Capacitor sub?

[Brian G]

I have a Cobra 2000 I need to replace the 2 pf cap in the receive strip between L7,8,9,10. I believe it is C2.

I don't have one right now but I have a 1 and a 3 pf. Can I use either one of those or do I need to wait and order the 2? I can retune the coils if needed after.

Thanks,

 

[nomadradio]

I would just install the 3pf and repeak the slugs in those two coils. I'm pretty sure you'll be hard-pressed to see a difference.

73

 

[Brian G]

Great, thanks.

 

[Brian G]

I would just install the 3pf and repeak the slugs in those two coils. I'm pretty sure you'll be hard-pressed to see a difference.

73

Any idea why when I tune L4 which is the last 27 mhz can that it barely does anything? I can turn the core all the way down and all the way up and I really dont see any difference on the voltmeter, in the receive, etc. Really weird.... 5 and 6 definitely make a difference.

 

[Brian G]

I would just install the 3pf and repeak the slugs in those two coils. I'm pretty sure you'll be hard-pressed to see a difference.

73

And now something seems to be not right with my receive. Strong signals (like if I key a walk-in talkie near the radio) pin the needle fine. But otherwise there is little ground noise and lower to moderate stations are not moving the needle at all. I retuned all the coils according to the receiver alignment in the Sam’s facts but doesn’t seem to help. My other radio has about 7-9 Db of ground noise on the same antenna. I replaced FET 1 and TR 15.

 

[Brian G]

And now something seems to be not right with my receive. Strong signals (like if I key a walk-in talkie near the radio) pin the needle fine. But otherwise there is little ground noise and lower to moderate stations are not moving the needle at all. I retuned all the coils according to the receiver alignment in the Sam’s facts but doesn’t seem to help. My other radio has about 7-9 Db of ground noise on the same antenna. I replaced FET 1 and TR 15.

And I put the 3 pf cap in c47 as we discussed above. You think they could be hurting?

 

[Brian G]

I would just install the 3pf and repeak the slugs in those two coils. I'm pretty sure you'll be hard-pressed to see a difference.

73

You think maybe TR14 could be the culprit?

 

[Brian G]

I would just install the 3pf and repeak the slugs in those two coils. I'm pretty sure you'll be hard-pressed to see a difference.

73

Sorry another update. I tested voltages at TR14, 15 and FET1. All are normal. Do you think it could be for using the 3pf instead of 2? Maybe that circuit is really sensitive to the correct value?

 

[nomadradio]

If L4 doesn't show a peak, this suggests two possibilities. Either L4 is bad. Rare, but possible. Or a component connected to L4 is bad, preventing it from showing a peak.

D12, D13, D14 and D16 come to mind. Simply unhooking one leg of D12 and one leg of D13 would tell you if that circuit is causing trouble. If the sensitivity increases, and L4 now shows a peak, next task is identify the failed parts.

L4 should have a clear peak with the slug at least one or two turns below the rim of the hole, usually more. Until it does, the receiver will be weak.

I really don't think that concentrating on C41 is important.

Um, remind me. Do L5 and L6 both show a clear peak, WITH THE SLUG BELOW THE RIM OF THE HOLE?

Yeah, I'm shouting. Just because the peak position of those slugs is a diagnostic issue.

73

 

[Brian G]

If L4 doesn't show a peak, this suggests two possibilities. Either L4 is bad. Rare, but possible. Or a component connected to L4 is bad, preventing it from showing a peak.

D12, D13, D14 and D16 come to mind. Simply unhooking one leg of D12 and one leg of D13 would tell you if that circuit is causing trouble. If the sensitivity increases, and L4 now shows a peak, next task is identify the failed parts.

L4 should have a clear peak with the slug at least one or two turns below the rim of the hole, usually more. Until it does, the receiver will be weak.

I really don't think that concentrating on C41 is important.

Um, remind me. Do L5 and L6 both show a clear peak, WITH THE SLUG BELOW THE RIM OF THE HOLE?

Yeah, I'm shouting. Just because the peak position of those slugs is a diagnostic issue.

73

Yes both l5 and l6 both easily peak below the rim.

 

[Brian G]

If L4 doesn't show a peak, this suggests two possibilities. Either L4 is bad. Rare, but possible. Or a component connected to L4 is bad, preventing it from showing a peak.

D12, D13, D14 and D16 come to mind. Simply unhooking one leg of D12 and one leg of D13 would tell you if that circuit is causing trouble. If the sensitivity increases, and L4 now shows a peak, next task is identify the failed parts.

L4 should have a clear peak with the slug at least one or two turns below the rim of the hole, usually more. Until it does, the receiver will be weak.

I really don't think that concentrating on C41 is important.

Um, remind me. Do L5 and L6 both show a clear peak, WITH THE SLUG BELOW THE RIM OF THE HOLE?

Yeah, I'm shouting. Just because the peak position of those slugs is a diagnostic issue.

73

I was able to better tune l4 with those 2 diodes out. I have left them out for now. What does that suggest them if I can better tune it now? Do I replace all 4 of those? I have 14 and 16 but I’d have to order 12 and 13.

 

[nomadradio]

If you have a way to test them, that's a start. If they test as a short circuit, no need to look further.

And if they test okay, another possibility is that the AGC voltage is too high, causing the two PIN diodes D12 and D13. Less common that poofed diodes, but possible.

Solder D12 and D13 back into the circuit. Now unsolder one leg of D14 and lift it from the pad.

If this gets you the same improved behavior that taking loose D12 and D13 did, that means those two diodes are not bad, just being activated too soon before the signal strength is high enough. Those two MC301 PIN diodes reduce the signal level coming into the radio as a part of the receiver's automatic gain function, the AGC.

Much better if they turn out to be okay. And if soldering them in reduces the receiver sensitivity, even when D14 is loose, one or both of them is bad. Closest sub anyone might stock is NTE553.

73

 

[Brian G]

If you have a way to test them, that's a start. If they test as a short circuit, no need to look further.

And if they test okay, another possibility is that the AGC voltage is too high, causing the two PIN diodes D12 and D13. Less common that poofed diodes, but possible.

Solder D12 and D13 back into the circuit. Now unsolder one leg of D14 and lift it from the pad.

If this gets you the same improved behavior that taking loose D12 and D13 did, that means those two diodes are not bad, just being activated too soon before the signal strength is high enough. Those two MC301 PIN diodes reduce the signal level coming into the radio as a part of the receiver's automatic gain function, the AGC.

Much better if they turn out to be okay. And if soldering them in reduces the receiver sensitivity, even when D14 is loose, one or both of them is bad. Closest sub anyone might stock is NTE553.

73

So if it turns out D12 and 13 are ok after doing the above, then what ?

 

[nomadradio]

If disconnecting D14 got you the same boost in receiver sensitivity that unhooking D12/13 did, this points to the circuitry upstream from D14.

And if this is not how it plays out, not a lot of point to looking upstream of D14.

Reminds me of the old joke about a guy who comes out of a bar and finds and old drunk on his hands and knees on the sidewalk. Asks the guy "looking for something?" The old guy answers "lost my wallet". Next question is "where did you last see it". The old guy points down the block "Over there".

Gets asked "So why are you looking over here?"

Old guy responds "The light's better".

73

 

[Brian G]

If disconnecting D14 got you the same boost in receiver sensitivity that unhooking D12/13 did, this points to the circuitry upstream from D14.

And if this is not how it plays out, not a lot of point to looking upstream of D14.

Reminds me of the old joke about a guy who comes out of a bar and finds and old drunk on his hands and knees on the sidewalk. Asks the guy "looking for something?" The old guy answers "lost my wallet". Next question is "where did you last see it". The old guy points down the block "Over there".

Gets asked "So why are you looking over here?"

Old guy responds "The light's better".

73

Question- I had used a signal generator to try and tune the receive cans. One of the connections is injecting the signal at the antenna input. Is it possible to overload the signal when injected that it could toast those diodes or some other component near that area between L4 and the antenna jack if the injected signal gain was turned up to much?