Cobra 29 won't come on

[Road Runner]

My Cobra 29 may have recieved a brief short, or reverse in polarity, and now it will not come on. I've probed around and found that, when plugged in, it has 12 volts going to lots of points on the solder side of the board, and even as far as the final. I'm thinking that this is not right, but don't know for sure, and don't have a working 29 to test right now to see. Can anyone steer me in the right direction on what to look for here, or is my radio toast?

[beerbarrel]

Have you checked the diode where the power comes in this is what normally goes first get your meter on it and you should only get a reading one way of it on( continuity) if you get a reading boths ways its gone also look for tracks that may have burnt out too

[Rick]

The diode is D23.
Also check that you are getting volts at Pin 1 of the audio amp IC4 bolted to the side of the radio.

[Road Runner]

Ok, with the meter on continuity (ohms), it shows continuity either both ways. So this means the diode is bad?

[Road Runner]

Ok, I took the diode out, and it tested bad. So I replaced it with a new one. Now, when I tested the new one before I put it in, it tested good (only showing continuity one way). After installation, however, it shows to have continuity BOTH ways. I tested it before power was applied to the radio. And the radio still does not work. Any other suggestions?

[Rick]

If the diode has gone short again check that you have connect the power cable the right way round and the supply is at a max of 13.8 volts DC.

[zodiac]

If the diode has gone short circuit then how come the fuse hasn't blown.
You have checked the fuse to see if it's the correct value.
Dave.

[ramblingman]

In order to check the diode properly,you must disconnect
one lead..with it in circuit,it will always show full scale on
your v.o.m.,as if it shorted..Also make sure you have
installed the diode properly..Should be marked on pcb
which direction the cathode goes.Also sometimes when
a reverse polarity diode shorts,it will burn a foil pattern
in area of where diode was and power wires.If it has burned
a foil trace into.Just jumper the pattern out with a short
insulated jumper wire.Also make sure you have the proper
fuse in line.I have seen people install a 20 amp car fuse
in line..It should have a 2 to 3 amp fuse inline.Also i have seen
people replace the reverse polarity diode with a 7 amp diode.
The proper diode should be a 1n4002 or 1n4003 diode,which is a
2 amp diode.In combination a 20 amp car fuse and a 7 amp diode
inline means for alot of distruction..And even worst than that
some folks just cut the reverse polarity diode out all together
Put this in combination with a 20 amp car fuse..allmost total
distruction..fried circuit foil patterns everywhere.

[zeppfrog]

also make sure the new diode is installed proper... ie: the white stripe to the right if the arrow on the pcb is to the right.

[Road Runner]

Sorry it's taken so long for me to respond, but I've been on vacation. I did install the diode with the cathode (stripe) toward the arrow, like the one I replaced. The fuse in the cord is a 2 amp, and the diode I used is a 1N4004. Is that acceptable?

[ramblingman]

Yes the diode you used is o.k.,Now before we just start
randomly changing parts.Lets take some voltage readings.
You will never learn much about troubleshooting your radio
just parts changing.First before we take any voltage readings.
Lets start by checking the simple stuff. First check to make sure
your p.a. switch is in the cb mode..next check to make sure your
power cord is making good connection.I have seen this many times
you get no power symptom,and you automatically think the diode is
gone out.But in actuality its just not getting power to the plug.
SO first check with your v.o.m at the hot and ground points
inside the radio and make sure were getting 13.8 v.d.c.If so
then move to the power switch..There should be 13.8 vdc on
one side of the switch only,until you turn power on then you
should have power on both terminals.This will determine if
the switch is good or bad.If switch checks good then lets next
check for proper voltage at the p.a. switch.If your not getting
voltage thru on the cb side of the switch.Then the switch is bad
and your stuck in p.a. mode.I have seen this a few times.
Also there could be a wire off the switch.check for this.Then next
check voltage at the audio amp.you should have 13.8 vdc on
the input and output of the chip.also you should have 13.8 vdc on
the collector of the driver and final.Also check c-112,when this
cap shorts it will kill your 6 volt source.

[Road Runner]

Ramblingman........... I did think to check power inside the chassis before I posted, and I did indeed have 13.8 volts present where the cord plugs in (INSIDE the radio). So I just now checked the power switch (it's the squelch also). It has 6 visible wires coming from it, and they are ALL hot whether the switch is on or off. Have I found my problem? If so, where do I come up with a new one? Right now, I don't have another cobra (29 or otherwise) to rob any parts from.

[zeppfrog]

if the switch is the problem, they are around on ebay... or http://www.pacific.us.fm/pacific2005/store/index.html maybe

[Rick]

With you VOM/DVM check for 13.8 volts on the switch connects. The connects are right on the back of the switch, one should have 13.8 volts and when you turn the switch on the second connect should have 13.8 volts, if the volts are present on both connects then the ON/OFF switch is OK

If you only have volts on one for the ON/OFF switches connects, then it look like the switch has gone south.

You can bypass the ON/OFF switch by soldering a wire across the switch connects, the radio should now have power.

You will most likely need the replace the audio IC (IC4) TA7222AP, due to D23 failing.