Trouble shooting help

[2FB327]

Many thanks to Radio Tech and others with their help. I'm looking to learn more on the subject of troubleshooting the audio chain. Their post have help me bring a couple of cobras back to life that had no receive.
Now I'm stuck on a couple (cobra 25's) with no carrier no modulation. The radio receives well but no carrier on the scope or wattmeter. However when you key the mic. the bench radio gets quiet and the tx/rx light changes.
I have 10.240 on the crystal and frequency on the so-239 and it changes when I change the channel selector.
When I went to cb tricks I noticed there are many different type boards and audio chips.
My questions are there any articles/videos that will show me a systematic approach to this repair?
Could someone show or explain to me their approach to this problem?

 

[Robb]

Check the driver; then the final.
If you have receive and audio; then it has to be in the final/driver stage.

 

[2FB327]

Thanks Robb, I got all the tools I need to make the tests including a transistor tester. Should I just use my DVM and the voltage charts and test for voltage to the final and then the driver and work my back or should I use the scope before I pull each out. Forgive my newness but I need a step by step.

 

[Robb]

I usually unsolder the driver or final leads from the board and do a simple transistor test with a DVM in diode test mode.

 

[vkrules]

Me,I prefer to use an rf probe and go down the rf chain before unsoldering parts or testing dc volts. You can usually tell in minutes what has rf and what is amplifying or not.

 

[Robb]

Me,I prefer to use an rf probe and go down the rf chain before unsoldering parts or testing dc volts. You can usually tell in minutes what has rf and what is amplifying or not.

I agree. But the reality of CB radios is that driver or final failure is a commonplace event.

 

[vkrules]

Absolutely Rob. But as far as testing an rf probe is a great way to go.and they are as cheap as chips and very easy to make.

 

[2FB327]

Can I use the scope in place of a RF probe, if so how?
I think I made one a few years back, if I use the probe do I set the DVM to ac and what am I looking for?

 

[Robb]

Unless this radio has been hacked, I'll bet a shiny new dime that the problem is either the driver or the final and nothing more. Seen this many times.

 

[2FB327]

The radio looks like new, nothing burnt everything intact. Back to the probe, When I get the probe near say a final or transistor what am I looking for?

 

[BBB]

Watch the video in this post closely:

http://www.worldwidedx.com/threads/65-shop-talk-sniffing-rf-signals-safely.203853/#post-559698

 

[Radio Tech]

Thanks BBB for the video link.


2FB327

You can use an RF probe or scope in either case.
RF Probe connected to volt meter in AC mode 1.5 volt scale. I prefer an analog meter to do this because it is much easier to see the needle move over a digital display.

On the Cobra 25 key mic and probe the base of TR9 and note the reading. Now probe the collector and note the reading. You should see an increase in the meter.
Now move up the TR8 (driver) and do the same. Base then collector. note readings.
Now to TR7 and di same noting the readings. You should see an increase on each transistor if working properly.

If you are using a scope it is about the same. But since you are looking at a signal instead of voltage you will use the scope in AC coupling mode. You will have to play with volts/division and time base to get a decent waveform.

I really should do a video on this subject since I am asked this so many times.
I really feel like the driver may be your issue due to the fact you have no modulation.

 

[2FB327]

Thank you my friend, I this is what I needed,found the RF probe I had made using a marker, just have to solder on some clips.
I'll keep you posted.

 

[2FB327]

I didn't have much luck with the probe. I hooked up a good working 25 side by side with the repair radio to do my test. As soon as I touched the rf probe to the good radios collector it popped the inline fuse on the power cord, so I dropped that idea, must be something in the probe.
So I connected my dvm and the only voltage difference between the two was that on the repair radio had 5V compared to 14 on the good radio at the collector on the driver.
Took out both driver and final and tested with a peak transistor tester (tested ok).
So it will have to wait till the weekend.

 

[2FB327]

Here is where my confusion starts.
Here is the probe: http://n5ese.com/rfprobe1.htm

Half way thru the article it states:
"The first thing you'll always do in using the RF Probe is to connect the banana-plug end to the +/- jacks of your DC Voltmeter; set the Voltmeter to DC-Volts (not AC)."

In Radio tech's post it states to "AC mode to 1.5"

Is one a typo? and is there a difference between a sniffer where you don't want to touch the circuit and a probe where the tip makes contact with the component?