Need help with side mic cobra 29 Ltd classic

[loosecannon]

Red97, i don't want to get in the middle of where Andy has you going, so please take my posts as a separate set of tests.

follow along with Andy, and what he is trying to get you to see and do, and if you get stuck, or want to try a different set of instructions, read my posts.

I am going to start you right back at the beginning.
please check the basics first, using another working radio, and don't assume anything yet.
first, are you using a known good mic? tested? ok good.
second, is your external watt/swr meter working properly? tested? ok good.
third, will the radio receive an actual signal from another radio, not just static, and check different channels, not just one.
forth, do you have a GREEN light on RX, and a RED light on TX?

please post back soon and let me know this is all working.
once that is done, get the radio set up on the bench with mic plugged in, wattmeter connected, another radio set up to be a signal source (like with a rubber band around the mic on it so you can just turn it on and off for when you want to see your radio receive a signal). that's going to be your fake signal generator. try to find a way to get it set at a location and power level that will show you somewhere between S1 and S5 on your broken radio.

get a DC voltmeter set up and ready, and get your oscilloscope ready to read AC volts.

once we are there, we will start going through the radio from the very beginning.
LC

 

[Handy Andy]

@loosecannon - thank you...

 

[red97k1500]

Thank you loose cannon, I will confirm everything you posted when I get home from work tonight. I’m still waiting on the scope probe I bought on eBay to show up, it should be here soon.

 

[Handy Andy]

I have to post this or I will forget. Im getting 7.72volts on the middle pin of tr16 in tx. In rx just mv jumping around. I couldn’t find any mods in the d17,d18 area. In rx I get 4 something volts on I believe the banded ends of d18/d17, then in tx I get 4 something volts on both sides of the diodes

I had swapped out tr16 out of the parts radio a few days ago but never tried to take any voltage measurements.
.

AWESOME!

That means your 10.240MHz Xtal is working both sides.

So you are good to continue.

One of many symptoms of "can't get any signal ..." past the Pre-drivers R62/R61 mess I've seen are from two faults.

Blown TR16 - that means the "Base" is shorted to one lead of the other two - Collector or Emitter - or blown completely in two (open) and the Base just sits there.

You can see these results on a DVM - DC Volts...
If the Base is blown - you either have 0 V or 7.7 volts on it - nothing in between. R61 and R62 make the signal, and the power good bias supplied by R62. Float in between 7 votls to as low as 1 volt. But that is in DC, so if the CENTER leg shows nearly 8V DC - another should be 0V DC while the other would go to 1.2~1.7V DC - the 1.7V DC line is your Base.

IF you can, just post your results...the issue just may be that the PLL is not in lock and forcing TR16 to stay off when you try to TX.

I'll let @loosecannon take over - but I just wanted to make sure you had support as well as a good start in fixing this thing.

 

[red97k1500]

Thank you for your help Andy, I really appreciate you and lc taking the time to help me, I don’t know any other way to learn, than with at least a little knowledge and advice from some people that have experience fixing a radio. So thanks again to both of you

 

[Shadetree Mechanic]

Tons of good info here, I am trying my best to follow along. I am in the process of setting up a test bench in the back room, I hope to be going through some radios soon. Thanks guys!
Chris

 

[red97k1500]

Dr Frankenstein voice on: its alive it’s alive. I repeaked l22 and l19 and l18. I have tx and audio I can hear my self on another radio through a dummy load in the same room. It still needs an alignment but im very happy. The carrier power is not balanced between 1-40 but it’s alive

 

[Handy Andy]

Congrats!​

Ok to help you along, this is where people tend to direct tune ups differently but the premise is the same...

In the typical CB radio - they have to use THE SAME OSCILLATOR for both TX and Receiver function. Hence the problems with tunes ups you've had because the oscillator has to shift - quickly - to obtain the modes' you selected (TX or RX) and stay locked to track the output without thinking something failed and then it crashes.

So that is where you'll need to refer back to some graphics I've posted in this thread.

One in particular is the VCO coils labeled - L21 - that "Twin core" unit.

That one works with the TX Mixer, and is designed to be a broad banded filter for the two frequencies it's trying to mix together. The 10.240 and the ~17MHz arriving from L22/L19 mess...AND - keep it all on the same frequency.

So - if you get the receive to work, that's fine, we need to look at how to make the TX side switch over quickly.

That is where you pay attention to a performance issue the Radio Designer put into their Tune Up Manuals.

​

Which Mode your tune up first?

That's the clue...so yes, you follow what that MANUAL says, but the reason is for where the coil you adjust in the TX modes "peaks out" at, it's at a different frequency than the RX side.

What is on the very next page of the 29's SERVICE MANUAL???

CLUE: It's not where the Emergency Exits are Located In The Building...



It's for the TRANSMITTER!

This may not matter much to some - but it provides you the guidance of what is more important - TX alignment for TX quick-switchover - or ordinary RX alignment?

How is your perspective, care to take a trip into another chassis for a moment?
This is what @loosecannon was trying to have you avoid...but there is a reason why I'm showing this...

This is for a COBRA 25 radio - like the PC-66/68 series too...


The smaller cousins had to do as much work in the Switchover - but with less parts. So they made one side the most important part of the radio to work effectively.

So - you are working on the TRANSMITTER - yes, but remember the peaking and where you need to focus your CHANNEL selector on?

After initial - it's Channel 40 - then equalize that to Channel 1 - why?

Harmonics and spurries - EMISSIONS - yes, these things need a Catalytic Converter, so they purposely designed the coils and the band spread to FADE AWAY (READ:LESS PEAKNIG) at higher frequencies to a greater degree than to lower ones...reduces the harmonics that gets into TV's hearing Aids - Cop Cars sitting next to you at the light - you get the idea...

So they wanted you to work on TX first, peak it out on Channel 40 thru to Channel 1 - make it as equal as possible

• - this includes the entire TX CHAIN
• - from L22 your 17MHz stuff,
• - L23 your 10.240MHz stuff,
• - then L19 To TX MIXER which stuffs it into,
• - L21 Band Pass PART OF THE TX MIXER which once done...sends it to,
• - L20 OUTPUT OF TX MIXER which then uses,
• - TR16 - PRE-DRIVER and L17

So what would I do? Focus more on channel 40 performance first, then tweak the power to make it down to Channel 1.

Then this is where that AGE factor of the radio starts to rear it's ugly head.

This is where you may need the Oscilloscope, refer back a post or two - I highlighted a green line to a jumper that comes off of a coil - as well as the coil itself - you need to balance this L18 coil and L19 coil performance together to obtain a balance between RX performance and TX performance. This can be tapped to see how the signal is doing as well as the open lead NEXT TO R88 back at the VCO MIXER closest to the VCO MIXER.

Even though you have replaced the Electrolytic caps - you may still have tuning issues in the TX strip as well as elsewhere in the radio where ordinary Non-Polarized caps may have drifted off their values - a factor of TOLERANCE...

Best to focus on the 3rd Topic reference...because that is what is happening to your radio as it ages...

So please continue, we're here to help.

This may seem like a lot, and yes it is. But I wanted you to look further into the research I do to help me find the best balance between what works in power and reception.

Take your time...

 

[loosecannon]

Red97,

that is great news!

it was also exactly where i was going to have you start LOL.

since your next step is an alignment done without all the professional gear, maybe start a new thread for that part of the process.
that way this thread doesn't get too broad, and might be of help to others.

maybe title it something like 29LTD backwoods tune up. OK OK just kidding.
but something that denotes to future searchers that this is an alignment procedure for a 29LTD that can be done by someone who doesn't have all the big toys.

I don't remember whether or not you have a frequency counter, but without that, there are just going to be some things in the radio we can't adjust, so try to get something, even if it's one of these:
https://www.ebay.com/itm/Dual-Frequ...=352893859329ffa10aee4fd3471d9ad77d3169d671f3

the ones i've bought have actually been pretty close to accurate. certainly enough to get a 29LTD working right.
LC

 

[red97k1500]

Hey lc, Yes I will start a new thread for the alignment, I’m going to have a bunch of questions for sure, my cheap Chinese oscilloscope probes from eBay finally showed up in the mail. I’m still studying about how to use the oscilloscope, it does not seem to difficult but I’m still figuring it out.
And yes I have a frequency counter. It’s an Mfj-886.

Maybe the title of the new thread should be called everything you ever wanted to know about alignments but were to scared to ask lol.

 

[loosecannon]

LOL i actually like that title but i think the thread would drift aimlessly pretty quickly.

please shoot me a message when you start the other thread.

as for the scope, find the manual online, and look through it for a section called something like "quick setup" or "pre setup for first use" or something similar.

this will give you a way to pre-set all the controls, and then you can just go to the section about "measuring AC voltage" and then do those steps.

really what you are going to be doing is just setting the scope for different levels of sensitivity depending on the level of the RF voltage at that particular stage of the radio.

when a signal is so strong that it goes past the edges of the CRT, adjust the sensitivity down so it will fit and you can see it get bigger and smaller.
if the line doesn't seem to change during a test, then increase the sensitivity until you can see the wave form.

once you get used to it, you will find that it has a bunch of controls that you will most likely never use.
LC

 

[red97k1500]

One last little update, I replaced the green channel display with a red one, since the green one had some burned out segments.
Now it does not miss a beat. I also polished a bezel from the parts radio because it was in better shape and bought a new faceplate for $20 bucks lol.
Soon I will start the new thread for the tune up and alignment.