Ranger 1 issues

[AB2EZ]

I'm surprised that you would see such a large 120Hz component on the B+ side of R23.

Correction... I'm not surprised, because the 120 Hz component of the 300V B+ supply would probably be around 1% (peak) = 3 volts peak.

Are you sure that this is not pickup from the probe? Try using a 0.1 uF capacitor to ground the probe (simulating the measurement at R23, but not actually touching R23). See if the 120 Hz sine wave goes away.

If you really have that much hum at 120 Hz on the B+ side of R23, see how much 120 Hz hum you have on the B+ side of R24.  Since R24 is only 47k ohms, the combination of C55 (0.1 uF) and R24 isn't really that good a filter at 120Hz... although it should provide some attenuation (about a factor of 4)... since its 3dB roll off frequency is 34 Hz.

If the 120 Hz signal you are seeing on the B+ side of R23 is real, y You might want to increase the capacitance of C55. That will remove reduce the 120 Hz signal on pin 6.

However, this does not explain why the hum becomes audible when the audio gain control is turned up.That still points to a problem in the first audio stage.

Since the "distorted" waveform goes away when you ground the mike input (still leaving 4.7k ohms between the grid and ground), check to see if R17 is still 1M ohm. If the grid leak resistor (R17) were open, that would mess up the biasing of the tube... but grounding the mike input would restore the biasing to a reasonable level.

Also try this (sort of a work-around).

Using a microphone that does not have to look into a very high impedance (i.e., almost anything except a D-104), put a 10k ohm resistor from mike input to ground (DC coupled) or (if easier) from pin 2 to ground. See if that fixes the hum problem (by lowering the impedance, at low frequencies, between the grid and ground)

Stu

[K5OG]

Good morning.
Checked the 1 meg resistor and it is high at 1.5 meg.  I'm feeding audio to the Ranger with a Daiwa RF-440 so impedance is not an issue.  Installed the 10K resistor and the noise is gone (I think).  I can't hear anything on the demod on my HP spectrum analyzer or on a receiver.  The waveform still has a very low level of AC (far less than 1%) but the highly distorted waveform is gone.  No time for air check until Thursday night but hopefully the reports will be good.
Thanks to everyone for all the helpful suggestions.
73 de K5OG
Fred
www.k5og.com