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

[Keto]

"Keto", I'm still trying to figure what a Keto diet is No pasta but lard is good?
I'm with you on preserving the equipment for the long term.

Actually I think it's low on carbohydrates isn't it.
I just had a workout partner several years ago that nicknamed me Keto.
I can't tell you much about a Keto diet, except mine, I had breakfast burritos this morning. Haha

 

[Recon]

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

Is there such a thing as a separate / stand-alone pre-amp without an amplifier?

 

[TM86]

Is there such a thing as a separate / stand-alone pre-amp without an amplifier?

Yes. More than a few manufacturers churned them out. I've got one made by Pride that I got as a package deal with a radio. CPI had the "Range Plus" and they built a preamp into their BC-2000 base console. Pretty sure there's others although none come to mind at the moment.

 

[Keto]

Is there such a thing as a separate / stand-alone pre-amp without an amplifier?

Mine on post #10 pulls a week station in, better with some radios than others.
Might not be called a pre amp but worked great with a cobra 2000.

 

[ShakyMcQuiver]

Ok.. I'm about ready to admit defeat and agree that it's the pre-amp itself has failed and not something else... Just bugs me that it's not happening on one of my amps, but two. No only that, these amps have been sitting in my closet for years.

So...

1. How does the pre-amp fail while sitting <un-powered> in a closet?

I had an old Stihl Chainsaw do that... the magneto nuked itself somehow, but apparently that used to happen on that particular model. $200 later and I got it running again (then promptly sold it).

We had no EMP's that I'm aware of or any other electrical devastation in or near my closet, so how do they sit and break?

2. What component(s) typically go bad when the pre-amp fails? I'm only guessing it's a bad/broken solder joint, dirty contact, etc.

No, I haven't opened the amps up yet. Just wondering if a giant hand will come out and slap me upside the head and say LOOK HERE! ?

LOL!

I know... probably not worth bitching about. I don't use the pre-amp that often. I just don't like broken stuff. I'm sorta OCD like that. I can always buy an external pre-amp. Fleabay has some that bolt inside your radio as well as others that are stand-alone, or build my own.

Thanks for all the replies on this one!

 

[SuperLid]

Ok.. I'm about ready to admit defeat and agree that it's the pre-amp itself has failed and not something else... Just bugs me that it's not happening on one of my amps, but two. No only that, these amps have been sitting in my closet for years.

So...

1. How does the pre-amp fail while sitting <un-powered> in a closet?

I had an old Stihl Chainsaw do that... the magneto nuked itself somehow, but apparently that used to happen on that particular model. $200 later and I got it running again (then promptly sold it).

We had no EMP's that I'm aware of or any other electrical devastation in or near my closet, so how do they sit and break?

2. What component(s) typically go bad when the pre-amp fails? I'm only guessing it's a bad/broken solder joint, dirty contact, etc.

No, I haven't opened the amps up yet. Just wondering if a giant hand will come out and slap me upside the head and say LOOK HERE! ?

LOL!

I know... probably not worth bitching about. I don't use the pre-amp that often. I just don't like broken stuff. I'm sorta OCD like that. I can always buy an external pre-amp. Fleabay has some that bolt inside your radio as well as others that are stand-alone, or build my own.

Thanks for all the replies on this one!

The RF amplifier transistor and the relay are the most common failures. Usually, it is oxidation of the relay contacts or TX RF leakage. Most of the pre-amps I have come across are too noisy. I have modified a few of my own, with low noise figure devices, that actually work. I don't use them, I just did it because I can.