Cobra 135 doesn't transmit

[Turbo T]

Today I picked up (for free) a Cobra 135. This is a 23 channel base rig. So far it powers up and receives, but does not transmit.

I've tried a few different mics, one of them I wired for the radio per the instructions on CB tricks...that didn't work. I tried the mic from my Uniden 510...that didn't work either. (the same mic I just wired worked on the 510 though)

I tried jumping pins 1 and 3 as if I didn't misread the schematic on CB tricks, this should have made the radio go into TX mode....but it didn't.

What else can/should I look for?

Thanks in advance.

 

[Robb]

Don't those 23 ch radios use a relay to get TX?

Uses the same mic wiring as any standard Cobra 4-pin mic.

 

[midnight special]

check the pa/cb switch

 

[mechanic]

The key up of this radio is done by the use of a transistor, 2SA562, a "common" (good luck finding it) PNP switching transistor. Denoted as Q24, it is directly controlled by the key circuit of the microphone.
I have found this to be a common fault of circuits of this type....


73 mechanic

 

[Turbo T]

Thought I'd give an update. I found the PA system works on it. So at least I'm pretty sure I have the mic wired right.

I do hear a little small popping sound thru the internal speaker when the mic is keyed.

Mechanic, is there any thing I can do to check the transistor you suggested might be bad?

 

[mechanic]

Thought I'd give an update. I found the PA system works on it. So at least I'm pretty sure I have the mic wired right.

I do hear a little small popping sound thru the internal speaker when the mic is keyed.

Mechanic, is there any thing I can do to check the transistor you suggested might be bad?

A transistor checker would help or you could check junctions with a VOM or DVM. I would try to find a replacement if possible, Mouser would be a good start and they have no minimum quantity. Rat Shack had these once but no more.... you could rob one from a parts radio if you have one.
By the way this base is a winner and is well worth the effort to repair! With a single crystal swap and adjust the voice lock spread, you will have over 46 channels and all in betweens!

There is a diagram on CB tricks....

73 mechanic

 

[Turbo T]

A transistor checker would help or you could check junctions with a VOM or DVM. I would try to find a replacement if possible, Mouser would be a good start and they have no minimum quantity. Rat Shack had these once but no more.... you could rob one from a parts radio if you have one.
By the way this base is a winner and is well worth the effort to repair! With a single crystal swap and adjust the voice lock spread, you will have over 46 channels and all in betweens!

There is a diagram on CB tricks....

73 mechanic

Mechanic,

I do not have a transistor checker. I do have a multimeter. Perhaps there's some way I can check for voltages on the legs to see if they're what they should be, with the multimeter?

As for a replacement, the only parts radios I have are a Realistic TRC-422A, though it doesn't RX or TX so who knows if it has any useable parts. That and I have an old Royce 1-612 that I could perhaps rob a few parts from, if it shares any identical parts.

I checked Mousers website and they show no matches found for "2sa562". I did search around online and found this on eBay, is this the correct transistor you speak of?:

20 PNP Plastic-Encapsulate Transistor 2SA562 A562 TO-92 | eBay

As for your last comment, yeah, everything I read about the Cobra 135 says it was top of the line....back in the mid 70's! And that it sold for over $400....I did an inflation calculator, and that would be just over $1600 in today's dollars!

But of course I was "given" this radio for free, by a CB repairman who was "just going to chuck it in the trash" (along with the Royce and Realistic I mentioned). I didn't get a power cord, or a mic for it. But if anything, I do want to bring it back to life and perhaps keep it in my little collection, even if it is just a 23 channel radio. It's just a nice looking piece.

 

[Turbo T]

Also I found this one too...

2SA562 PNP Transistor