[AMPS] amplifier help

Tue, 15 Jan 2002 22:07:56 -0800

Hi Ron,
-
:The Amp is from a Ranger 2990? ...the 100 watt unit....so 
:much for quality but it should be useable. 
-
Not without a bit of work... Ranger stuff is notorious 
for poor quality and performance.  They started out 
making really broad band 12/11/10 meter multi mode 
radios and are now pushing similar products for 
hams like a current six meter version.   
I don't really think much of their cosmetics (radio 
ergonomics..?) but aside from that, every one I've 
played with at a friends house or car has had a 
stinky receiver (serious front end problems) . 
-
: I would think that the amp would be considered HF 
: but used at or near 30 Mhz.
: The "pills" (why do they call them that) run on 12 to 
: 15 volts and the drive is applied using a 50 ohm line.  
- 
Pills are the stupid name used by the CB crowd for RF 
transistors.  
-
: I placed the LPF in the 50 Ohm line but I think that is 
: wrong in that the impedance network is located
: on the driver section and so the line is terminated 
: in the input transformer.  It might be close but I get 
: different settings when loading the driver into 50 
: ohms vs the input transformer.
: Both input and output are push pull.  I  will look again 
: but I think there is not a stitch of filtering on the output.  
: If there is it is just an LC single pole.
- 
Reads like a common HF emitter balasted push pull 
design.  First, put the driver into a 50 ohm termination 
and look at it's output.  If you don't have a clean 
driver section... throw it away and get on with your 
life.  
- 
On with the circuit...
The first input transformer provides a step down 
impedance ratio, separate drive signals for 
each transistor base and a method to provide a 
the base bias current which should be voltage 
regulated.  One often sees active voltage regulator 
circuits, but I do have a diode current mirror 
circuit that works well.  It ends up being some large 
resistors, bypass caps and a silicon diode.  
- 
In your repair work on/with the amplifier, you can 
make up a simple half or 1 dB pad to install/insert 
at the amplifier stage input. It will provide a 
relative measure of stability while you trouble shoot 
the amplifier circuit. 
-
The zero signal bias current for the transistors are 
set using the bias supply and is set to a proper 
mfgrs data sheet value.  Going the real extra mile 
includes adjustments of the bias supply in operation 
for best practical working performance (lowest 
distortion). The best bias supply setting is often 
different for static, low drive and active full tilt 
operation. 
- 
Your other post mentions large values of zero 
signal collector current. Something is wrong, it's 
probably the base bias circuit or it's adjustment 
or an oscillation problem. 
Most likely the circuit has a problem which 
might have been the reason for the previous 
device(s) failure(s). 
-
Oscillation can also cause large (excessive) Collector
current (Ic) values.. obviously this is an unwanted signal. 
The output of the amplifier should be stable into a 
termination and slightly reactive loads as found in 
most common antenna systems. 
- 
A series RC network is often used to provide feedback 
in broad band amplifiers. The gain of broad band 
3-30 stages is often much higher at the 3 Mhz end 
of operation.  It's common to see feedback to reduce 
gain a lower frequencies and provide an overall "flat" 
amplifier gain to the end user.  
-
The "chip caps" at or near the base of most 
transistor amplifiers need careful attention for 
proper type, value and condition.  This is a very very 
big deal.  Many amplifiers I've worked on have 
serious short comings in this very important area.  
Capacitors have RF resistance, dilectric Q and 
dissipation factor (DF)which should always be 
carefully considered.  Shortcuts can end up as 
excessive heat...  I have seen more than one 
amplifier with self de soldering caps around the 
transistor.  
- 
The output transformer provides step up back 
towards 50 ohms impedance and an easy 
method to Apply the collector voltage.  One will 
often see silver mica caps or arco style trimmer 
caps on one or more of the transformer. From 
my experience, the arco type trimmer caps are 
common problem generators.  
- 
I've got a nice little 2 pole low pass filter for use 
at 10 meters. It fits into a small space and will 
handle 150 watts no problem... drop me a note 
direct if you need details. 
-  
So you have amplifier problems from IMO 
poor design and problematic parts.  Service 
manuals for that unit can be had easy enough. 
If you can find one and share a copy, I'll walk you 
through some of the repair issues as time 
allows. 
- 
Your results will probably vary..
- 
cheers
skipp 
-
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