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Cobra 148 Night Watch Sound Tracker no am-low ssb

dss56

Active Member
Sep 27, 2010
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Hi all
I have a cobra 148 NW-ST and it has no Am out and the SSB power is about 8 watts.
Radio has the 2sc 1969 and the 2sc2166 for finals.

What would be the first thing to check I did try another 1969 and the same low ssb and no AM.
There is audio modulation on the ssb and good receive audio.

thanks
 

dss56

Active Member
Sep 27, 2010
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So I checked for the 13v at the middle leg of the final and driver and both have 13 volts until you key the mic then nothing o volts. There is SSB power but no AM.
I also set it up on my bench next to another radio and receiving the ssb audio but the AM is a cold nothing not even any audio. So would it be a final.
I checked the regulator and seems to be working and also the 7809 regulator is working also. Puzzled on this one
 

dss56

Active Member
Sep 27, 2010
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Further detective work points to the AM regulator so I ordered a 2sb754 regulator its a much better replacement and hope it does the trick.
Will report back on my success or failure. LOL
 

guitar_199

Sr. Member
Mar 8, 2011
715
959
103
Deer Park, TX
Hi all
I have a cobra 148 NW-ST and it has no Am out and the SSB power is about 8 watts.
Radio has the 2sc 1969 and the 2sc2166 for finals.

What would be the first thing to check I did try another 1969 and the same low ssb and no AM.
There is audio modulation on the ssb and good receive audio.

thanks
AM - no
SSB - yes ( I assume both upper and lower)

I would check the final and driver collectors and see if they have DC volts when you key up in AM.


Reason: generally in SSB the mode switch pipes full voltage to the driver/final because the SSB modulation comes from the balanced modulator then mixes up to 27 mhz and rides up through the transmit strip.

In AM the mode switch generally routes the AM regulator down to the driver/final but they will be lower than SSB voltages because the AM regulator feeds modulation down to the two collectors and it will be lower in voltage.

So like I say.... put it in AM .(be sure to use a dummy load or..... shudder.... an antenna).....use your DMM to measure the collector DC volts on the driver or final..... key up XMT. Even with no modulation you should get something.

This would be something I would do.
 

dss56

Active Member
Sep 27, 2010
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I did check and there is 13v in middle of the final and both sides nothing.
When I key up the 13v in the middle goes away.
 

dss56

Active Member
Sep 27, 2010
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So I replaced the AM regulator and still no AM output power ssb is fine.
Any further help appreciated
 

Handy Andy

Do Your Research First, Then Decide...
Apr 23, 2018
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Did you happen to check and see if the AM mode RX/TX light shows Green?

Did you also happen to verify that the AM Power pot is adjusting carrier (its the one right by the large power choke) - if it opened - this would prevent the AM side from working.

You also need to check the AM mode switch - it may have opened due to a power mod jumper that opened the switch contact on that - but SSB would not be affected because it bypasses the AM Mode and it's AM Regulator anyways.

Usually SSB means AM is ok, but when it takes out the 1419 - you'll need an NPN in there. PNP might not work.

1668884970384.png


The above view is as if you were looking on the Foil down thru the foil (X ray) so the power choke. When the component side is up - is the opposite corner..

1668885411653.png
 
Last edited:

dss56

Active Member
Sep 27, 2010
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Handy Andy thanks for the reply

*the radio from what I can see has never been touched all pots are sealed with red glue or something.
*Yes the light is green and will turn red on transmit.
*Radio has no mods

I will check out the vari resistor and see if open.

thanks
 

Handy Andy

Do Your Research First, Then Decide...
Apr 23, 2018
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IT would be the one of many in the "AM Power - No Carrier" problem - for if it was not supplying any power - no carrier - because it would not be able to "know" how to modulate - for no power to know where to set its mid-point - no carrier

Also would be prudent to double check the post jumpers that feed the audio into Driver and Final - those posts - if their wires opened up or not soldered correctly or broken off from age - makes an open circuit - but you say you have SSB so it means the Jumpers work - the only other scenario is to look for those caps (those round can-style Blue or Grey caps) the electrolytics may have dried out or failed in one way or another - the one cap I'm referring to is C173 by the AM Regulator you've been working on.
 

dss56

Active Member
Sep 27, 2010
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Handy Andy I replaced the C173 and still no AM carrier.
I will be doing some voltage checks and report back here of what I find.

thanks so far on helping out
 

Handy Andy

Do Your Research First, Then Decide...
Apr 23, 2018
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Are you able to verify the AM Regulator is getting power in AM mode?

Reason is, if the switch has got a bad pole, the AM Regulator just sits there waiting for power.

Another aspect, a 1419 is an NPN - not a PNP so if you're into replacement - double check the 1419's legs, if it checks good, HFE shows it's operable - why not put it back in and see what happens?

You'll need to follow the traces/wires from the board to the Mode switch, then follow them back to make sure the AM mode is correctly wired - you can have SSB, but if the AM mode is not working - you need to verify the power is getting to it, then if no, the problem is the switch - not the AM regulator itself.

A dropping resistor of 220ohms is used here, if the resistor measure voltage on one side, but not on the other in AM mode, follow those traces to the rear - driver and final - you have something sapping power from the AM regulator in AM mode

Else the 2SB754 can do the job, also a bit oversized for the required - it's not exactly a perfect fit - the radio wants a TO-220-tab style - so if the shoe fits...
 

dss56

Active Member
Sep 27, 2010
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I think its tr41 in this radio and its a 2sc4369 pnp and a replacement is a 2SB754.

i will re-install and see what happens even though it seems to check out ok after I removed it to replace with the 2sb754

I did swap the legs in installing this new regulator but will install the old and start doing some voltage check also

this 148 NW ST is a bit different with some parts and again radio was never touch until I opened to start checking parts.
I will check out the am/ssb mode switch and see what happens there.

thanks again for helping.
 

Handy Andy

Do Your Research First, Then Decide...
Apr 23, 2018
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The NW models are one of the last refinements of the older 148 chassis before they switched over to their RCI-based 6900 and 3600 boards and did away with the 148 due to the parts and support components were mostly depleted - there is a back story with this but will have to wait for a later time.

Ok, you have the RCI board not the older Cobra board - else the PNP would have fought you tooth and nail.

So, when you install the original part, and still no luck - then suspect by default, that you may need to investigate a keying transistor in the AM mode and the Mode switch used to power that part. (The Green to Red in AM Mode)

IF fact, you can even remove that AM Regulator transistor completely and are able to see no change at all - then suspect the Driver to the AM regulator as having issues - you can verify that by see if a voltage appears on where the Base of the AM regulator should be.

It should also show adjustability too - using VR10
 

dss56

Active Member
Sep 27, 2010
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Handy Andy.

Just want to thank you on pointing me in a positive direction. I replaced TR42 with a 2sc945 replacement for the KTC3198 and now it transmits on AM. I put the original regulator TR41 KTC4369 back in and all set for transmit AM/SSB I will tune and align radio in next few days.
Again thanks and Happy Thanksgiving Holiday.
Bob Here in Boston Ma.
 
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Handy Andy

Do Your Research First, Then Decide...
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Another radio saved from a recycle pile...

Wish you the best - Miles of Smiles!
 

Attachments

  • KEC NPN Bipolar for AM Sub of 1419 Cobra 148.pdf
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  • KEC KTC 3198 NPN sub 945 Cobra 148.pdf
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