Assymetrical modulation and distortion on the RCA BTA-1S

[VE1IDX]

Well I am now starting to beat my head against the all on this one. I can get 100 % negative modulation but only about 50% positive before the modulation becomes distorted.I see an imbalance in the audio driver cathode current of the 2E26's that drive the 4-400's. One runs 2.8 mA the other 5.7 mA when they should both run 3.5-4.5 mA each.All indicators including the trouble shooting guide in the manual point to a bad resistor or capacitor in the feedback ladder. I checked each component twice in each of the FB ladders and everything checks out fine. I even swapped the audio drivers and the mods to see if the problem changed but no luck. I even switched the polarity of the incoming audio. The problem is still with the positive peaks crapping out over 50% mod level.In all my 22 years in the broadcash biz I have never had a problem with a feedback ladder. What am I missing? Anybody encounter this problem before? Everything seems to check out fine at least with the power off. With the audio driver cathode current being unbalanced it does look like FB ladder problems or could it be an input transformer problem? One side with shorted turns? I need some advice so I can take this head on again on my next days off starting next Wednesday.

[Bacon, WA3WDR]

Try disconnecting the feedback, and cut your gain way back and try some more.  If the problem remains, it will probably be worse, and that would mean it's not the feedback but maybe a bad transformer, a bad driver tube etc.  But if the problem basically goes away, then it is the feedback.

[kc2ifr]

Had almost the same problem with my 500. Its highly modified using 572b's as modulators and the 572b's are driven by 5881's cathode followers. The bias on one of 572's went way negative and caused the same problem.

[flintstone mop]

Is it the original mod iron? I had a 250 GE Tx that did not have the original mod transformer and it did the same thing. It turned out that it needed the Heising choke/cap to take the DC off of the primary. The orginal iron could handle the DC and current. Some one just jammed something that half assd worked in there.
Fred

[W3SLK]

Bacon said:

Try disconnecting the feedback, and cut your gain way back and try some more.  If the problem remains, it will probably be worse, and that would mean it's not the feedback but maybe a bad transformer, a bad driver tube etc.  But if the problem basically goes away, then it is the feedback.

I will admit that I never worked on one but I have to agree with Bacon on this one. It sounds like some kind of impedance unbalance in the driving circuit. Instead of swapping drivers, swap the leads on the driving Xformer if possible. If you wind up with 100% positive vs. 50% negative, then you know you found the problem. Gud Luck.

[VE1IDX]

Thanks I'll try unhooking the feedback ladder and see what happens. The TX is still all original.I want it working 100% before I convert to 80m.On a good note I was just given a brand-new-never-taken-out-of-the-shipping crate modulation transformer for it. Dave VE1UJ,my former boss, happened onto a fellow retired broadcast engineer who had a brand new xmfr in his basement left over from waaaay back when.It is the original RCA BTA-1S mod iron so I now have a spare just in case.

[VE1IDX]

How lucky can you get!?

I don't know but I did just run out and buy a lottery ticket after that.