symptoms of a bad amplifier pill oscillation?

[KD2GOE]

What are some of the symptoms of a bad amplifier pill? I am referring to my amp in an RCI-2990
I am getting full power on SSB, AM should be 50 watts high but will jump to 150 watts at 30% of the RF power on dead key. only on my or mine or my friends antenna, on my dummy load RF power is smoth all the way to 100%.. I can't think his Imax 2000 is faulty...

I took it in to a shop and they told it was all set. I think they never tested it on an antenna just the dummy load... just took it to a different shop told me he wont work on Galaxy's or ranger radios...

what other things can happen like self oscillation? when power is turned up?


If my amp is messed up I can do 1 of 4 things

1. Fix the amp
2. Get a RM Italy KL203 and replace it with that if it fits.. ran my friends and it seemed to work good...
3. put dual final CB in the radio. and run no amp... < may just be easier to run the KL203...
4 Sell the radio off and look for some thing different.... :angry: don't want to give up on this one....

 

[2RT307]

5. Don't talk on AM. :laugh:

Seriously, a local tech is working on these and the 2995's all the time, and it's always the finals giving people problems. It does sound like oscillation, but are you sure you're not getting RF back into the radio causing a squeal? Since it's not doing it with a dummy load, this could be something to look at, even though it did it on your buddies setup, too. I would check that angle... ground the rig, use ferrites on the coax, etc before replacing the finals.

73,
RT307

 

[KD2GOE]

Ok well I not hearing squeal through my Uniden when it happens to the RCI if that helps.
Every thing is grounded i was thinking of running 1 more ground to the radio from a copper water pipe..

I have 1 ferritet on the coax ill try some more

 

[hotrod]

What are some of the symptoms of a bad amplifier pill? I am referring to my amp in an RCI-2990
I am getting full power on SSB, AM should be 50 watts high but will jump to 150 watts at 30% of the RF power on dead key. only on my or mine or my friends antenna, on my dummy load RF power is smoth all the way to 100%.. I can't think his Imax 2000 is faulty...

im not completely sure what your asking. but i think your dead key adjustment is set
too high.what is your deadkey all way down on A.M.? i had 1 2990 and 2 2970 with same
setup as yours and AM POWER was 5-10 watts on low and 50-60 all way up
SSB POWER was 25 or 30 on low and 100-125 on high.i would only check this with
a dummy load ,meter, and 2 very short pieces of coax

 

[KD2GOE]

to 100%.. I can't think his Imax 2000 is faulty...

im not completely sure what your asking. but i think your dead key adjustment is set
too high.what is your deadkey all way down on A.M.? i had 1 2990 and 2 2970 with same
setup as yours and AM POWER was 5-10 watts on low and 50-60 all way up
SSB POWER was 25 or 30 on low and 100-125 on high.i would only check this with
a dummy load ,meter, and 2 very short pieces of coax

On my dummy load I get 7 watts on low and 50 on high
On the antenna I get 8 watts low and turning it up getting to about 30% on the dial it get to 50 watts quick the jumps to 150 watts.. I have tested this with a low pass filter before my meter.And with out. I have a video ill post it...

SSB is just like you listed

My SWR on low is 1.5 high 2.0
My friends antenna is 1.3 on low and 1.5 on high

 

[KD2GOE]

ok update it works fine on my friends A99

so I am going to say it is my A99

I did test my coax with a dummy load and that tested good...?

and after the cable it was still doing that crazy swr plugged right in to the antenna so I think it is safe to say there is some thing not right with my A99 GRRR!!!!! {Cry_river}

from low power to high his is 1.5:1 SWT MAX

 

[Torino]

I had a similar problem with my AR3500 100-watt model on my A99 antenna. Turned out it was crossover distortion. I was able to fix the problem by increasing the bias to the 2-pill final amp until the problem went away. Checking the amp's idle current (keyed with no rf input) afterward, it was about 4.3 amps. I increased the bias a little more to give it some margin, so my final bias setting gave me somewhere between 4.5 and 5 amps idle current. My AR3500 amp has a convenient adjustable bias setup. I don't know what kind of bias setup the RCI-2990 amp has, but if the bias is set for a low (or zero) idle current, crossover distortion could be the culprit.

 

[Shockwave]

You have perfectly described an amplifier that has self oscillation problems. They almost never show the symptom on a resistive dummy load but often show it on a resonant antenna. Just using a different length of coax can change the reactive properties enough to cause or stop this oscillation because the amplifier is so unstable. I'll bet the finals in the amp are low quality TO-220 or TO-3P cases and nothing like a real RF MOSFET capable of making significant power at 30 Mhz.

 

[KD2GOE]

this is the amp in the RCI-2990 it runs on 2 2SC2990 the 2 caps they where exploded so I put new ones in and the 22ohm resister on the right was burnet and I replaced that 2 but that's the old one in the pic...

I need to fined out what the adjustment in yellow is for it was burnet up..

 

[KD2GOE]

bump

I need to know what this trim pot and transistor control on this amp ...

 

[CrazyFin]

bump

I need to know what this trim pot and transistor control on this amp ...

I am so sorry to nekro post on such an old thread but instead of creating a new thread with identical question I thought I would post my question here.

Did you ever get info on what function the trimpot R901 and TR903 transistor (2SD1406) on the RF amp board on RCI-2990 has?

 

[nomadradio]

TR903 switches on the "idle current" base bias for the 2SC2290 amplifier transistors while transmitting, shutting it off in receive mode.

R901 is a trimpot that sets the zero-drive idle current for the linear transistors.

They didn't give you a way to put a current meter in line with the linear transistors to measure this. We take the red wire that feeds power to the amplifier board loose, and put the meter in line with it. Set the radio to either USB or LSB with the mike gain at zero. Key the mike and set the trimpot for minimum current. You'll be reading the relay's coil current now. Turn the trimpot up to 100 mA higher than that reading.

73

 

[CrazyFin]

@nomadradio:
Wonderful! Thanks for the info, all clear now on that part. I´ll check the current draw tonight.

I have 3 of the RCI-2990 equivalents (one Galaxy Saturn Turbo and two CRT Hercule Turbo) and all three needs various kinds of repair due to being severly modded and misaligned/"overtuned".

I have all the necessary measurement tools such as a Rigol DS2202 oscilloscope, Rigol DSA815TG spectrum analyzer, IFR 2945A radio comm test set, HP53131A freq counter, HP8904B all options function generator with dual tones and all kinds of smaller instruments for measurement and component testing. I also GPS based 10MHz freq standard that feeds 10MHz reference to all my instruments.

And this is just my hobby...

I've been able to fix some smaller issues that I've had but some others still remain. I´ll post them in separate threads though instead of messing this one up.

 

[Onelasttime]

For the record, the transistor is not what usually causes the oscillation itself when talking about bjt's normally. It is normally an issue with the rest of the circuit fix the circuit and the problem goes away. I can make an amp with perfectly fine transistors oscillate. Many so-called "builders" have issues out of the box especially on class c builds because they do not check each capacitor and match them before install. It is even worse when they use 3-4 cheap disc caps instead of one proper value cap or in place of silver dipped mica or metal clad's. Then each cap that is not with intolerance or is damaged by heat is multiplied by the number of caps being used.

Sometimes I think they are smoking crack on the job because they will not even bother to use the proper capacitance for a given transistor.

You pay for quality control and often cheap Chinese disk caps are so far out of specification and the OEM does not check. For the price, you are paying for the parts it is a given you need to do the quality control on your end.