The AM Forum

I built the 6SN7 product detector for my nc-183d.  I am not quite sure of the operation though.  I have to turn the RF Gain down to about 2 for the audio to not be distorted.  I thought the idea of this was to be able to leave the RF at max and it boosted the BFO level up to that and mixed it and the IF together.

thanks

Tom - AB3FL

You have way too much IF injection, Tom.  Build up a capacitive divider.  10 pF in series from the IF line to the grid of the 6SN7.  220 pF to ground from the grid.  Play with the ratios til you get it right.  More shunt capactiance to ground yields less IF signal at the grid.

other problem could be not enough BFO injection. 

If it is a true product detector, low BFO injection should result in low audio output.  Turning off the BFO completely should result in zero audio output. Anything X 0 = 0.

If you disable the BFO and can still hear some signal coming through, that means you have intermodulation distortion causing it to partially function as an envelope detector. This will result in distorted audio in the product detection mode.

The stock built-in product detector in my 75A4 produces extremely poor audio on SSB.  With the outboard detector I was surprised at how good SSB audio can sound, if good audio is transmitted. Not sure if something in the stock PD is malfunctioning or if the design is just poor.  It is disabled right now, so I don't worry about it.

Of course, regarding the audio quality of SSB signals, you can't put lipstick on a pig and expect anything more than a pig, and garbage in = garbage out.

I still have too hot of an IF level.  I have a 1pf in series and a 390pf to ground.   Any other ideas.  I have a 47K between the two plates.  There is still too much gain on the IF.  Even with the BFO off, it does bleed through.


Tom - AB3FL

Hey Tom,

  Perhaps you could pull the IF from a preceeding stage. That
should knock it down a bit.

/Dan

Tom, are you using the NBFM socket in the NC-183 for this? 

You shouldn't have plate resistors at both sections of your tube.   The half of the tube that's seeing the IF input needs to have its plate bypassed to ground.

Any way you coult post the schematic you're using?

Yes, I am using the NBFM socket.  Here is the schizmatic I have from this site

http://amfone.net/Amforum/index.php?topic=24268.msg180391#msg180391


Tom - AB3FL

I may have figured out some of the distortion problems.  The (2) 6BE6s test weak.  These are the converters.  I have to lower the RF gain to about 5 for the audio to be undistorted even on AM


Tom - AB3FL

OK, you have an AVC problem somewhere.  Remember the AVC line is attached at that NBFM socket - you need to make sure you've disconnected the proper wires.  Also, as I recall, the IF signal at that toob socket is actually from the AVC amp tube.  If you messed something up at the socket you'll hack up the AVC action.  Did the AVC work OK before you built this?

That schematic oughta work.  I'd add a 22K or so in series with the 47K, so the left half of the 6SN7 has a bit of voltage drop to work against.  Also I'd bypass the plate of the left half with a .01-.05 to keep the IF signal from coupling back into the output.

Check yer voltages to make sure you don't have somethign messed up.  Should be 0V on the grids, between 2-10V on the cathode, 250V on the left plate and perhaps 200 on the right half.

I have been following this post, contemplating building it into a minibox to plug into the NBFM adaptor socket, and maybe adding a buffer stage to increase the BFO injection and decrease any chances of strong signals "pulling" the BFO frequency a little. I am thinking 12AU7 detector and adding a 6C4 as a buffer. (12AU7 ans a 6SN7 are basically the same tube, just different envelopes)


There is a very similar circuit in the 1963 ARRL handbook using a 12AU7.

OK...I may have found something.  Pin 7 of V8 (6AL5) detector reads almost -40V to -16V depending on wherer the RF gain control is.  Manual says -2.6V at 10 on RF gain and I have -16V at that position.  I have removed the product detector entirely for now.


Uhhh....voltages are with no signal.  Once I disconnected the antenna I got closer to the correct values.  What would happen if the 6BE6s were weak?


Tom - AB3FL  

My first experience with synchronous detection was using a receiver with the BFO too tightly coupled to the diode detector. Strong signals pulled the BFO into sync with the AM carrier.  I discovered it by accident once when I was trying to receive a piss weak AM signal through the atmospheric the background noise.  Tried turning on the BFO and zero-beating the carrier.  Suddenly the audio from the AM signal popped right out of the noise, at least 10 dB.  The AM  carrier had pulled the BFO on frequency and in phase with the carrier.  It worked, but was very touchy, and the AM carrier had to be practically zero beat to start with, then it would lock in phase.  It was amazing how much the audio came up out of the background as soon as lock occurred.

Generally not much except poor sensitivity and the HF oscillator maybe kicking out on parts of some bands. In that radio both are acting as converters which was a first for National in a higher end set and a dissapointment. Neither is on the AVC line but any tube with leakage can be causing the distortion. For improved signal handling replace them with 6BY6's.

Id opt for a tube swap starting with V-1 and concentrate on getting AM working properly and then follow JN's lead on the 6SN7.

Carl

You win the prize!  Even though V1 (6BA6) tested good on my tester, when I replaced it with another one, the AVC started working fine. 

Now I am still getting too much gain with my product detector, but I will work on that tomorrow

thanks all

Tom - AB3FL

OK, make sure that you can receive undistorted strong AM signals with the RF gain at full.

You may need to do a slight re-alignment of the BFO, last IF, and the tuned circuit in the plate of the AVC amp tube.

Yep...Did that.  I have WBCB 1490 about 1 mile from my house.  I think they are 1KW.  No distortion at all now.

Now how do I get the gain down on the product detector. 


Tom - AB3FL

OK here is what I have, and I have tried several 6SN7s

Left Plate 260V
Left Grid 0V
Right Plate 70V
Right grid 6V
Cathodes 6.5V

Tom - AB3FL

You gots an issue with the right grid.  Should be zero volts.  Make sure the grid leak is going to ground instead of the cathodes, and that there's no leakage in the grid blocking cap.

Also, I don't know how you're switching the audio from the AM detector to the product detector, while simultaneously turning on the BFO.  Or maybe you're just flipping the radio/phono switch and turning on the BFO.

Did you remove the audio wiring from pin 1 of the accessory socket?  That's the audio output from the AM detector and noise limiter.

Last, did you remove the wire from C88 (coupling cap at the plate of the BFO tube) at the 6AL5?  You should run that directly to the right grid of the 6SN7.   

IT WORKS!!!!

thanks jn

The problem was that I did not remove C88 from the 6AL5.

thanks

Tom - AB3FL

Quickie question to both of youse:

Do you have to crank the RF gain way back when listening to SSB, or does the BFO have enuff BA's to handle strong SSB signals without a bunch of distortion??

I did that mod in a HRO 60 and it works FB.  I rewired the switch so it turned on the Beef-O and switched the audio simultaneously, and also added a cap in the AVC line to slow the decay down a bit.

Johnny Novice strikes again. Now go downtown for a nice Greek dinner or watch a riot...its been too quiet in Athens recently....go start one ;D.

How was Christmas? Brian made it home with his wife and I had all 3 boys here for Christmas for the first time since 97. When you coming home next?

Xmas was pretty good, all things considered Carl.  Attended a party xmas eve with some colleagues including some of our Marines, then xmas day another party in my building this time with colleagues that all had little kids.  It was fun seeing the kids enthusiasm and excitement!  Christmas week was pretty tough in Athens, with multiple strikes affecting most of the transit system so commuting was a real bitch.

I'm coming back to the States in Jan/Feb for some training and leave either side.

Musta been great having everyone there.  I see the same thing in my near future - wayward sons going off to make their fortune in places unknown  ;D