HB ATU puzzler

[G4YVM]

Ive built several ATUs before, and if needs be I can strip this one down and start again, but I thought I'd ask first...

I recently bought a HUGE ATU in a SK sale, nicely made, silver roller coaster, two big caps, RF sniffer to drive the SWR and power meter etc etc. BUT it doesnt tune anything! Ive cleaned the coaster and checked all the joints...by and large, not a jot of difference no matter what I do. Ive tried other ATUs here in the shack and they work well, they'll tune and de tune a 20m dipole quite convincingly.

Oddly, the SWR start at a certain point then sloooowyl creeps up when connected to the "new" ATU. I cant figure that one. Its as if capacitance is changing, but I cant see how. It may be a red herring of course, the antenna might be doing that somehow.

Well there we are, before I treat this as a pile of components and start over, anyone got any thoughts?


David G4YVM

[WB3BEL]

If it were mine, I'd start out by drawing up the schematic. I'd look it over and see if it makes sense.

I'd hook up my antenna analyzer to the radio side and hook up a dummy load to the antenna side and see if I can get a match. If not, I'd look for bad connections. If the 50 Ohm match test is successful, then I'd insert a power meter in between the tuner and dummy load and connect a transceiver to the radio side and measure the insertion loss of the tuner. If that looks reasonably low, certainly much less than 1 dB loss, then I would connect some other resistor loads to the antenna port and drive the tuner with the antenna analyzer as the source. Maybe some values like 10 Ohm, 25 Ohm, 100 Ohm, 200 Ohm 400 Ohm, etc. See what range it will match.

I'd suspect bad wiring or an open connection. The roller inductor contacts may not be reliable. You can measure the reactance of the inductor by itself if you want by using an antenna analyzer. Likewise you can measure the reactance of the capacitors. Even if all that looks good with the low power of the antenna analyzer, you may find some problem at higher power if the connections are not clean or lossy.

[G4YVM]

Thanks for the help. I have added a 50ohm dl to the output and it tunes fine (more accurately, de tunes fine). I'll try the power meter as suggested then different loads.

Thank you.

David

[KE3WD]

...Oddly, the SWR start at a certain point then sloooowyl creeps up when connected to the "new" ATU. I cant figure that one. Its as if capacitance is changing, but I cant see how. It may be a red herring of course, the antenna might be doing that somehow....

Hi David,

If you have other Antenna Matching Networks that do work with the same rig and antenna, then suspect this tuner.

What you describe could easily be the result of a bad connection, a dry joint (as you chaps call it, we yankees call it a "cold solder joint") or a mechanical disconnect that may be in the Capacitor wiper brushes, indicating that a good cleaning of just the brush wiping area is in order, my personal favorite substance for doing that is to use a small amount of denatured alcohol delivered just at the brush points and then rotate the capacitor back and forth full travel while the alcohol is still wetting the contacts. The reason I prefer the denatured alcohol it that it will thoroughly evaporate, leaving no residue that may serve to detune the capacitor.

Also check the S0-239 connectors for tight connections at the ground screws to chassis, as well as the center conductor should be inspected for possible stretching that does not make good contact with your PL-259s.

I wouldn't think that there would be a need for teardown and start over here, provided the thing worked properly at one time, which may be the unknown here, so the suggestions to trace the circuit and draw the schematic is something I would also do. Being able to examine the schematic should also allow for the ability to analyse the thing and compare to known working designs as well.

73

[KO6WB]

Opps, never mind
73
Gary