DTB Radio in Carlisle, PA

Inside the bipolar amplifier finals are multiple cells built upon a single Si substrate where each emitter has a fine gold wire bonded to a common point. As Mr great white north over-drove his amplifier into a likely poorly thought out and adjusted antenna system multiple small intensely hot spots developed until one or more emitter wires vaporized spewing metal ions out upon the substrate. This causes current flow to rise due to increased internal reverse C-B leakage. Resulting in a final which has reduced power gain at HF frequencies while exhibiting rapid thermal runaway resulting in added modes of self oscillation at VHF frequencies. Typical watt-meters for HF do not see these high frequency modes they merely indicate lower power than normal at 27 MHZ, a Spectrum analyzer rated upwards of 1GHZ will clearly show this 'phantom power' in the 300 MHZ range. The COR circuit in the amplifier however does see this self oscillation which begins due to rapid thermal runaway of the now leaky final. Resulting in an amplifier which keeps itself keyed up after a few seconds of operation. Cure is A - new final(s), B - repair the antenna system as well as properly matching the gear, and C - never sell an amplifier to someone unable to properly operate their station. Have another brewski eh. For clarification this references the operator with the bad amplifier not Big M who from reading his posts for years clearly does know how to run an amplifier.

 

Glad to see this thread returned from hijack land is back on track, thank you.

 

Phase angle of the cable is exactly 180 electrical degrees. That's the cable that was used on the bench during the tuning process which presented the perfect wave form. Any other length alters the waveform. And I'm sure you are correct about the input impedance not being exactly 50 ohms. That's precisely where creative engineering comes into play. And conventional wisdom gets tossed out the window.

 

Outlaw cb..I bought it from cb man and took it to dtb for a chk up..I never installed it and just for curiosity i had this other shop compare dtb's results..Were not the same results..I dont claim to know anything about radios and amps but im learning as i go..Now as for asticky trans..I was told it would stay in transmit mode for a few seconds after the mic key was released..It needed some cleaning..Staples under the pills were just that..I was looking over the other guys shoulder as he was showing me stuff and as he was cleaning before testing two new pills we saw the staples in that white cream stuff under the old pills..
I didnt blame dtb for anything but i just wonder how he got power from pills that were all but burnt out(i saw them test and compare)and didnt notice the sticky transmit issue..I only asked cause i was watching 2nd tech through entire process and asked questions....Im fairly sure i exained this and that it didnt need to be rehashed..Maybe go back over the thread and see what i said..

 

You were using what I like to term 'shade tree tweaknician' terms and if you were merely repeating what the 'tech' said, then consider the source. As to power measurement differences this is a combination of many things. Differences in impedance resistive and complex the amp saw at the time, varying meters, varying drive levels, varying power sources, bench wiring, etc.. What I did was explain events inside a failing final and why it will cause an amplifier to remain self keyed. As to reading the thread I have no time, as it is I barely post here once a year if even that. I do know staples under finals is sabotage by the installer of the finals, I have encountered it in the past. Concerning DTB I have no idea how his bench is set up nor what equipment he has. I do know from experience going back to the 80's his knowledge of electronic theory in radio is very good.

 

Using a different transmitter with a different output impedance the coax would show a different output. A better approach would be to tune the transmitter to 50 ohms output then tune the amp for a 50 ohm input. Now everything is a balanced.

leon
kc0iv

 

That sounds too much like work for most of these radio technicians trying to rush their customers out the door although I agree it's a great idea.

 

Id be interested to know what formula he used to calculate coax length.
300/center freq x v-factor is one I use myself. Is there another way of doing it? Or maybe more to it?

It was a good vid to show what can happen if not properly setup. Ive seen guys just plug'n'play, and cant figure out why they fry their equipment, or the ecm on their truck!

Just to add, thats where a good tuner between x-mitter and amp works well also. I know t-stars are notorious for having an input of something other than 50ohms, as im sure others are just as bad...

 

Speed of light in meters per second divided by frequency equals freespace wavelength in meters multiplied by velocity of cable tells you where to cut for 360 electrical degrees

 

same formula different wording