142GTL Final

[Robb]

Cobra 142 GTL Schematic:
http://www.cbtricks.com/radios/cobra/142gtl/graphics/cobra_142gtl_main_sch_b_size.pdf

NTE-601 for replacing the MV1Y (about $.60/ea). You will have to mount it as the one you are replacing:
http://www.datasheetcatalog.org/datasheet/nte/NTE601.pdf
http://www.worldwidedx.com/cb-radio-modifications/144787-mv1y-diode-can-substituted.html

If the MV-13YH is bad; that doesn't have a direct substitute replacement AFAIK. Maybe the NTE-605A:
http://www.wearecb.com/store/cobra-1530119001-diode-for-cobra-148-gtl-cb-radio-mv-13yh-p-1056.html
http://www.nteinc.com/specs/600to699/pdf/nte605a.pdf

 

[azscr4mbl3r]

Hey Thanks , I had read those posts of yours here and on other forums. Still that MV12YH seems pretty mysterious doesn't it? I have my bench DVM and scope setup now and my generator I just got on the 24th.. So I did poke around on it yesterday briefly. I am leaning toward there being no carrier. I am going to try to look at waveforms I have on the 148 and try to find a similar point on the 142 to compare with because I see modulation back at the predriver are but it doesn't seem to be riding on anything... Thanks again for taking time to help!! These mods done and not done and the age of the unit makes this a fun challenge.

-Allen

 

[azscr4mbl3r]

WooHoo I got another one ( Ebay Treasure ) 44 bucks works good and unmolested. I have a "go by" now LOL. other than a hum is rx and tx fine 3-4 wats out in AM. Listening to LA right now.

-Allen

 

[azscr4mbl3r]

Back on the old 142gtl I removed the crystal switch mod. I put the L37 and 38 back to their proper places. I touched up a few shady solder joints. I checked the regulator it seemed odd to me as it checked 489 and 508 on the diode setting... two legs lifted I have seen them a few off like 509 507.. I'll keep it in mind..40Ma and 45Ma on the biasing TP's but ZERO watts DK and OK modulation... Still trying to find any and all mods that have been done to her and put them back to stock.

-Allen

 

[azscr4mbl3r]

At the base of TR38 I get a nice waveform but it seems to be folded over and is all negative going and about .2 volts I pulled it and checked it and it tested good. IC6 SO42P is the source I will check those in and out and see if it is bad.

-Allen

 

[Grinder74]

I just bought a midland 13-895(Cobra 139) and have the same problem. Some modulation but no power. I'm hoping this thread will help me also. I have no bench equipment so I can't help.

 

[azscr4mbl3r]

Grinder you can do quite a lot with minimal equipment. I multimeter is under 20 bucks which you can use for car and home repairs as well as electronics. An inexpensive soldering pencil and some solder also very inexpensive. Your eyeballs... That is most often all you need. Of course some decent hand tools screwdrivers mostly. Probably the most common thing is a bad final. Probably just a 2.00-5.00 dollar part

good luck,

-Allen

 

[azscr4mbl3r]

Well I have taken a break and I am working on a 148GTL which isn't as modified. I did pull a few possible bad parts to check or replace. TR 40 2sc496. was good. TR 38 2sc1973 was good. I replaced a few caps that might have caused issues with higher voltage ones. I had one oddball that was a 1000 uF but the schematic and parts list said was 470uF C118 over by the audio chip. MV13YH D47 I ordered because it seems to be questionable in both of these radios. We'll see! I rewired my Barjan noise cancelling mic which never worked right and now it does. It never occured to me that the mic turns the speaker on and off LOL I read that they don't receive if the mic isn't plugged in now I know why it turns the speaker on and off.
-Allen

Happy New Year.

 

[azscr4mbl3r]

I checked the collector voltage on the driver and final and I have only +3VDC in AM transmit. 13.xx in SSB. Should be 5.4 VDC I think. Any ideas ?

TIA

-Allen

 

[David Waits]

On the NPC mod. Do you connect to the same points in a 142GTL?

 

[David Waits]

First of all, remove that 1969 and put a driver in there. you can't get enough bias voltage to make a 1969 work as a driver for another. Like Mark said, you need a 2SC2166, they're about four dollars at RFparts.