The Electronics Universe

Oh yeah. I found what this is. It was called the card kit.  Never used any of those myself.

Hello   

Your welcome,I have a ques.does the tx run thru the center pin of coax or the shield?
I kno its a stupid one but on the diagram it looks like the rx is coming thru the center conductor

Good question,
The receive and transmit go through the center conductor. The receive is picked off through block labeled L.P.F

Here is another attachement that shows the receive, trans mit paths, along with the frequency and audio paths.

I will complete the other half of the schematic shortly

Now, what to do now eh?
You are probably thinking "thats not much to go on, you left out half the radio".

Well, yes and no.
This is the basic workings of a receiver and transmitter. If you will take the block diagram, the schematic, and the above info, you will see a whole bigger picture. But we are only starting with the basic radio.

It\'s the difference in knowing how to fix a radio or being a parts changer by the "shotgun approach". What is the shotgun approach? It is going in and changing parts without forst determining what is and what is not working. A lot of times you get lucky and replace the bad part. Most of the time you end up throwing the rig to the side.

The big three.
Radio may light up.
But has no transmit and no receive. That is what we call a dead radio.
Radio transmits but has no receive, Dead receiver.
Radio receives but has no transmit. Dead transmitter.

By using the above info you should be able to determine the problem in a matter of minutes.  By process of ellimination.
Find the stages that are working. Then elliminate each one.

Again, this is where your test radio comes in handy.
Dead radio. Do you hear anything in the test radio when transmiting? Or hear anything in the receiver while transmitting on the test rig?
The most possible cause of a Dead radio is the PLL and voltage rail.

The dead receiver or dead transmiter can also be caused by a missing voltage but maily one of the mixers  or OSC not working.

Weak receive can possibly be TR & bad or the protestion diodes just ahead of TR 7 bad. This happens when the front end is over loaded by strong signals such as some one keying a big amp up. Even static electricity on the antenna can take these devices out.

No output but can hear a 27 mhz signal in a test radio can tell you the final, driver , or prediver is bad. A completly dead transmitter can be as simple as the TX mixer has gone bad.

Other circuits we will discuss is :
RF Gain
Squelch
NB/ANL
AGC
Audio Circuit
Phase Lock Loop

The Receiver

This looks so complicated.  And over whelm the new person trying to read the schematic. So lets block this together to make it a bit easier.

 Let’s get to know what is happening in the AM receiver of this Cobra 29.
Go here and look at the block diagram:
http://www.cbtricks.com/radios/cobra/29ltd_29ltd_st_29wx_st/graphics/29ltd-st_29wx-st_sm_pg1_pg11.pdf

RF signals are picked up by your antenna, converted into energy and sent to the so-239 on the back of you radio. We will call these 26.965 to 27.405 mhz signal.  This signal is brought to TR 7. This is your RF amp. This amplifies the signal to make it more usable. The signal is then fed into FET 1. This is the first mixer. Lets say you radio is tune to channel 1. The frequency is 26.965 Mhz.. At the same time the PLL (IC 1) with the respect of the channel selected, IC 2 the VCO mixer sends the signal of 16.270 Mhz to FET 1 also. The signal produced at IC 2 will be from 16.270 to 16.710 (1 – 40).

The signal of 16.270 is then subtracted from the 26.965 Mhz signal yet producing our 10.695 Mhz signal.
 26.965
-16.270
=10.695
From there the signal passes through the 10.965 Mhz filter (FL1) and delivered to FET 2. This is our second mixer. At this point we are getting a 10.240 Mhz signal from TR20 (10.240 OSC) This is mixed with the 10.695 Mhz signal
 10.695
-10.240
=  .455 Khz
The signal is then passed through the 455 Khz filter (FL2) and then sent to the second IF amplifiers, TR8, TR 9, TR 10.  After the amplification the signal is passed through D4. This is our detector. Its purpose is to demodulate the signal for speech signal recovery.  The recovered speech signal is sent through the volume control, to IC 4 (audio chip) and finally to the speaker.  Where we plainly hear “Breaker 19, anyone got yer ears on comone?”

Ok, Back to the radio...

No Receive.
Each question will be followed by a NO and YES answer.

Is there hash in the speaker?

NO Run you finger over the audio chip or do the metal probe on pin 4. If still no noise check pin 10 for 13.10 volts. You can check rest of the pins and make sure the voltages are as followed:
pin 1- 13.7 pin 2- 6.63 pin 5- 1.14  pin 9-6.86  pin 10-13.10 Rest of the pins should be at zero. For some reason you vave the same voltage on many pins look for a bad ground. If you have 13 volts (supply voltage) on 3 or more pins most likly the chip is bad.  If there is no sound at all and all the above is good then make sure the mic is wired correctly.  If ok then check ffrom the output of the chip towards the speaker.  work your way towards the audio transformer and check C46. This is a coupling cap. Now work you way through the PA switch and the external speaker jack and then to the speaker. Make sure you check all caps along the way. These cap isolate DC from the audio line.
YES

Is the Receive indicator light on?
NO The receive LED is fed from R113. Check the 8 volt receive supply regulator TR23. Check C119. It is  underated and fails often. Replace it with a higher voltage cap. Check the zener diode D22. It has a high failuer rate also. If D22 or C119 fails this will remove bias from TR23. That is all the voltage rail for the Receive supply.
YES

Does pin 11 of IC 1 have 5 volts?
NO Check D19. There should be 5 volts on it. If R124 is warm then most likely C112 has shorted.  If the voltage is low check C97.
YES

Is the 10.240 oscilator working?
NO There are a lot of components in this circuit. We know the voltages are ok because of the checks that got us this far. Either the crystal is bad or there is a bad solder joint. D6 is a varactor diode. This is a diode capacitor combination. If the internal capacitance changes the circuit will not oscilate.
YES



More later

Tonight I am looking at showing you all how to convert a cobra 29 into a stable signal generator.
I will put what I can together and try to get this layed out. The generator will need to be 50 ohms, and produce a clean 100uv signal. This will be unmodulated. You will also have the option to drop the 100uv at 10 increment levels. this will be done with a simple divide by 10 network of resistors.

What we need.
A clean stable 100 microvolt signal. ( 100 uv is enough to set a S-meter to S9)
A way to drop the signal for testing the sensitivity and alignment of the receiver. (peak the stages)

The idea of the signal generator is that when you have a radio on nearby, and you turn the generator on with out being connected to the radio you want to test, you should not hear anything coming from the generator. This is going to take a bit of work and a lot of shielding to stop this from happening. The one I built years ago from the RCA co-pilot was mounted inside a steel chassis. The tuned circuits you build will also have to have there on fabricated boxes. These can be made of very thin tin sheets. As long as you can bed it and solder to it. No aluminum. You can also fabricate the boxes from double sided circuit board. This is easy to work with and has good sheilding.

So how do we start?
Looking at the cobra 29 schematic find the TX mixer. This is IC 3. Pin 9 of the mixer chip is your rf out. This goes to L20, the first tuned circuit in the output stage. From L20 we go to R63 then to C65. This is as far as we need in the circuit. Everything from the antenna side of C65 towards the antenna is useless. This is the section we will pick our signal off at and convert it to a usable signal.

At this point in the radio we are going to have around a 1000 ohms impeadance @ about 1 milivolt. As you can see we have to go a long way to get this to 50 ohms @ 100 microvolts (uv). The idea is to get this signal through a cable and to the radio under test and not through residual radiation from a poorly built leaky generator. So what we will do is take our 1 milivolt 1000 ohm signal and force it to 50 ohms @ 500 uv via a 1/2 divider network. To do this we need 3 47 ohm 1/4 watt resistors, 2 100 ohm 1/4 watt resitors.

If anyone see\'s error in what I am saying please jump in and let me know..
More later