Cobra 18 Ultra No TX and no RX - WX works OK

[stinkijeans]

Radio Make and Model: Cobra 18 Ultra Weatherband CB
PCB Number: 411-032-d and number underneath that one: # 4a1030-d
PLL Number: LC7185

Test gear available: Freq counter w/probe ability, Digital swr/power meter, 10a bench power supply, digital VOM w/transistor check.
I also have the schematic, but it doesn't show the voltages.


Symptoms: No TX or RX. No RX audio, No meter deflection in TX or RX modes.

What is working: Weather channels work. You can hear the stations just fine. The TX light will turn red when mike keyed.
Channel selections works, channel 9 switch works fine.


What has been tried:

Final (2SC2078) getting full voltage at collector. When keyed, the collector voltage will increase slightly and then go back to bench voltage.
The driver is getting full voltage at its collector.
I tried transmitting with a good working radio, but the 18 won't pick up anything.
I tried the basic push/tug around the board, but it didn't work.
Tried (known) good microphone - no help.


Did it suddenly happen ?: Unknown. This is a friends radio. He didn't say what happened to it or when.

Anything else that may be of help?:
When you turn the radio on, you can hear the speaker 'pop' slightly, so I can assume the audio IC is fine. I'm also assuming the weather portion of the radio works with the audio IC, so I can assume it's OK.

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In a nutshell, this radio acts JUST like it would if you had your PA turned on (Unfortunately, this radio doesn't have a PA). I think that's the best (and quickest) way I can describe the failure.
My next step is to re-flow solder joints, but I wanted to run this by your web site before I did.



Thanks for your help and GREAT web site!


Homer

[The Defpom]

I would start by looking to see if the PLL and VCO are functioning correctly, I would definitely be looking for things common to both RX and TX.

Can you please post a copy of the diagram here, I do not have that one.

[stinkijeans]

Thanks for the reply...

I'll give the PLL and VCO a look.

I got the schematic off this site: http://www.freeinfostuff.com/CBs.htm

Cobra 18 schematic: http://www.freeinfostuff.com/CBs/18-Ultra.pdf

I would upload the schematic here, but I didn't want to get you (or myself) into any trouble in case it's protected, etc...
The site says 'free stuff', so I can assume you can grab the schematic for your purposes.


H.

[stinkijeans]

I sniffed around the board with my frequency counter and the only life I can get is from the 10.24 reference crystal.
I typically don't work on these rigs, so I'm at a loss.

I'm a 148, 29 and 25 fan and my old SAMS photos!
Not having pin-outs and voltages makes it tough too.


The radio is completely virgin and looks like it just came out of the box, so it's hard to imagine what happened to it.

I think you're right... it's in the VCO or PLL area. Question is... what should the pin-outs be? What voltage, frequency, etc...

Darn I miss my SAMS!

H.

[The Defpom]

OK, if the VCO and PLL are functioning, you should see 16.270 MHz on CH1 in RX, and 13.4825MHz in TX on PLL pin 23.

If the signal is missing, then I would suggest you start by checking power supplies, the radio uses Zener diodes, it is possible one has failed, or a capacitor is shorting dragging down the supply.

Check pin 24 of the PLL, you should get about 7.5V.
Check R415 at junction of R415,R402/R403/R404, there should be about 8V there.

[stinkijeans]

Pin 23 shows nothing in both RX and TX

Pin 24 shows 5.57 volts

R415 shows 7.79 on one side and 4.57 on the other.

All the zeners appear to be working as they should.

[The Defpom]

it looks like the VCO signal is missing, check for the same frequencies at Q402 (it is just a buffer/amplifier), check all three legs, one is 0V, so wont have anything, the other two should have the frequencies on it I mentioned.

If there is nothing, then check Q403 and the alignment of L401.

[stinkijeans]

WOO HOO!!!

Found the problem...

Several solder joints around Q402 were busted loose.
After melting out all the waxy-gooey mess, I noticed the joints moving about. A quick re-flow fixed it all.

Thanks again for your help Scott. I couldn't' have done it without you!

Homer

[The Defpom]

Glad you got it sorted.