[Boatanchors] Receiver Amplifier Test Question

[Whitebear1122]

No response to my first request.  I looked at it again tonight, tracing out the circuit for the umpteenth time, making sure it’s wired correctly, and making measurements.

https://i.postimg.cc/VNvH9bPT/IMG-2797.jpg <https://i.postimg.cc/VNvH9bPT/IMG-2797.jpg>

Note that the receiver works quite well so the signal is getting through this RF amplifier, and the RF gain controls the overall gain.  I cannot measure the signal output at the V1B plate pin 1 nor at the coil L2A.  I have to measure it indirectly further down the chain.

I have the signal generator tied to the ANT and Ground connections, using capacitance coupling.  At the RF amplifier V1B pin 6 input grid, I can easily see the signal generator signal so I know the signal is getting there.  Pin 2 measures around 100 volts DC.  This is generated by the 212 VDC applied the 100K/100K voltage divider.

Filament voltage on pin 4 and ground on pin 5:  yep

RF gain control Cathode pin 3 gets its voltage through the RF gain potentiometer voltage divider. It is 212 volts applied to the 2000/68K voltage divider, and yields between zero volts to around 6 volts DC.  

Pin 1 plate output gets it’s voltage from the 212 VDC line as well.  This time it’s fed through an 1800 ohm current limiter, then L2B pins 1,5 of the coil, then a small hand wound RFC consisting of maybe 20 turns of wire, then to the tube pin 1.  I measure 212-ish volts DC at pin 1.  The AC signal at tube pin 1 or coil pin 5 looks pretty much trash.  I might see some minuscule waveform in the hash,  not even the size of the input grid signal, distorted.  I was expecting to measure a signal several times larger than the input grid signal.  

All I’m trying to do is visually see the amplified input signal on the output of the RF amplifier stage but I’m not.

I’m pretty sure I swapped the tube during a previous investigation but I didn’t do it today.  I will do that tomorrow and double check that it’s not due to the tube. 

I’d be grateful if one you experts could look at the schematic and either confirm what I should be seeing or pointing out where I am ignorant about tube operation….

Thank you!  73, Scott WA9WFA



> On May 14, 2020, at 6:15 PM, whitebear1122 at comcast.net wrote:
> 
> I have a test question that I hope you can shed some light on.  
> 
> On my HBR-13 receiver, the RF amplifier stage is a 6AZ8.  With a clearly
> visible signal going into the tube, I am unable to see an output signal on
> the plate of the 6AZ8.  I know the stage is working because I look at the
> signal further down in the 1610 KC IF stages.  When I adjust the RF gain
> control, it adjusts.  
> 
> What is puzzling to me is that I cannot see the signal at the output of the
> 6AZ8 or on the other side of the 1st mixer coil L2A.  What I did here was
> pull the 1st oscillator so I was only looking at the output of the 6AZ8 as
> it goes into the grid of the first mixer.  Is the oscilloscope probe loading
> the output?  I'm using a divide by 10 Tek scope probe and a Tek 2213 scope.
> What the heck am I doing wrong?
> 
> I've included a link to a photo of the this RF stage schematic.  HBR-13 RF
> Amplifier Schematic <https://i.postimg.cc/VNvH9bPT/IMG-2797.jpg> 
> 
> Here is a link to a few photos of the HBR-13.  Scott's HBR-13
> <https://postimg.cc/gallery/y7qmvfV> 
> 
> Thanks.  73, Scott WA9WFA