Some Help Needed Please!

[Kommissar73]

Hello everyone, I am brand new to the forum and would like to take this opportunity to say hello. A little background, I used to CB about 10 years ago and got out of it, a few month's back, I decided to get back into the hobby. I installed my mobile rig and it was working great, but I decided to upgrade my radio. I came across a NIB Motorola Mocat 4020 on craigslist. I picked it up from the gentleman yesterday and he was a nice guy. He said his father bought the radio brand new and never installed it. I looked at it when I met with him and this seemed correct. All in original plastic, still in the factory box. So I installed the radio in my Jeep, checked the SWR's with a new meter and thought all was good. On my way to my friend's house, I could not get a radio check off any of the locals, so I asked my friend to fire his rig up and I parked at a part about 500 feet from him. I received his signal well as usual, but when I went to key up he got no audio from me. His needle moved, and he could hear me keying up,but no audio. I tried on both my antenna and his antenna to no avail.

I also hooked it up to his PA system, and again, you could hear the click of the Mic keying up, but no audio. My setup is a 4' Firestik on a triple magnet mount, centered on the roof of my Jeep, brand new coax cable, and the radio is mounted to a cigarette lighter adaptor, radio receives phenomenally. SWR us 1.1 on Channel 1 and 1.5 on Channel 40. Any ideas folks? Thanks for your help in advance.

 

[K7MEM]

It sounds like you checked and swapped everything you could, except for the microphone. If the element was bad, or the microphone cable was wired incorrectly, it would cause the problems you describe.

 

[KC4RAF]

+1 with K7MEM posted.

Is it by chance a powered mic? If so, battery, gain,etc. If not the mic, it'll be in the audio circuit.

 

[Kommissar73]

From what I see there are no Mic gain adjustments on the Mic or on the radio. The antenna is new as well, worked wonderfully with my Uniden Pro-510XL. I would like to rewire the Mic, but it has a different connection than my Uniden or GE. 4 pin male on the Mic to a 4 pin female on the radio. How difficult would it be to replace the element? Thanks again for all input as well!

 

[TheSpaceMann]

Replacement element is cheap! You may want to try a new mike for the unit.

 

[Dawn]

I'm going to take a shot at this. The law enforcement Mocats were about the last ones that I ever worked on professionally during the 70's. While we had little to do with the CB installations with the municipalities that received grants for them, if the radio was in the shop with another repair, we'd check it out.

I don't know if this applies to the civilian versions, but the connector was a 4 pin internal D shell that was common on many of their early 80's radios like the Syntors, Moxy's or Maxars. The mic itself was I think the same one used on the Marine units. Cheaper mic and was powered by the radio. I'm not sure if there was an internal amp on a dynamic or if it was an electret, but the radio provided the power to the microphone. They did go bad with reports as above of no modulation even back then. It could also be the reason you found a literally NOS radio as off the shelf replacements were not available by others and had to be ordered from national parts.

Someone could have tried to modify it by putting a conventional connector and disabling the Bias. Check to see if the mic connector is a black composite inside with thin pins and had a D shape to the internal shell. If this is the same as the police versions, it's not a difficult mod for a decent tech to either put on a power mic with a cobra or other style connector on the radio and remove the Bias if hasn't already been done or maybe find an amplified cartridge from another mic and substitute it. Try finding another moto mic first.

IIRC, these style mics were essentially backwards compatible replacements for a carbon button mic with an amp made by shure and were originally controlled reluctance elements with a build in amp types in the commercial radios. The marine and CB radios got a work alike that was much cheaper and smaller.

Wish I could see a schematic. Hope this applies to your civilian version.

 

[k8krh]

I would get a new mic, you can pick them up on e-bay many at a low price.
Just make sure the wiring is the same.
DOCTOR/795

 

[Dawn]

Is a working mocat mic with their special connector that common on Ebay?

I was looking through some of the mod indexes last evening. There's a mod using a cobra CA-50 mic element in secret cb 24 PP 51 with a conversion for the Mocat 40 mic. Seems to be what I remembered.

 

[Kommissar73]

Thanks everyone for the help on this. I've been busy with work and haven't had too much time to look deeper. I did get a phone number from a nice group of guys who talk on Channel 32 locally, a local tech with good equipment and knowledge. He does a lot of ham work, but still does CB stuff as well. I've got a Midland, a Uniden and a GE I'm going to have him look at in addition to the Motorola. It'd be nice to find the mobile I want (Sears Roadtalker 40 or a Cherokee Nightrider), but I'll get these fixed or sell them off. Too many broken radios and not brought working ones! Again, thank you all for your replies!