29LTD has no RX. Please Help!

[simmonds1970]

I recently found my dads old cobra 29ltd and wanted to use it in my truck. I purchased a new power cord and tried it and the recieve seems to be dead. There is a small hiss that is hard to hear from the speaker when the mic is plugged in. When the mic is keyed the Green led goes to red and the needle swings and spikes as I speak. When the mic is un keyed there is no sound. I also tried with an extension speaker and still just got really soft hissing. Also there is no movement of the needle on RX on any channels. If I plug the extension speaker into the PA socket and key the mic the audio is LOUD and clear. Yes I have tried the radio with the CB and PA switch in both directions and have the squelch turned all the way counter clockwise This is an older 29ltd that does not say classic and has the side mount mic. Please help me figure my dads old radio out. Chris

[Rick]

Check that volts are present at both sides of R12, 15, 18, 26, 32, 42, 43 and 127 on the hot end of the resistors in should be about 8 to 8.5 vdc also check FET2 and TR8. Let us know how you get on.

Rick.

[simmonds1970]

Rick, thanks. I will check those values and let you know.

[Rick]

Also check that the VCO is lock in the RX mode. To check this the voltage at TP1 should be the same in TX and RX ? 5%. My diagram on CBTricks maybe easy to read the URL is http://www.cbtricks.com/radios/cobra/29 ... ic_sch.pdf

Rick (Euro Radio Co).

[simmonds1970]

Forgive my ignorance but I cannot get a steady voltage from any point in the radio. For example, when I touch the negative lead of my meter to the frame (ground) and the poistive to a point such as R12 or TP1 the voltage on my meter starts out very low and goes down quickly until it reaches zero volts...like a discharging cap. What am I doing wrong???

[Rick]

Do not use the frame as ground as it is not negative, only use the negative lead which comes from you power supply (Black lead) or one the metal tuning cans in the radio like L2, L3 ect.

Rick.

[simmonds1970]

Rick, thanks for setting me straight. I am used to working on AC powered antique radios. I will re-check those values (the right way this time).

[simmonds1970]

The voltages tested are as follows:

R12 - 8.51 , 1.66
R15 - 8.51 , 8.33
R18 - 8.51 , 8.44
R26 - 8.51 , 0.55
R32 - 8.51 , 1.58
R42 - 8.51 , 0.69
R43 - 8.51 , 1.47
R127 - 8.51 , 0.08

FET2 = 12v , 1.3v , 0v
TR8 = 1.45v , 7v , .74v

I got a reading of zero volts on TP1 in both RX and TX. Here is probably my problem

[Rick]

Ok then check some pins of the PLL IC1 (uPC2816) check pins 7, 11, 12, 13, 15, 16, 17, 18 ,19 and 22 also check pin 9 in RX/TX mode. It sound like the PLL as failed. One point if there is on voltage at TP1 in TX this means it is not transmitting do you have second radio so you can check it is transmitting. Let us know how you get on.

Rick.

[simmonds1970]

Rick, thanks again. I will check those values in the morning and go from there. I dont have another CB to test the TX but I think my programable scanner covers the CB band so I will check TX too. I will let you know. Chris.

[wolfbro_74]

I am having the exact same problem, hopefully these tips will help.