The AM Forum

Greetings gurus of the green.

Our next subject, a TX-1. Radio loaded up without issues, SWR and P.O. were good. When I began to transmit, was told I had a nasty hum on the audio signal. Listened to it through headphones and a monitor reciever, sure enough, the best way to describe it is that I sound like I'm talking into a fan (Like Chris Farley does in Tommy Boy) an even keeled warble. (I'll try and post a sample soon) It was suggested I had a bad filter cap. Wrestled it out of the cabinet and took a look underneath.

Well, every Electrolytic and Paper cap in it was original Benton Harbor issue. So with a little time and a little more cash (amd some help from Tom at Hayseed Hamfest) I have replaced every 'lytic and paper cap in the power supply and modulator section.

Got home from work and warmed it up to see if my problem was solved...alas no. Still the warbly "fan" hum on the audio. The rig is being run into a dummy load of appropriate size (1.5 Kw oil filled).

Although I only have a cheesy Heathkit TC-3 Tube Checker, I will begin to check all the tubes in the power supply and audio departments. While I have that underway, does anyone have any other suggestions as to what might be causing this? Rig was working FB from the time I got it a month ago till this issue sprang up.

Thanks in advance and 73's

With a scope, it should be relatively easy to track down. The alternative is, "guess the bad component", "guess the broken wire", "guess the etc". A scope is a very useful trouble shooting tool.

Those mic connectors sometimes have a habit of the shield being above ground at the connector should the nut loosen.  Check all that wire in the spiral shield to be sure the shield is grounded.

For the record, the mic connector was replaced with a 1/4" jack. And I need to find a reasonable Scope! Cover me, I'm goin' in...

What happens with no audio (i.e. in the CW position)?  If it is also present in CW mode it could be the VFO, keyer tube, or clamp tube; any heater to cathode leakage in these tubes will create the problem.

If it is in the audio stages, first see if it is affected by the gain control and this will help you to start isolating the problem.  Without a scope your best bet to trace it through the audio line is by pulling tubes (with the power off when you remove or insert tubes) to determine what is the originating stage.

Rodger WQ9E

...i had the same problem, years ago...that stupid pot under the rig, fer modulator resting current is junk...i replaced it with a similar value. i placed it on the chassis...as well, the pot on the front panel, fer mic gain, is actually the clipper. the pot behind the key jack, is the real audio gain. again, a poor plastic pot...try to eliminate these possibilities first. and remove the clipper toob as well..dont need it...
...tim...sk..

That is what was on the two that I found were not making good ground connections.  The hole in the chassis wasn't big enough to accomodate the jack mating with the front panel.

GL

My Apache was stripped out for a complete audio re-do with new tube sockets etc.. .and I started with the first tube circuit, a 12AX7 and was not happy with the hum out of the first two stages! I changed the jack, shielded mic cable and its routing, pot and tried single point and two point grounding, DC fills. etc... and never got anywhere. So I put it away for the year. I will try again this winter. I fear I have a ground loop on an unrelated power circuit or a transformer issue.

Mark, WA1QHQ promises to come over and show me where I am doing or measuring something wrong - HI.

Mike WU2D (On Travel)

For the record, this rig has major audio mods, clipper circut is LOOOOOONG gone. And I did find two bad toobs, but did not rectify the issue!

rip it all out n rebuild it the way you want it. Never waste time fixing someone elses idea of hta hey wabted the ig o be.   damn typing

I was happy with the audio, and it was a by the book mod...till this happened!