S/RF Meter Problem

[sdmahr]

Radio: Cobra 146 GTL

S/RF meter works when transmitting (swings as it should). It doesn't do anything on receive. No swing at all with incoming signals.

Rest of the radio works great ! Any suggestions where to start to cure the ill??

Thanks

 

[sp5it]

Check D7, 8, 9 and VR1
Mike

 

[W5LZ]

... and if you don't have the equipment to do the checking/setting of that 'S' meter, and if you don't know exactly what you're doing, don't mess with it. Take it to someone who does have the stuff to do it with and knows what they're doing.
Not a very nice response, huh? Well, it certainly good advice though. Oh well...
- 'Doc

(Not being a 'smarta_ _', but if you have to ask how to do it, you shouldn't do it. Sorry 'bout that.)

 

[sonoma]

on this old of a radio the VR could be bad. as W5LZ said use your discretion as to what your knowledge of working on a radio is. just by turning it the signal may come back.

 

[w9cll]

I agree with doc.

 

[Robb]

I agree with sp5it.
Go for it.

 

[sdmahr]

Wow.....what a variety of responses......

So I have attempted to move VR1 but it makes no different. The legs of VR1 are dead.....no voltage. Same is the diodes leading up to VR1.

And just for the record, I have the equipment to adjust the receive side of the S/RF meter. My question was about why it may not be working.

 

[sonoma]

here is the schematic for your radio. just trace the feed across the top of the schematic for the s meter. also you need to check the diodes for function not voltage and see what ohms the VR is reading or if it even works.

http://www.cbtricks.com/radios/cobra/146gtl/graphics/cobra_146gtl_sch.pdf

 

[Robb]

IC1 might be bad; it is used for AGC, Squelch, and the meter movement. Looks like a LM-723, very common and pretty cheap ($.50 - $1).

Check C17 for being shorted; it is a 10v cap and 10v caps are the most prone to failure. In fact, I would change out all of the 10v caps as a rule of thumb. C113 too. Seen too many of them waaaay out of tolerance and some shorted.

Also check out TR8 & 9; D5, 6, and 10 too. The only other part in that circuit is the pickup coil. If I read the schematic right, check for a signal at the junction of L10 and C19. Track forward to IC1 from there . . .

http://www.cbtricks.com/radios/cobra/146gtl/graphics/cobra_146gtl_sch.pdf

 

[sdmahr]

Thanks for the insight Robb. I'll check those components out. I just got done recapping the entire radio....(and the meter was dead before that as well), so I'll be checking out these related components.

Once again....appreciate the leads ! !

 

[sonoma]

be sure and look for a cut trace also. have seen this more than once.

 

[sdmahr]

Thanks Sonoma............I'll check it out.

 

[sdmahr]

Radio was freshly re-capped, (complete), last week but the S/RF meter is dead in receive only, (swings during TX)

Update: Doesn't appear to be any cut traces.

TR8 and TR9 voltages appear within spec in service manual.

D5, D6, and D10 check ok.

There is activity on the scope at the junction of L10 and C19.

I ordered IC1 and hope that takes care of it. We shall see.

 

[sdmahr]

Chip arrived today....installed and meter is still DEAD during receive. Oh well, back to the drawing board !

 

[sonoma]

did you verify the VR is good?