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Why does my pre-amp decrease signal instead of increase?

S

ShakyMcQuiver

Active Member
Apr 11, 2018
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So I dug out some of my old 2-pill amps today to play around with on my Cobra 2000.
The amps work fine; however when I flip the pre-amp switch, my S-meter will drop and get quieter (NOT louder) as it should.
Flip the pre-amp off, and all goes back to normal. I tried this on 2 different amps.

I don't normally use a pre-amp so it's not a big deal, but why aren't they increasing my receive instead of decreasing?
I've never ran an amp on my Cobra 2000, so maybe the last owner did something inside, I don't know (yet). I've used these amps before in my truck and they've always worked well (pre-amp too).

Thanks,
 
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A preamp that drives the S-meter down when it's turned on is just broken.

I blame it on the "SSB" switch found on most mobile linears. When the preamp is turned on, a relay places the preamplifier circuit in line with the linear's coax sockets. When you key the mike, there is a slight delay for the relay contacts to wake up and throw over to the transmit side. During that delay, a short burst of your transmit power is being crammed up the "rear orifice" of the preamp circuit. A glance at the tiny parts in the preamp will suggest that this short burst of RF power has to be pretty darned brief, or those tiny preamp parts will get toasted. The circuit will usually include at least one diode that's meant to "clamp" or absorb that short burst before it can reach sensitive parts.

But the SSB switch does more than delay the relay's return from transmit to receive mode. It also delays the "pull-in" time for the relay to activate. This means that the short burst of RF from your radio just got longer. My experience tells me that this is just long enough to blow out the preamp transistor and/or other parts.

Might not be what happened to your amplifier, but I have seen it over and over.

73
 
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Interesting. One of my amps (Desperado 100) has a manual SSB switch, but the other (Commander 150A) does not unless it's one of those automatic SSB type I read about. I don't know much about that amp and don't even remember where I got it.. lol. It just says, "Class AB linear amplifier".

Both are doing the same thing with the Cobra 2000 S-meter (decreasing signal). I wonder what the chances BOTH amps are doing the same exact thing?

I'll try and hook one of them up to a different radio and see what happens. The amps have been on the shelf for years, so who knows.

I also have a couple others (Texas Stars), but I haven't tried them.


Thank you for your reply!
 
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I tried a different (completely stock) radio on the amps and the pre-amp did the exact same thing. It caused the S-meter/signal to drop.

I will accept maybe one amp having some issues, but not two. I will try one of my others that are known to be good, but I'm betting I'll get the same issue. Using the Commander 150 amp, I've been talking with people from Hawaii to Florida and all have said my Cobra 2k sounds excellent, so that can be ruled out. Just don't know why the pre-amp is being wonky.

My SWR is excellent, so I can assume it's not an antenna or coax issue, but of course I'll check them anyway.

It's a mystery Scooby!
 
Any time a customer asks to have a broken preamp fixed in a linear, I quote an extra ninety bucks for that option. Bypassing it to prevent receive problems is no extra charge. I also point out that it will fail fairly soon, just like it did the first time.

I will also suggest that the only decent preamps are sold in a separate box. If you really need one, that's the way to go.

Out of the last several hundred linears repaired here, less than a third of them had a functioning preamp. People who build them won't spend what it takes to make a preamp robust and dependable. Wouldn't make the linear sell any better, or command a higher price, so there's no incentive to spend the money. Broken preamps are more the rule than the exception.

73
 
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nomadradio said:
Any time a customer asks to have a broken preamp fixed in a linear, I quote an extra ninety bucks for that option. Bypassing it to prevent receive problems is no extra charge. I also point out that it will fail fairly soon, just like it did the first time.

I will also suggest that the only decent preamps are sold in a separate box. If you really need one, that's the way to go.

Out of the last several hundred linears repaired here, less than a third of them had a functioning preamp. People who build them won't spend what it takes to make a preamp robust and dependable. Wouldn't make the linear sell any better, or command a higher price, so there's no incentive to spend the money. Broken preamps are more the rule than the exception.

73
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That's interesting Nomad. In all my years I've never had one fail on me. The one in my 667 that I've had since 1996 still works just fine, and I'm betting that amp is all of 35 years old. I've just had better luck I guess. (I'm not questioning you in the least. Experience is experience.)
 
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The CB friend 316 out of Wichita gave me this several years ago and it worked great with my Cobra 2000.
I think this is one of the separate boxes @nomadradio mentioned.
 
Keto said:
@nomadradio what this makes me think is if 1/3 or so preamps might fail, maybe I should leave it preamp off an amp purchase?
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Not sure where you're at? Pre-amp or not?
If your radio is capable of serving with sufficient RMS output then, No.
If your CB 4W radio, with a variable, 2-4W, then, Yes.

Really, what is the cost of the added pre-amp?

The finals might fail, the whole thing might go up in smoke :eek: Mites grow on chickens. Don't sleep with chickens.
 
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Alabama Buckeye said:
Not sure where you're at? Pre-amp or not?
If your radio is capable of serving with sufficient RMS output then, No.
If your CB 4W radio, with a variable, 2-4W, then, Yes.

The finals might fail, the whole thing might go up in smoke :eek: Mites grow on chickens. Don't sleep with chickens.
Click to expand...
Might, may, whatever the case have a good day.
I like my chicken cooked.
 
Keto said:
Might, may, whatever the case have a good day.
I like my chicken cooked.
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Oh, I see! The cost of replacing a single driver (pre-amp) transistor is "$90", holi Osama bin Biden :eek: I missed my calling! $90 to replace a MOSFET, sad that is :(

Yes, it's likely to fail again. Meat-heads put too much power to 'em! I'm told repeatedly, 2W, 2W, 2W...I usually keep it at 1.5.
 
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Alabama Buckeye said:
Oh, I see! The cost of replacing a single driver (pre-amp) transistor is "$90", holi Osama bin Biden :eek: I missed my calling!

Yes, it's likely to fail again. Meat-heads put too much power to 'em! I'm told repeatedly, 2W, 2W, 2W...I usually keep it at 1.5.
Click to expand...
I kind of was taught years ago 2w also. I'd kind of like to have good gear that I enjoy and be a little bit easy on it, make it last.
 

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