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Cobra 134 low TX & RX

Skyrider

Skyrider

Member
Apr 9, 2010
21
0
11
33
Leon, GTO. Mexico
Hey everyone,

I got myself a very nice old radio. A Cobra 134. Market find. The thing powers and transmits and receives, just very very low. Nothing more than 1 or 2 S-units on receive and no more than around 200mw power output or less on output. It's so low on the transmit that i can't even calibrate my meter, when I attempt to calibrate the needle only goes up about 1/4 of the way up with the calibrate knob at max.

Driver final and pre driver are okay. Relay was checked over for bad contacts and that was eliminated as a possible cause.

Currently using a schematic for a 138 as I can't find anything for the 134. Not sure how good of a guide this will be. Would really appreciate any info on this chassis.

If someone knows of another radio that uses this same chassis "PC-003A" let me know.

Much appreciated for any help!
 

Checked antenna path from relay for any bad capacitors but found none. Behind relay there is no capacitors or any components that are shared directly between both of the modes that could be causing some kind of issue at the antenna. I'm trying to go for the easy stuff for now until I could hopefully find some service info. Checked all the transformers from receiver and transmitter. All is good. All transistors on the transmitter path are okay.
 
In Mexico there is a huge thing for personalizing and decorating personal things. Everything has to have a personal touch of some kind. Houses are very colorful for example. It's a type of folklore. It's fun. I suppose it comes from as far back as the natives of the country.

Just like buying and using enormous 30,000 Watt transmitters that splatter up and down the entire 11 meter band from excessive pinched off modulation, installing enormous train generators in SUVs like suburbans, putting them on 6, 28, 11, and more it's just a trend. All costing multiple thousands of dollars. Guaranteed that there's plenty of people who would wonder why on earth you would do such a thing and also plenty of people who get annoyed by it when it flocks over this way and over to other countries as well.

That's just a thing some people enjoy doing here. And you have your own things you guys enjoy doing over there. No one deserves to get slapped. Everybody has their own thing. Otherwise it would be boring. ;) Personally I respect and enjoy hearing the variety of crazy inventions and modifications I hear on this very unique band. Over all the only thing that ever annoys me is bleed over/splatter. And even that is a kind of characteristic a lot of CBers like on their radios.

All of those roger beeps are hand made by a few technicians down in Mexico (mostly Mexico city) who have done a lot very interesting work. Whether or not you like them (I personally sort of half like them depending on what channel i'm talking on) you gotta give these guys some credit for inventing new circuits almost every year to keep up with their competing technicians.

Most of the guys using that stuff are truck drivers and some taxi drivers.

Anyway, I had already checked out google and CB tricks and did not find anything. Also spent some time trying to cross reference this chassis to another radio. I still believe it's pretty much identical to the Cobra 138 with the exception of a few extra features like dynamike. I suppose so far this schematic has served me well but nothing like a good ol sams photofact to help you trouble shoot something!


Went ahead and checked every transformer in the radio with no problems found. All of the receiver side transistors are also okay. I think that's it for "easy" trouble shooting and anything beyond that is going to require a service manual. Will be doing diodes next.

Thanks.
 
The Cobra 134 (Black Face) was a 'budget' model 138. Sister radios included the Cobra 28. At the end of the 23 channel era Dynascan had a bunch of unsold radios and parts so they removed the dynamike control and sold 'em at rock-bottom prices just to get rid of them.

Any tips, tricks or schematics for the 138 will apply to the 134 with the obvious omission of the dynamike control (which can be added as the hole in the chassi exists for it)

While we are on the subject of Mexican operators I'd like to ask this: What is up with the frequency position? Why are they almost never on the 0s or the 5s, but instead somewhere in-between? Example: I often hear stations on 27.702 or 27.637. Also there seems to be no agreement as to what mode is being used. Often I will hear AM, USB and LSB on the same frequency.

I'm not a real big roger beep fan but even I have to admit some of them are kind of cool. Just not on sideband. Never use a RB on sideband, it just pisses people off.

Good luck with that 134
 
Atlasta said:
The Cobra 134 (Black Face) was a 'budget' model 138. Sister radios included the Cobra 28. At the end of the 23 channel era Dynascan had a bunch of unsold radios and parts so they removed the dynamike control and sold 'em at rock-bottom prices just to get rid of them.

Any tips, tricks or schematics for the 138 will apply to the 134 with the obvious omission of the dynamike control (which can be added as the hole in the chassi exists for it)

While we are on the subject of Mexican operators I'd like to ask this: What is up with the frequency position? Why are they almost never on the 0s or the 5s, but instead somewhere in-between? Example: I often hear stations on 27.702 or 27.637. Also there seems to be no agreement as to what mode is being used. Often I will hear AM, USB and LSB on the same frequency.

I'm not a real big roger beep fan but even I have to admit some of them are kind of cool. Just not on sideband. Never use a RB on sideband, it just pisses people off.

Good luck with that 134
Click to expand...

Hi, well keep in mind it's a big country and a lot of what ends up over on your side is probably all kinds of skip clumped up from different places. It sounds the same over this way from your side. I hear side banders on the upper 30s at the same time sometimes I hear truck drivers in the south with their southern accents. I assume the mode changes might be just a collage of all kinds of signals being transmitted at the same time from multiple places without necessarily any relation. Can't tell you about the odd number frequencies. I assume maybe two parties are sharing QRM problems in a local contact or just a weird way some clubs have decided to set up shop. Keep in mind that there's probably a lot more CB radio users here than over on your side. It's still used as a common utility for taxi drivers, and that will probably be most of the traffic you guys hear from Mexico and other parts of Latin America in the normal band. Big relay and check in stations usually go to the lower band. I believe large amplifiers are probably common there.
 
Atlasta is correct, the Cobra 134 shares the same board as a Cobra 138, minus the mic gain. Sounds like the radio just needs a good alignment on TX and RX and it should get better. Also, replacing all the electrolytic capacitors would help too, as those are some old components in the radio.


And since it got brought up.... while few people like hearing all the traffic from "South of the Border", they have every right to use the radio as much as we do. If ya don't like hearing it, turn the radio off. That's what I do. Jumping on someone from Mexico about what his fellow countrymen do when he's asking a legitimate radio question is a bit juvenile, IMO.


Let's all just get along. :)


~Cheers~
 
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Reactions: 3 people
And since it got brought up.... while few people like hearing all the traffic from "South of the Border", they have every right to use the radio as much as we do. If ya don't like hearing it, turn the radio off. That's what I do. Jumping on someone from Mexico about what his fellow countrymen do when he's asking a legitimate radio question is a bit juvenile, IMO.


Let's all just get along.


~Cheers~
Click to expand...
Agreed.
Thread cleaned.

73
Jeff
 
Skyrider

Saludos y bien venido a nuestro foro...
Nuestra Casa es Su Casa.

You write in english very well, much better than I can speak or write spanish but I try.

Hope to catch you on the air waves.
 
AudioShockwav said:
Agreed.
Thread cleaned.

73
Jeff
Click to expand...

I'll remember that next time the contingent from England bad mouths our country because of our politicians. Calling me and whoever juvenile for pointing out the obvious with a low key comical remark is not very cool.
 
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Thanks to all for the comments and suggestions.

It turns out we definitely do have some alignment issues which I missed on my initial trials.

On my first test I remember I went through all of the channels checking them against my normal receiver and they all worked. I also noticed some issues with the receiver where sometimes it would sound louder. I simply credited this to dirty contacts from a long time of no use. Another thing I noticed on one occasion was channel 11 not working, however that started working again on another run through. Again I credited this to dirty contacts. It just seemed to have settled down after that until I hooked it up for a transmitter test and I discovered the issues which made me open up this thread.

All the controls have been cleaned up now but the issue remains.

I gave it a nice run through again and discovered the following channels are not working,

23
22
19
15
11
7
3


All of them seem like the may be related to the same crystal combinations. A run through the mixing chart is in order.

Also discovered something very peculiar and that is that in order for it to operate on the selected channel and sound almost on frequency the voice lock has to be all the way to the left. However it doesn't look like there has been any kind of clarifier modifications done to this radio, infact it all looks totally original.

Okay so now we know where to look!
 
Alright guys, was able to get a hold of the service manual for the Cobra 138, should work for any other radio using the same chassis PC-003AA. Cobra 134, Midland 13-893, and 13-895. Obviously there are differences between the radios. In this service manual you'll find information that will assist you on all of them however.

Thank you very much to the person who sent me this.

Anyway, currently I unfortunately do not have access to my frequency counter and my buddies frequency counter is no good. I'll have to wait til I get my hands on my own to do this alignment. Since the radio seems untouched I will try and find any bad components now that I have a more detailed idea of the circuitry and how it's working.

I hope this information is of help to some people.

EDIT: Not posting the service manual online as requested by the person who sent it to me. Might be able to share it in case someone needs it.
 
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