Cobra 139XLR receive tune up or mods?

[Adamf]

Hi all, I am sure this has been asked before but I have a Cobra 139XLR (D858 chip), receive seems "decent" but it is somewhat noisy and I know I can improve the receive. Can anyone tell me what to align for receive or perhaps some simple mods. I have a signal generator, VTVM, and solder/desolder equipment so any information would be great.

Thanks to all, Adam

 

[TM86]

Best site for info on the 858 chassis is run by a member here, Unit_399. His page: https://unit399.wixsite.com/858ssb

 

[Adamf]

Hi, thanks for responding, I have that site and it has some great information but also hoping someone knows the alignment info for what to adjust for the receiver and also perhaps what VR5 AM AGC is for? Thanks again for any help. Adam

 

[Holydvr]

These radios are by their nature more noisy than most newer types. You may get some tips that may whittle you in a good direction, but these radios are just that way to a degree. I have five Courier 858 base stations and two 139XLR’s. They are all fairly consistent with one another on the inherent noise they make in receive.

 

[Shadetree Mechanic]

Most people would suggest to replace all the capacitors in a radio that old. I have an 858 President Madison done up by our member @loosecannon, the receive is very quiet and it flat out smokes any cb receiver that I have. Set up right, these are the best radios ever made.

 

[Adamf]

Hi, thanks for all who posted, I appreciate that. Anyone familiar with alignment of receive and what VR-5 does? Thanks, Adam

 

[Handy Andy]

Most of the 139 follow suit with any other tune-up of the 858 chassis. So Unit 399 webpage is about the best if not ONLY source for the most current information available - aside from forums like this.

But you did ask a question dealing with VR5 and the AM AGC.

This might sound "weird" but it's really made to help balance the signal IF from both the SSB side and the AM side, making them more EQUAL in signal performance - noise floor, and signal meter - as well as SELECTIVITY and SENSITIVITY the AM side has - COMPARED TO - the SSB side.

The SSB side of the receive is considered "full bore" amplification strip design - maximized more for the Detection and then when decoded - heterodyned with the built in IF - which they knew would be noisy.

It's simply for - so when you switched modes, the thing doesn't go deaf...

You set VR5 as more of a personal preference - be lucky you have it, not all chassis have this.

Tune up is pretty straight forward.
Adjust CT1 and VR5 TOGETHER for BEST LINEARITY
of signal amplification 50uV 27.205MHz Signal
with MIC input 50mV 2-tone AF signal
(Listening for clipping distortions and noise level volume)
You do the best compromise between CT1 and VR5
for "same sound" processing
CT1 trims VOLUME for SSB
Change C242 to a HIGHER value if you
have TOO much SSB and not enough AM
Do the opposite with C242 if you have too much AM versus SSB signal.
You adjust DELTA-TUNE to match S-meter results for Volume of TONE
Adjust VR1for S/RF METER results
​

 

[Adamf]

Hi Andy, thanks for the response. Adam

 

[unit_399]

Adamf -

Check if L6 and FT2 are installed in your 139. Absolutely necessary for lowest receive noise. They're missing in most 139XLRs.

- 399

 

[Adamf]

Hi, I have an L6, where would the FT2 be and what is it, a coil? Thanks, Adam