate the knob thru it's full rotation (that wiper - use it rub the contacts of the substrate under it) a few times - then leave it fully Clockwise (no attenuation full on)
Pin 2 is Audio GROUND Return - should be Foil Ground ohmic reading.
Note, VR405 SUBSTRATE - all pins are connected. So you can do your first check from the comfort of the front Pin 1 and 2 on the Radio...
Set up DVM for 10K -
- Read ohmic reading ACROSS Pin 1 to Pin 2 - is it high, above 1.4K ohms?
- Rotate Dynamike knob to full Counterclockwise - Full Attenuation
- Repeat Step 1
- Step 1 should be high ~1.5K but not infinite
- Step 3 should be near ZERO ohms.
- If the above two steps Pass...continue...
IF these steps pass, the INPUT from the MIC seems to be ok. not out of the woods yet though...
I'll wait for your response before we go further - if any of the above doesn't pass, this will show up in the extremely attenuated/distorted mic audio...
The 7222 chip, it may be set up for talkback - so brace yourself and wear earplugs as you work thru, remember hearing that audio thru the speaker, it may not be getting thru to the AM Regulator AF Amp, that is where it sends it to the Final and Driver what they need, so that is why I had you check the CB/PA switch to see if Audio routing is correct - since you hear squeal - the Radio in TX mode should MUTE thru it's own speaker, but you will hear it pipe thru into and be heard thru the other radio close by.
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Ok modulation is back but there is a very loud background hum when you key up. Issue appeared to be around the mike jack. You can get rid of it by turning down the dynakmike control but you have to basically shut it off to get it to stop. So aggravating.
Ok, slow down, don't get aggravated, it's just part of being there and the one to fix it.
Be Patient...else you'll wind up being a Patient yourself...
Ok, if it's really loud, that may indicate a shorted or failed cap - go back to the post about that mic amp circuit.
I'll put this here to help us later if we need to...
Since 2000's are rare these days, built well, but they are of an older design and no one is making them, they don't really need a lot of attention but a little care, except for there are some parts like the AMBER circled above - that are Electrolytic, or POLARIZED capacitors' that, as they age, tend to act more like DEAD SHORTS than a capacitor they are supposed to be.
It affects the Gain of the stage they are in, locate C100 a Tantalum, Usually a Orange color "dipped" cap body, that are polarized but as they age can fail shorted, they are not supposed to be "shorts" to DC, only to Audio frequencies - the Resistor right next to them in the same circuits are biasing the amp to run right, and correctly so you hear strong good audio.
IF those caps failed- open dead, the stage won't amplify very loudly - if they went - shorted, they put that stage of the amplifier in a "high gain" state - really loud distorted audio is the typical results - crunchy, squeals, even muffled boomy sounding are some some of the adjectives people use when they come across these caps as failed and cause the stage of the amp to overamplify and makes the audio sound distorted and too loud.
Be Patient...else you'll wind up being a Patient yourself...
Ok, if it's really loud, that may indicate a shorted or failed cap - go back to the post about that mic amp circuit.
I'll put this here to help us later if we need to...
It affects the Gain of the stage they are in, locate C100 a Tantalum, Usually a Orange color "dipped" cap body, that are polarized but as they age can fail shorted, they are not supposed to be "shorts" to DC, only to Audio frequencies - the Resistor right next to them in the same circuits are biasing the amp to run right, and correctly so you hear strong good audio.
IF those caps failed- open dead, the stage won't amplify very loudly - if they went - shorted, they put that stage of the amplifier in a "high gain" state - really loud distorted audio is the typical results - crunchy, squeals, even muffled boomy sounding are some some of the adjectives people use when they come across these caps as failed and cause the stage of the amp to overamplify and makes the audio sound distorted and too loud.
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Hows' the patient?
You have two threads of the same subject ... going on here...
So what is one to do?
You have two threads of the same subject ... going on here...
So what is one to do?
Problem fixed. Issue was a bad cap in the rf amp strip. Thanks for the help.
3 year old post and i have no modulation in am, ssb or out the pa speaker in pa mode as well
A question for Hand Andy, When you ohm test from pin 1 and pin 2 at the mic connector and the ohm's don't change form 1,038 ohm's no mater where the Dyna mike control is turned to. I never red what to do of the ohm's never change as you said.
I did remove the shielded cable from the dyna mike control where it's soldered to the board and ohm checked the center conductor to pin 1 at the mike connector and with the dyna mike control full CW i get Zero ohm's and with the control full CCW i get 1.038 ohm's. Ohm checking the shield show's Zero Ohm's.
I tested C108 and it is .47uF at 50VDC and i swapped it out with a new one for easy install with the long new lead's and reinstalled the shielded wires from the dyna mike to the board.
I'm currently going to pull and test the remaining cap's you recommended Brian to check.
It would be nice to know the exact cap number that Brian changed to fix his 2000 as well.
Thank's in advance.
2WR/NB267 Scott from Colorado
A question for Hand Andy, When you ohm test from pin 1 and pin 2 at the mic connector and the ohm's don't change form 1,038 ohm's no mater where the Dyna mike control is turned to. I never red what to do of the ohm's never change as you said.
I did remove the shielded cable from the dyna mike control where it's soldered to the board and ohm checked the center conductor to pin 1 at the mike connector and with the dyna mike control full CW i get Zero ohm's and with the control full CCW i get 1.038 ohm's. Ohm checking the shield show's Zero Ohm's.
I tested C108 and it is .47uF at 50VDC and i swapped it out with a new one for easy install with the long new lead's and reinstalled the shielded wires from the dyna mike to the board.
I'm currently going to pull and test the remaining cap's you recommended Brian to check.
It would be nice to know the exact cap number that Brian changed to fix his 2000 as well.
Thank's in advance.
2WR/NB267 Scott from Colorado