What type of disc capacitor did you use as a replacement?? See:
It might be worth the effort to use metalized film caps in place of the ceramics for use in the audio stages.
Regards,
Jim
Logic: Method used to arrive at the wrong conclusion, with confidence.? Murphy
On Tuesday, April 23, 2024 at 01:29:42 PM CDT, Floyd - K8AC <floydsense@...> wrote:
HT-44 - one last query for ideas before moving on.? ?The problem is a lot of "noise" within the audio passband when in LSB and USB modes.? I previously had thought the problem was in the mic amplifier stages, but it seems that's not true.? The nature of the noise is best seen in the attached spectrum pictures.? These were all taken from a tap on the dummy load with the HT-44 adjusted for about 10 watts output.? The first two photos show the output in CW and AM mode.? The second and third show LSB and USB mode.? The noise appears in the active sideband and shifts to the other sideband when changing from LSB to USB or vice versa.? The mic gain is set to 3-4 with somewhere around 5 mv sine wave audio input at 1400 Hz.? Before discovering the noise, I made one SSB QSO and the report was excellent audio but with some noise in the background.
What's been done so far:? All new NOS tubes.? Replaced .01 and .001 caps in the area of V1, V2, V3, V4.? That was done after finding one defective disc cap in the audio chain output.? Biased off the audio chain by using a jumper between the bias line and the first mic amp stage as is done for CW mode.? The noise remained, so I assume that the problem is in the balanced modulator stage.? ?Looked at the output of T2 with the spectrum scope and the noise is present there, so ruled out a problem in subsequent RF stages.
The balanced modulator section is at least working as I can minimize the unwanted sideband fairly well, and CW and AM modes don't look bad.? I suppose it's possible that there's a noisy resistor or capacitor and will look more closely at that in the next few days.? Years ago I ran into a difficult problem in an HA-1 keyer and it turned out to be a poor ground through one of the riveted tube sockets.? Hopefully this isn't that problem.
Here's how the output looks on CW and AM.??
The horizontal lines are 10 dB, so pretty clean down to 50 dB or so.? Now for SSB:
LSB.? Carrier point under the red M, LSB to the left, USB to the right.? Audio input was 1400 Hz sine wave
Jim Whartenby