Cobra 25 LTD Classic (ver3 with TDA2003 chip) no RX/TX

[443]

PA works, sounds strong audio.

In CB Mode, No RX and No TX Audio or RF Output. Bought it missing the PLL chip. I assume it had the branded Cobra 25LTD PLL in it.

I do not have a spare, so I dropped in two different TC9106P (or BP) PLL's. I get 4.6VDC or so on pins 13 and 14. I have another exact copy 25 LTD Classic with TDA2003 chip that WORKS, but has the Cobra PLL in it. I DO NOT get the 4.6VDC on pins 13 and 14 when checking that radio.

I have not tried pulling the Cobra branded PLL from the working radio to try in the non working one that was missing the PLL.

Any idea what could be going with this thing? Is TC9106 not compatible? Something wrong with the channel selector circuit board maybe? I haven't pulled that yet to check it over. Seeing as how Pins 13 and 14 are labeled P3 and P4 in datasheet, it appears I might have something wrong in the channel selector. With the PLL removed from board, powered up radio, I do not have the 4.6VDC on those pins, only when PLL is installed, which is probably the way it should be.

Thanks for help in advance. This radio was a basket case, it was a rainy day so I thought I'd take a stab at it.

 

[443]

Swapped the Cobra Branded PLL over. Did NOT get the 4.6VDC on pins 13 and 14, still radio no worky. No TX at all, light changes on front panel from RX to TX when keyed, PA works, but there is nothing being received and not a mW of RF going out on TX.

 

[Handy Andy]

Well, check this thread...

https://www.worldwidedx.com/threads/1995-cobra-29-ltd-wx-classic-no-modulation.254565/#post-718898

That one is for a '29, but that doesn't mean YOU can't suffer that same fate of missing parts...

So why not a pic (or two) of the board and let us help you in seeing what's under the hood...

 

[443]

You are probably right Andy. Here's some high-res pics different angles, the light in here is a little limited.

I see already it may be missing D2, which I would assume it needs that. I am going to look over the board and the schematic and see if many of the apparently missing parts are actually in the schematic, lots of extra holes in this board, especially near the front panel. The PLL is of course missing, it WAS in there an hour ago, I took it back out and put it back in the working 25LTD and already put that radio back in my truck, may have to go take it back out. Do you think the TC9106 PLL will work in this China 25 LTD? I have some of those, just none of the Cobra branded PLLs that belong in this radio.

 

[Robb]

I think one of the biggest problem you will have here, is getting a non-existent schematic for it.
Just went thru that yesterday.

Handy Andy helped me out of that jam (radio works just fine now, Andy!).
You might be better served to take close up pics. Say, in four separate quadrants? In that way, Andy can get a much closer look.
You just gotta give Andy a fighting chance, and he will get 'er done.

 

[Handy Andy]

As far as the 9106 - yes, but the ENCODING of the Truth table it needs, that is a different story.

I'm referring to the BCD Rotary Channel selector.

It may work, just someone took out the Chip in this one to fit another -

Got an old PC-66? SM5124?

The 25 "chip" - there are two, the NEC (SM5124) version and the Toshiba one (9106)

The parts missing up front are the Weather ones - each "revision" always seems to have either the ST chassis or the WX chassis. This one's Weather.

Don't sweat those details - I think it's possible!




Here's' The SM5124...


Now, not sure if the SM and 9106 are Pin to Pin - what I wanted to point out, is the IF frequency - the DIVIDE ratio is a little different. But it is another chip you can try in there.

By what I see elsewhere, your Varactors' are intact (Hurray) You Just don't have the PLL - The "wire" issue may be kinda moot, the 25 does not come with a TON of features that need a Ground of a particular type or a Signal from There to go together with this over here, onto the Nosie Blankers' "Er, Whut?" scenarios.

Remember too, the caps are old, so if it don't fire up - don't give up - remember Scotty needed Batteries to get Kirk out of a Jam - so when in the need - Use Peanut Butter and Chewing Gum as much as you can...

 

[443]

I need to find a Service Manual for this radio with detailed alignment instructions and test point info. I did test/set the VCO on CH 40 to 3.5VDC, and I checked the crystal, it is oscillating around 10.2392 Mhz. I didn't get anything on TP3, (JP14) on the scope while in TX mode, only some fuzz in RX mode. That's as far I got, about to go to bed now. Oh, and checked for missing parts against a working radio, no missing parts. The ones that are missing are supposed to be missing. Thanks for the help.

 

[Handy Andy]

If that radio has a Channel 9 switch, turn it on and see if ITS PIN programming can make the radio Fire.

The Switch overrides the main PCB thru a toggle line and Diode Matrix. It turns off that BCD encoder.

If possible - until we know more, you may need to use that Channel 9 switch to help put it back in lock.

Remember the loop, it uses those Two caps. C101 and C102 on Pin 6 and 7 and Pin 5 "Excites" it and Pins 1 to 3 use another set too C103 is a RIPPLE filter cap so it's kinda critical in value 0.22uF while C104 is a BIG Honker of a cap used for the Power supply input - you should see about 8.2 to 9.1 Volts on Pin 1. If the 470 is weak/Dried out - that may hold down the voltage and it's ability to stay locked...

Pin 4 on the PLL is the "power good" - so if any power glitches or filter caps in the RX side or TX side get goofy, they hold this line low.

Pin 8 is the RX/TX toggle / Shift line that looks at TX Power to shift that 455kHz.

Are you getting enough voltage on Pin 1 for that thing? Watch that line when you Go from RX to TX and back again, a big glitch signals that the 470 cap as well as, some others further towards the main Power feed branch, by TR17 may be weak or drying out affecting the main board regulation.

 

[443]

Thanks Andy, will try it more with the Ch 9 switch on, but so far no luck. As for voltage on pin 1 of the PLL, checking it with the scope for sudden changes, and a DOM for accuracy, it only changes from 8.09VDC to 8.08VDC when I hit the TX mode.

Will look at all those caps around the PLL closer.

 

[Handy Andy]

@443 - IF you don't mind, I need to segue a moment to show @Robb something...

@Robb - you wanted to know your W85 jumper locations - here ya' go, not ON TOP of the board...

UNDERNEATH on the Foil side -

​

Hey - we learned something - this is a ground loop issue - look at the Foil trace it mounts to, right by the "screw hole - board mount - that large land is for the WX and it's unpopulated - a perfect environment for noise loop problems - so the W85 wire goes BELOW on the Foil side.

 

[443]

I gave up for now on that 25LTD Classic China model.

INSTEAD, I found this one in the stack and got it all fixed up. It powered on but nothing worked, not even the display, after replacing the regulator transistors, zener diodes, and some caps and tuning on it, it came back to life. Actually not a bad little radio. 5 watts carrier 20 watts modulated. RX seems good, and was really suprised at how well the ANL works on this model.

Cobra 21 GTL

This one I tried the two different PLL chips I had both 9106. One is a Toshiba, the other is TI maybe? Not sure. Anyway it only worked with the Toshiba one.