OK Brandon,
I'll pull D64.
In the mean time i redid my original; picture in Message 33. All reading's are in SSB RX
I'll pull D64.
In the mean time i redid my original; picture in Message 33. All reading's are in SSB RX
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Cobra 2000 no modulation.
- Thread starter Brian Gosper
- Start date
Be sure to test VR12 when you have D64 out too, because if it went high Ω, it could explain there only being .272v across that diode. The lower the forward current, the lower the Vf you will measure. If the pot is in the MΩ range, these readings would make sense.
I pulled D64, Unable to read what it say's on it,
Tested as a diode.
uf - 701MV
C 4pf
Multimeter set in Diode mode and with the + lead on anode side and - on cathode side it read's .586. Reverse lead's and it say's .of
Tested the radio with D64 removed No Transmit Modulation anywhere,
I did test VR12 by soldering a wire to the cathode side of where D64 was to ground and with VR12 full CW i got 30.43M ohms and Full CCW i got 10.86M Ohm's.
The schematic say's VR12 is 500 Oms, So mabey VR12 is fubar.
What's the chances both VR11 and VR12 are both fubar, Just thinking out loud on what may cause both AM, SSB and PA to have no transmit modulation
I was thinking the 1N194 diode interchanged with 1S1588, But i may be wrong and need to intell that more,
NTE say's the NTE519 interchanges with 1S1588.
Tested as a diode.
uf - 701MV
C 4pf
Multimeter set in Diode mode and with the + lead on anode side and - on cathode side it read's .586. Reverse lead's and it say's .of
Tested the radio with D64 removed No Transmit Modulation anywhere,
I did test VR12 by soldering a wire to the cathode side of where D64 was to ground and with VR12 full CW i got 30.43M ohms and Full CCW i got 10.86M Ohm's.
The schematic say's VR12 is 500 Oms, So mabey VR12 is fubar.
What's the chances both VR11 and VR12 are both fubar, Just thinking out loud on what may cause both AM, SSB and PA to have no transmit modulation
I was thinking the 1N194 diode interchanged with 1S1588, But i may be wrong and need to intell that more,
NTE say's the NTE519 interchanges with 1S1588.
Agreed.
If the meter black lead is for sure on a good circuit "signal" ground.... then the cathode of D64 absolutely should pull down to near 0 v. If it doesn't something is wrong.
Another handy number I would like to have is the collector voltage of TR26 when it is IN and the radio is in FAILURE mode..... meaning the mic audio is not working...... I would just like to know what voltage is being fed back to the 1.5k near TR25...... when the radio is failing.....
If the meter black lead is for sure on a good circuit "signal" ground.... then the cathode of D64 absolutely should pull down to near 0 v. If it doesn't something is wrong.
Another handy number I would like to have is the collector voltage of TR26 when it is IN and the radio is in FAILURE mode..... meaning the mic audio is not working...... I would just like to know what voltage is being fed back to the 1.5k near TR25...... when the radio is failing.....
Hey Bob.
I need to put a diode in D64 location before i can test this.
I don't have the 1S1588 Diode and it's questionable if the D64 diode was correct and i can't read it.
NTE say's the 1S1588 interchanges to the NTE519, as well 1N914 and 1N4148 interchange to NTE 519 and i stock the 1N914 and the 1N4148 I don't have the NTE519
VR12 is for sure bad. Replace that. The diode is probably ok with a Vf of .586v. If you are unsure, a 1N4148 should work fine there.
With R131 and VR12 totaling 10500Ω connected to the 6.49v source, there would be (6.49-.6)/10500Ω=.562mA through the diode. The datasheet for the 1S1588 says that Vf at .56mA is about 0.68v. The datasheet for the 1N914 puts Vf for .56mA at 0.57v. The diotec datashet puts the 4148 in the same ballpark as the 914.
At the end of the day, you have roughly .56mA through a 500Ω pot (once replaced anyhow) which gives a possible voltage swing of .281v. Being there is roughly .1v difference in the Vf of the old diode and the ones you have, it might not adjust right (or be right at the end of the pot range). In that event, changing to a 1k pot should make it work. Something tells me .281v is enough room for such a small change in diode Vf. And I think the original is ok...
At the end of the day, you have roughly .56mA through a 500Ω pot (once replaced anyhow) which gives a possible voltage swing of .281v. Being there is roughly .1v difference in the Vf of the old diode and the ones you have, it might not adjust right (or be right at the end of the pot range). In that event, changing to a 1k pot should make it work. Something tells me .281v is enough room for such a small change in diode Vf. And I think the original is ok...
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Hey Bob.
I put a 1N4148 in the D64 spot and at C of TR26 i have,,,
4.362 VDC AM RX.
4.535 VDC SSB RX
1.825 VDC AM TX
4.535 VDC SSB TX
That is when the mic audio is not working and Failing.
You didn't say what mode to check C of TR26, so i did them all.
Hey Brandon,
Look's like it's time to get on the hunt for a replacement VR12.
It still has me wondering. If VR12 is fubar (and it appears it is) Will this also affect SSB?
Mabey i need to check everything around TR34 as well and VR11.
The failed pot is forcing TR26 to stay on all the time, this keeps TR25 on all the time, and therefore TR24 shunts your mic audio in all modes. It wouldn't matter AM SSB PA, none would work.
Hey Brandon,
You are DA MAN. I pulled out VR12 and ohm check it and it's fubar,
I don't see that part number anywhere on the WWW, I have a donar part's cobra 138XLR and stole all the VR style semi fixed resistor's out and found one marked around 500, Installed it and BAM I have transmit audio every where, here's a video on it on my you tube channel
You are DA MAN. I pulled out VR12 and ohm check it and it's fubar,
I don't see that part number anywhere on the WWW, I have a donar part's cobra 138XLR and stole all the VR style semi fixed resistor's out and found one marked around 500, Installed it and BAM I have transmit audio every where, here's a video on it on my you tube channel
As I busy myself trying to get caught up in this thread...
Wanted to just stop for a moment and congratulate all of you for working so hard in getting this problem resolved.
Now to help you.
There are several ways the TX audio gets "stopped" - one you've already found - that is the D12/VR12 limiter sense.
D12 is designed to "see" the power arriving - but conduct only one way - so it clips Positive going (N to P - of the NPN of TR26) while only so much energy (per VR12) is kept behind to shot over into the Limiter TR24.
IF VR12 or the Diode is "open" TR26 base doesn't trigger.
VR12 does sound like it';s toast - but remember too, these are silver plated or chrome plated, they may engage and start working again if removed and cleaned inspected and possibly re-crimped - to see if the substrate and the wiper can make contact again .
IF you do a search here on the Forums, Inquire about Cobra 148GTL or R127 1 Meg Resistor - see if something comes up in the search engine here.
Why? R127 is oftentimes the forgotten man in this - as it - when replaced with a lower value - will oftentimes make the ALC and AMC "work more linearly" together because the VR12 adjustments you can get - is only going to work so far and everything else seems to clip/clamp a little too hard and can "swing negative".
Since you are working on a Cobra 2000 - remember also, that Pin 5 of the MIC jack, is the "ground" for the ALC and AMC - thru D60 - it grounds the otherwise Pinch off presence that quiets the limiter and mic audio out of the Audio Amp when it's in RX mode.
So, Pin 5 TX mode "grounds to" Pin 4 being Ground - Pin 3 is Speaker Ground and Grounds in RX mode to Pin 4.
So out of the Pins 3, 4 and 5 - 4 is the middle and is board ground - 5 goes to the MB3756 and also banded end of D60 - to open up the Limiter.
So check your board and your PA/CB Switch wiring - make sure Pin 5 and Pin 4 are NOT reversed in error - so trace the wires from that Mic jack back to that console board and verify the CB/PA switch is doing it's job right.
Per previous post, it sounds like you have it working again.
Wanted to just stop for a moment and congratulate all of you for working so hard in getting this problem resolved.
Now to help you.
There are several ways the TX audio gets "stopped" - one you've already found - that is the D12/VR12 limiter sense.
D12 is designed to "see" the power arriving - but conduct only one way - so it clips Positive going (N to P - of the NPN of TR26) while only so much energy (per VR12) is kept behind to shot over into the Limiter TR24.
IF VR12 or the Diode is "open" TR26 base doesn't trigger.
VR12 does sound like it';s toast - but remember too, these are silver plated or chrome plated, they may engage and start working again if removed and cleaned inspected and possibly re-crimped - to see if the substrate and the wiper can make contact again .
IF you do a search here on the Forums, Inquire about Cobra 148GTL or R127 1 Meg Resistor - see if something comes up in the search engine here.
Why? R127 is oftentimes the forgotten man in this - as it - when replaced with a lower value - will oftentimes make the ALC and AMC "work more linearly" together because the VR12 adjustments you can get - is only going to work so far and everything else seems to clip/clamp a little too hard and can "swing negative".
Since you are working on a Cobra 2000 - remember also, that Pin 5 of the MIC jack, is the "ground" for the ALC and AMC - thru D60 - it grounds the otherwise Pinch off presence that quiets the limiter and mic audio out of the Audio Amp when it's in RX mode.
So, Pin 5 TX mode "grounds to" Pin 4 being Ground - Pin 3 is Speaker Ground and Grounds in RX mode to Pin 4.
So out of the Pins 3, 4 and 5 - 4 is the middle and is board ground - 5 goes to the MB3756 and also banded end of D60 - to open up the Limiter.
So check your board and your PA/CB Switch wiring - make sure Pin 5 and Pin 4 are NOT reversed in error - so trace the wires from that Mic jack back to that console board and verify the CB/PA switch is doing it's job right.
Per previous post, it sounds like you have it working again.
SO Just to report (As many times it doesn't) I found TR12 to be at fault, I will now perform the alignment and do further over the air testing before returning the radio to the customer.
Thank's everyone for your help and i hope if someone else has this problem, going through these 5 pages, it will help them out as well
Thank's everyone for your help and i hope if someone else has this problem, going through these 5 pages, it will help them out as well
Wasn't just me, I would have never been able to jump in there and suspect VR12 had 4 thread pages of the diagnostic work not been done already. Quite a few people were involved in this one. Glad to hear you got it working, it's always exciting to bring one back to life!!
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