SWR is fine then spikes with amp on

[Turbo T]

Check this out...so I decided to do an SWR check on my CB set up, which currently consists of a 108 whip mounted dead middle of my truck, my Gemtronics GT-44 that no one wanted, and my Bimbo 100 amp.

I did an SWR check w/o the amp on, on channel 1 I got 1.2:1, channel 19 I got a 1.1:1 SWR and on ch 40 I got 1.0:1.

Turned on the amp and rechecked, I found on channel 40, with the amp on, as soon as I key up, the SWR pegs to 3. On channels 1 and 19, it reads 1.4:1/1.3:1, then after 10 seconds suddenly the SWR's peg past 3.

Only thing I can think of is if there's some sort of feedback causing it, but I had the garage 23 channel CB on 19 when I tested channel 19 with the amp on, and didn't hear any squealing/feedback.

Also the mic I'm using is a 600 ohm stock Midland mic, and I don't have talkback or mic gain on the GT-44.

Any ideas?

Thanx in advance

 

[IRock817]

You did recalibrate the meter when keying with the amp on?op:

 

[Turbo T]

Yes it did

 

[Shockwave]

The problem is 100% for sure a defective amplifier. It breaks into self oscillation after it's keyed. That oscillation is not on the fundamental frequency so it causes a sharp rise in reflected power. Most of its output power is also being wasted since only a portion of it is now on the desired frequency. The causes and repairs for this problem are well outlined in other treads on this forum. One was titled something like "higher SWR with more power added".

 

[Beetle]

The amplifier, OR something in the antenna circuit that's breaking down under the higher voltage.

 

[Captain Kilowatt]

The amplifier, OR something in the antenna circuit that's breaking down under the higher voltage.

I had that happen once on 160m. I found out it was the inductor in my tuner heating up. I found out it was heating up when I finally heard a loud SNAP!! and saw smoke coming out of it. The SWR was good at first key up and then gradually went south as I held the key down.

 

[Turbo T]

Here's an interesting twist...first I want to point out the Bimbo has a switch for choosing RMS power or PEP.

I decided to try my Uniden Pro 510 XL on this amp. My 510 is stock, D14 mod diode removed, and has a stock mic.

With the amp is the RMS position, when I key up the mic, I show no deadkey on the meter, yet when I speak into the mic I get crazy swing.

Furthermore I can also keep the radio deadkeyed as long as I want and the needle never goes even past 1.1:1.

Now if I put it in PEP and key up, I get squealing and the needle pegs out.


Now on my GT-44, with it in RMS, when I key up, I show a deadkey, and then it eventually drifts into the red on the SWR meter.

On PEP I get the same squeal issue as I do on the 510.

Any ideas why I don't get a deadkey on the 510 like I do the GT44, yet the SWR's never go up on the 510 if it's kept in RMS?

 

[Shockwave]

Classic solid state amplifier self oscillation symptoms. Pad the input trough some resistors (low power switch) and it breaks the oscillation. You could try checking the antenna connectors but I'm nearly certain this problem is within your amp. Seen it a thousand times before.

 

[Turbo T]

Thanks, I'm still digging on what to do to rectify this problem...if there's something inside the amp I need to replace or what.

 

[psycho]

Thanks, I'm still digging on what to do to rectify this problem...if there's something inside the amp I need to replace or what.

Buy the Bimbo a new car and the problems will stop. :laugh:
Sounds like it's time to put it on ebay as a Vintage, Classic, Old School, linear amplifier with a Buy it Now of $200 or best offer.
Now you know why Messengers are so desireable.

 

[Turbo T]

Actually I'd rather fix the amp.

 

[Shockwave]

Actually I'd rather fix the amp.

Start checking out things like the negative feedback loop which may be non existent in this amp and you'll be on the right track.

 

[Turbo T]

I'm going to presume it's non existent unless someone can spot it in the pics.

 

[555]

can you post a pic of how you have the antenna mounted and where

 

[Turbo T]

I have the antenna mounted to the tool box in the bed.