Cobra 29 Plus Downward Modulation unstable carrier.

[LeapFrog]

I recently got this Cobra 29 Plus model made in the Philippines during May of 1988/1989 (Hard to read.)

(Non peak reading) Meter swings backwards under modulation, & the carrier is unstable.
You can see it is 3 watts then 2 watts...

I haven't checked frequency yet, no bnc jumpers/probe.
A local told me the radio works over an on air check, ok but i'm not sold yet.
A Uniden 68XL in the next room over turned to channel 20 received my audio just fine. (No ant, rf gain down) PA Works fine also. The light show has failed.....
Any tips on where to look first? The caps appear to be good (all original radio) but that doesn't mean anything.
I have the factory service manual downloaded so I can go through and check voltages.
If anybody has experienced these symptoms and could point me in the right direction it would be much appreciated. Thank You & 73s'
-Leap

 

[Robb]

Acts like it is being starved.
Power issue perhaps.

 

[LeapFrog]

Acts like it is being starved.
Power issue perhaps.

15 amps power supply running through a 2 amp fuse @ 12 volts, maybe this is the bottle neck, also the wire for the fuse holder is a smaller gauge, I will post again after trying with the other 15 amp supply that is unfused and uses a single wire gauge.
Thank You Robb

Edit: With the unfused 12v 15 amp supply it keys a steady 3 watt carrier no stability issues anymore.
The other supply with the fuse holder section must have been starving it good call Robb, but she is still hungry laugh out loud!
It still swings backwards on the meter a little to the left, not much but I can see it's going reverse still.
Perhaps a 13.8V Supply would allow it to swing forward under modulation.
I'm going to have to figure something out for a variable bench power supply, my diy one only puts out 3 amps max and I doubt it does the whole (rated @) 3 amps!

 

[Robb]

So long as all of the current is getting to the board without anything creating resistance to it, it should be OK.

It could be bad caps too, as it is an older radio.

Another thing that is possible, is that the TX tuning cans are out of alignment.

Hard to troubleshoot radios w/o having hands on.
Your ability to troubleshoot will change as you get more experience. Just keep trying and don't stop until you nail it - IMO. There are a lot of guys on this forum with CB radio repair experience that can help . . .

 

[sonoma]

you are doing good Robb. one other thing he can check is to put a VOM on the power switch and see if the power stays above 13 volts when he keys up and talks. some of the old radios had a bad power switch in them and not carry the full voltage to the radio.some of the old washington radios had a weak switch and I have put a relay in them to give full voltage when turned on. the 858 president's had a problem with that.

 

[LeapFrog]

you are doing good Robb. one other thing he can check is to put a VOM on the power switch and see if the power stays above 13 volts when he keys up and talks. some of the old radios had a bad power switch in them and not carry the full voltage to the radio.some of the old washington radios had a weak switch and I have put a relay in them to give full voltage when turned on. the 858 president's had a problem with that.

The physical switch to power the unit on, apply volt ohm meter and see if the full voltage passes through the switch when keyed? The only supply with enough current on demand (minus a good automobile battery) that I have are older xbox 360 power supplies, the label reads 203 watt 15 amp 12 volt 5 volt, I havent been able to crack one open yet to see if any potentiometer exists to allow for adjustment of output voltage. They only output a12v line & 5v rail which shorts to ground to soft-power on the psu and bring it out of standby mode , I see now that I probably left excessive lengths of the original wire on the unit before attatching the radio power connector which had it's own several feet of wire to add to the original wires, In the morning i'm going to shorten the leads to a more reasonable length (short). This should stop any current bottlenecks to the radio at least.

I have been doing all my talking and rf power measurements on 12 volts for months, with a proper 13.8v I should see more output from some of my radios!

I cannot format this text very well on my mobile phone sorry, DOH i have over 6 feet of wire before the power even reaches the radio!! Huge error on my part, also these supplies say 16.5 A not 15 amps. When im rested in the morning I will begin my quest for D.C. power then it's R.F. resonating off my k40.

 

[LeapFrog]

Follow up, I shortened the leads to under one foot.
Next I will measure the voltage of the radio/psu while keying.
Now it throws a carrier over 3 watts steadily, still goes backwards but not as bad.
Thank You Robb & Sonoma, i'm convinced it's the power supply unit.

 

[LeapFrog]

you are doing good Robb. one other thing he can check is to put a VOM on the power switch and see if the power stays above 13 volts when he keys up and talks. some of the old radios had a bad power switch in them and not carry the full voltage to the radio.some of the old washington radios had a weak switch and I have put a relay in them to give full voltage when turned on. the 858 president's had a problem with that.

Should I check where the d.c. switches to the tx section of the radio (no relay on this model) or behind the main power switch of the radio?

I just measured the voltage with the radio off, its 12.10v when i turn on the radio it floats between 12.09- 12.08
When I key the voltage drops to 12.05 at it lowest, this is with a "nice & decent" gauge wire, no fuse holder. Thank You
here is the film to show

 

[sonoma]

check the voltage at the power switch after it goes through the switch to the main radio board.put the ground on the neg lead coming into the radio or at a neg location on the main board. do not put the ground on the metal case of the radio.
you should have 13.8 volts into the radio and the same at the power switch. one side of the switch is straight from the power plug. so put your test lead on the side feeding into the radio on the switch.

 

[Robb]

It is only dropping a few hundredths of a volt. Not a problem, and is expected. Also, check the voltage at the switch. If that isn't convenient; then check it where the switched voltage goes to the board and then key. What you are looking for is a volt or more drop in voltage.

Most guys use at least 12.5v to feed a radio with, but that shouldn't be much of a problem for testing purposes.

Another thing; if you make a video, the n be sure there is enough front light to see your meters and gauges move - JMHO.

 

[LeapFrog]

Check the Voltage
You should have 13.8 volts into the radio and the same at the power switch.
.

Forgive my ignorance here but im only putting about 12.05 into it so should expect this is why the radio "downward modulates" i have an old scope that is working (My 1st One) but I need a bnc jumper for an rf sampler to see the waveform.

 

[sonoma]

not really. just check to see if you are getting the full 12.05 volts pass the on off switch for starters.

 

[LeapFrog]

Ok Thanks, going to check it this morning after coffee.
The Factory Service Manual states the input voltage is 13.8V +D.C., I should start there I think.

 

[222DBFL]

I know this may not be correct, but I see no one has suggested using a different watt meter. A PEP reading one. And also some way of knowing what modulation is set at. Is it at 100% now? Something like a radio shack meter might be a better VSWR/power meter as it will show somewhat of a pep reading that is fairly accurate on AM and it has an AVG setting as well.
Like I said, I may be way off base, and yes I understand the concept of not enough power. But these radios can run off a car battery at 12.6vdc, as long as there is enough amperage to hold the voltage at this level or close to this level, with 12vdc being the min. voltage, I understand the power drop, but he is also using a NON-PEAK reading meter. Does this or will this make any difference using an active peak reading meter, with the same voltage (12vdc) and said amperage, or not?? Or again am I just off base?
And also, what type meters do others normally use when measuring power output on a CB radio? Normally some type of peak reading meter, whether an active one or not correct? I never used the AVG selection on my Radio Shack meter, just PEP mode, unless I am on SSB that is. Am on the right page here fellas or is there something I am missing? Just trying to learn like most others here. Thx for any responses. Good day.

 

[LeapFrog]

I know this may not be correct, but I see no one has suggested using a different watt meter. A PEP reading one. And also some way of knowing what modulation is set at. Is it at 100% now? Something like a radio shack meter might be a better VSWR/power meter as it will show somewhat of a pep reading that is fairly accurate on AM and it has an AVG setting as well.
Like I said, I may be way off base, and yes I understand the concept of not enough power. But these radios can run off a car battery at 12.6vdc, as long as there is enough amperage to hold the voltage at this level or close to this level, with 12vdc being the min. voltage, I understand the power drop, but he is also using a NON-PEAK reading meter. Does this or will this make any difference using an active peak reading meter, with the same voltage (12vdc) and said amperage, or not?? Or again am I just off base?
And also, what type meters do others normally use when measuring power output on a CB radio? Normally some type of peak reading meter, whether an active one or not correct? I never used the AVG selection on my Radio Shack meter, just PEP mode, unless I am on SSB that is. Am on the right page here fellas or is there something I am missing? Just trying to learn like most others here. Thx for any responses. Good day.

Alright, well I do have this.


Same microphone, same power supply, same antenna, & the same meter.
This is on my Uniden PC68XL, as you can see the needle on the average reading meter moves forward under modulation.

You made some good points in your post Sonoma, perhaps someone can film a stock radio on an avg. reading meter @ 12.1 volts, I don't know how but when this radio was serviced I found a 2sc1969 Final Output R.F. Transistor was installed.
(Is that a factory deal or is the 2sc2166 the oem spec part? thanks).
Without an 'oscope probe I cant show you the waveform, modulated carrier or measure peak voltages.
I don't have a stand alone modulation combo/power meter like paradynamics makes, that's why I have the scope, im just about to finish building the rf sampler, then i need a probe, and bnc jumper cable and i can get some peak readings for this thread (still using the 12v xbox p.s.u. for the radios).

I'm not going to give up on my 29 Plus, i'm currently looking for more D.C. power, 13.8v @ at least 5 amps. I still need to open it up, without a de-soldering pump it may be neigh impossible to get at the switch without destroying the front (led) pc board.

I noticed a faint response on the last audio check.