[Heathkit] Apache TX-1 Modulation

[Robert & Linda McGraw K4TAX]

Perhaps some can share their info on this issue.

To improve the audio I changed the coupling caps in the audio stages from 
510pf to 0.05 mfd and the 0.01 mfd to 0.05 mfd and removed the 6AL5 clipper 
from the socket.  Audio looks quite good in terms of frequency response.

However, in going through the "new" Apache TX-1 that I recently acquired I 
discovered a strange behavior.  When the modulation gets to near 100% on the 
positive peaks I see a small notch appear in the envelope.  Further 
investigation disclosed that the 0B2 regulators under the chassis, those 
that are in the screen circult are actuly firing.  It is at the point they 
fire that the notch appears.  Screen voltage on the 6146's is running about 
210 at this point.  I checked R29, the series screen dropping resistor, 
which in my transmitter is actually two 15K 10 watt R's and they are in 
tolerance.  All functions work as they should in CW and SSB modes.  Removing 
one of the 0B2's corrects the problem but then the screen voltage is not 
regulated for SSB AB1 operation.  I changed one of the 0B2's to a 0A3 and 
the problem is solved.  The positive modulation actually will now exceed 
100%

Any others observe this condition and if so, what were your findings and 
solutions.  Perhaps there's something I've overlooked.

Thanks in advance.

Bob, K4TAX