The AM Forum

I seem to have a wierd issue that just started with my apache.  When operating 75M, I would have the plate at about 7 and the load at about 2.  Now the plate is still at 7, but the load is all the way CW, which is min capacitance.  The other bands have the plate and loading where they always were.  Any ideas?



Tom - AB3FL

How about that antenna and all the line that connects to it?  Have they changed any?  Heard any arcs or sparks lately?  If so where were they?

all feedlines are good,  checked the antenna and balun(it is an OCF doublet 132') all good.  Did an on air check on 3885 and was 40 over in Long Island.  No arcs, sparks, or zorches.


All connections inside the HV/RF cage are good.


Tom - AB3FL

Forget the feedline, antenna, etc. for now. Put a dummy load on it and check the operation on 75 and the other bands. No dummy load; use a 100 watt light bulb. Check or disconnect the RF Choke, 2.5 mh, hanging on the output side of the tank coil. Verify the linkages are all engaging the switches on the proper bands. Make sure the forms holding the plate tank coil haven't melted and caused the individual coil wires to touch. You need to determine if the problem is in the rig first.

What kind of balun?  It may be bad or going to be soon.  Dump it and see what happens.

here is the balun: http://www.balundesigns.com/servlet/the-75/OCF-balun-4-cln-1/Detail

I checked that aand it looks fine.  I will have to hook up the 100W light bulb today

If you have operated very far off center frequency of that antenna then you can have damage that you cannot see.  If you have an MFJ 249 then use it to check that balun.  That will give you an indication if it has trouble.

As Pete said, load into a dummy load to see if your transmitter has damage.  Take steps that are easy to do first and move up the ladder of complexity.

Did the old buzzard 100W light bulb dummy load....same thing....Then for the hell of it i took the low pass filter out of the circuit......BINGO!   I will have to take it apart and look at it.  Luckily no more analog TV

thanks all

tom - AB3FL

Them damn Lopez filters wil getcha every time!!! :P

Congrats on finding the problem.  If you don't need the filter, ditch it.  It has insertion loss and can give problems like this.  You may have had a lightning strike nearby that punctured one of the caps, or high voltage at the antenna could have done the damage.

I took the filter apart.  It is an old buzzard Barker & Williamson model 425 made in Upper Darby, PA.  There are NO caps in it, just a bunch of coils made with 12 guage wire.  Everything looked good.  oh well, don't need  it

Tom - AB3FL

A good rule of thumb when testing a transmitter for a possible perceived problem is to remove all ancillary hardware connected to it (antenna, tuner, scope, filters, relays, microphone, etc.). If then determine the transmitter is not the problem, you can start adding back the ancillary hardware to see if one of them is causing the problem. Can save you a lot of time and frustration in trouble shooting the problem.