Ranger 1 gargling audio

[K6ZA]

Got a new used Ranger 1 recently. Solid stated, new caps, loads to spec. Traditional Johnson HV PTT mod by the book. No hum pickup through D-104 switching, audio sounds and looks good at low level, but as audio level is raised past 3-4  audio breaks into gargling-like sound oscillation. Any thoughts from the Johnson fans out there ?

Clarification (1/11): Oscillation only happens during modulation past 3-4. Also, replaced the 12ax7, 12au7, no change. 1614's (not replaced) run quite hot. Can't remember if they normally do. Tried different mic., Another D-104 with shorter cable. no change. If rf, not typical rf incursion symptom.

Will try cap testing next.

Thanks again for all your help,

Barry
K6ZA

[ke7trp]

Thats a tough one. Not much information.  Common issue would be a bad audio gain pot.  Every ranger I have had, gave me trouble with the pot.  I would change it out and cross your fingers. 

C

[K1JJ]

but as audio level is raised past 3-4  audio breaks into gargling-like sound oscillation. Any thoughts from the Johnson fans out there ?
Thanks,
Barry
K6ZA

Barry,

In addition to a bad pot, the audio oscillation could be the result of poor or inadequate capacitor bypassing somewhere in the audio stages.

Maybe you can use a cap checker to test each one.

Then try keying the rig, get the problem going and carefully clip lead in a similar value cap to each existing cap looking for a big change in the oscillation.  The caps you are looking for go from the circuit to chassis, but may be other caps too.

Be sure the caps you already replaced are polarized correctly and of proper value.

That's a start, anyway.

Tom, K1JJ

[W2PFY]

Sounds like you have RF in the audio. First thing to do is attach a wire to one of the base screws and then connect it to chassis ground. Then turn it up past where it begins to gargle. This wire need to be shorter than the cable coming out of the D-104. If that stops the oscillation, then you could shorten the D-104 cable or put on a better shielded cable.

[W3GMS]

Sounds like you have RF in the audio. First thing to do is attach a wire to one of the base screws and then connect it to chassis ground. Then turn it up past where it begins to gargle. This wire need to be shorter than the cable coming out of the D-104. If that stops the oscillation, then you could shorten the D-104 cable or put on a better shielded cable.

I agree with Terry.  It sounds like you have RF getting into the audio.  Stick a dummy load on the transmitter and see if you still have the problem. 

Bad
"decoupling caps" generally cause low frequency motor boating.  A lot of times folks will increase the size of the coupling caps and then have that issue.  The solution is better decoupling on the HV rail feeding the various plates resistors of the voltage amplifier stages.  Not doing that, you will likely have positive feedback from stage to stage and the resulting oscillation.

Good Luck and Congrats on picking up the Ranger!

Joe, W3GMS   

[K6ZA]

Thanks Joe et al.

Very much appreciate the comments. "Motor-boating" is a far better description of my issue then "gargling". I am using a dummy load on the bench. Can someone point me to specific decoupling caps to look at ? Audio is not my strong suit.javascript:void(0);

Barry
K6ZA

[K6ZA]

Found the problem. Appears I missed the C59a/b multi-cap. It was bad and the cause of the motor-boating. Audio is nice and clear now and the control works as expected. I want to say thanks again for the help. Hope I can reciprocate.

Barry
K6ZA

[W3GMS]

Fantastic Barry!  Glad you found the problem and the solution was an easy one.  Enjoy your Ranger.  I have one here and its one of my favorite transmitters. 
Fun to use, fun to modify and not to heavy!
73,
Joe, GMS