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Cobra 148 GTL S/RF CAL SWR switch and meter problem

Unit 75

Unit 75

"TRAINMAN"
Jul 29, 2014
526
1,018
153
Central Alabama
This radio transmits good in both modes. When I have the S/RF CAL SWR switch in the S/RF position the meter instantly jumps up to about a S-9 reading. It doesn't do that in the other 2 positions. The meter movement is ok. I have replaced the switch (it was nasty) ... D73, D79, D46, D45 were also replaced. I'm getting .433v on the orange and red wires on the one lug of the switch....nothing else. Anybody have any ideas? The receive is not functioning properly either but that is another issue. I need to find this fault before I can begin to align the receive with the meter.
 

Be sure the negative side of the meter when in SWR/RF is properly grounded if you have the same reading on both sides of the meter leads when in that position it maybe it's not grounded to the boards neg ground. Just an idea.
 
You could be correct. I can actually barely hear a close incoming signal from my other radio when I key up, much better sensitivity on SSB than AM though. It does not get any better when I try to tune the IF strip,
 
Ok. Took me a bit to troubleshoot this basket case. Here is what I found. I started with the detector diodes D21 & D22 then moved on and found TR48 (meter amp) to be defective. I replaced that and still had no S-meter fix. checked TR11 ( the meter detector) and checked D12, D13 for a receive problem but all ok. Then I found the orange wire from the squelch control pot to the PC board to have a bad solder joint at the board. It appears the wire was barely in the boards hole but not soldered well...you could pull it out. Soldered the wire back in. Still no receive but the radio sounded different. I found the squelch control pot to be bad. So I replaced the vol/squelch control pot and guess what? I could hear my generator tone faintly. I adjusted L4 on up so it got louder and I would be in the ball park to an alignment. I now have receive. The S-meter has still got me stumped because it is still moving the needle way up in AM mode only when switched to S/RF position. SSB is ok. But at least the receive problem was not connected to the meter problem. Back to work!
 
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That is kind of interesting. I am just going back in memory here...but I was thinking that both the meter and the squelch were driven by the AGC system. The idea there being, as your incoming signal gets hotter, the AGC would climb up wards and be wrapped back around in a manner that applied negative control over an earlier RF stage. But if you think about it, that AGC signal....since it varies higher with hot signals and lower with weak signals ...would be a natural candidate to use to drive a meter. So something in the path between the AGC circuit and the meter.

I could be wrong, but it wouldn't hurt to give it a look with that in mind.....

EDITED TO ADD:

And the same would be true for squelch. SInce the AGC signal is higher for hot signals and lower for weak.... that would make sense using it to drive the squelch level detection.

So I am thinking that, yes, squelch and the meter circuit are tied together but both are driven out of AGC.
 
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What I've seen, and it sounds like you may have already checked this - but are your PIN diodes pair ok?

Check C33, a 1uF 50V part - it's part of that section - this may simply be a trickle charge unbalanced state.

What does that mean?

Well the S-Meter for the RX side, is weighted by a small signal - simply to keep a set to two diodes forward biased. There is a resistor in the PIN diode section that keeps this trickle charge floating. It helps quiet the RX, but then also is 1/2 of a voltage divider - when strong RF comes in, the Balancer uses diodes to rectify the current to take it out of the circuit - in doing so, allows an amp / switch to turn on and allow more current to pass into the PIN section as an effort to attenuate the RF signal.

It's R46, a 4.7K - this is the other part of that balancer - it's there so RF "noise" doesn't do any overkill on the PIN Diodes - so they can do the job to keep the RF amp quiet but working and seeking CB band - not just everything else...But, if C33 is shorted out - R46 is taken out of the circuit, and current flows into and thru that TR12/TR48 amp section and the S-meter sees this "trickle voltage".

I clued in on this when you said you had problems with the Squelch. So I had come across this before and it was really from a re-capper I had the blessings to work on. Dried out caps cause simple problems and when you try to solve them, your off on a wild goose chance tracking this "phantom" down only to find that a cap that is supposed to stay open - didn't...

Now maybe I have the circuit description wrong, but I know the C33 very well.
Cobra148GTLC33.jpg

Hope this helps!
 
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Your posts are always informative and I like how you explain the circuit functions. It gives me a better understanding of the problem at hand and that understanding is a vital troubleshooting skill. I was actually thinking about the "phantom" problem thing as well and decided to re cap the radio. I figured this to be best because the radio is in great cosmetic condition.... but has also presented itself as a challenge in it's original state. So I wanted to rule out any other "surprises". I did the re cap last night and found the s-meter is still moving upward in AM mode only. I will look at the circuit again this morning and see what I can find. Thanks, again.
-75
 
In RX mode (into a dummy load...so no signal) I have no voltage at the meter until the switch is in the S/RF position in AM mode only. I then read 98.864 mV at the meter. In TX it drops down to 74 mV. Mind you, that voltage I'm getting is the cause of my issue for the meter reading. That trickle would be there to deflect the needle for a incoming signal and during TX for needle deflection I would think.


Edit: I found VR1 adjustment will adjust the meter reading when I have it in the S/RF switch position without a signal at the antenna jack (in AM RX mode). I can turn the pot fully clockwise and it zero's the reading. VR2 does not. When I check the green wire from the S/RF CAL SWR switch to the meters positive side I am reading 15mV in AM mode (ground to negative side of the meter)....which is causing the meter to deflect in AM mode only. So where might that be leaking from?

Handy Andy... I checked everything in your above post and all the components are good. I re checked the PIN diodes as well.
 
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