Base Linear SWR problem

[TonyV225]

I have an Amp here its a 110 AC Base Amp its kind of a Palomar Style it says GB on the front with a 4 position power setting knob. What it has for Final Transistors is 4 / MRF-492. The problem is no matter what radio I have inline the SWR is sky rocket high it belongs to a friend and it was doing this for him aswell what would cause this out of tune Biasing?? Where do I start its a mind boggling problem

 

[mackmobile43]

maybe something gone short in the keying circuit, transistor or a bad 239.

Do you have recieve when the amp is turned on?

 

[Beetle]

Are you using a good 50 ohm dummy load for testing? If not, get one and see what the result is.

My thinking is that a dummy load will look like 50 ohms over a VERY wide spectrum, while an actual, properly-adjusted on-the-air antenna will look like 50 ohms over a much narrower spectrum. Any spurs or harmonics on the signal that are outside the antenna's comfort zone will be reflected and will make things look like you have a high SWR -- which you would. A 50-ohm dummy load, fed with 50-ohm coax, should show very close to 1:1 on SWR at ANY frequency or group of frequencies.

If you have access to a spectrum analyzer (not just an oscilloscope), it might be interesting to take a look at just what this "amplifier" is doing.

 

[TonyV225]

Yes the Amp recieves and transmits fine other than that sky rocket off the charts SWR even the preamp works good on this thing its just a weird thing happening it shows high SWR in the radio and in the External meter between the Amp and Antenna. I remember years ago I had this happen with an old Pride 150 mobile Amplifier and there was a tuning setting in it for matching the Amp to the radio.

 

[mackmobile43]

Yes the Amp recieves and transmits fine other than that sky rocket off the charts SWR even the preamp works good on this thing its just a weird thing happening it shows high SWR in the radio and in the External meter between the Amp and Antenna. I remember years ago I had this happen with an old Pride 150 mobile Amplifier and there was a tuning setting in it for matching the Amp to the radio.

Well I guess you were'nt clear in your first post as to where the swr was and now you say it's showing up as input swr which would indicate mistuning in the input side of your amp and to check it there you'll need a half wave of 50 ohm coax from the meter to the amp and a good 50 ohm dummy load after the amp and make sure the meter is connected directly to the back of the radio by means of a double male pl-259 and then let us now what you're seeing.

If there is a compression trimmer on the input transformers someone may have had a twittle with it,(twittle) is a bob85 term for turning something they did'nt know what they were turning for and if it's a factory trimmer usually there will be some paint on the tuning screw there and it will show if it's been 'twittled' with.

 

[TonyV225]

Beetle I have to figure out who made this thing for starters LOL. It never ends with these things theres so many made and ones that are no longer made and the guy that owns it wants me to see if I can find out who made it. Ill have to send a pic to one of the Amp builders Greg or Adam and maybe they will have an idea or may have seen one.

 

[mackmobile43]

Beetle I have to figure out who made this thing for starters LOL. It never ends with these things theres so many made and ones that are no longer made and the guy that owns it wants me to see if I can find out who made it. Ill have to send a pic to one of the Amp builders Greg or Adam and maybe they will have an idea or may have seen one.

what's the big secret? why can't you post it here?

 

[Beetle]

Tony - just so it's clear in my mind (increasingly a poor assumption to make): Are you talking about sky-high SWR between the radio and the input to the amplifier, or between output from the amplifier to the antenna?

 

[TonyV225]

Im pretty sure it was in both spots I can double check it but as I said Im pretty sure it was in the radio and after the Amplifier