[HBR] Another Receiver Project -- HBR-4, Part 10

[waltah at earthlink.net]

The AGC problems seem to be straightened out.   The big problem 
was slight gas in two of my three 6EH7 IF tubes.   I need to pull 
out the tube tester and go through all of the spares.  

The one month HBR  had the same problem so I was a little 
quicker to try substituting tubes this time.   The symptom is that 
the AGC buss doesn't want to sit near zero ... either it goes to 
positive voltage (from the grid of the gassy tube) or enough negative 
voltage that that tube doesn't flow positive current.   But of course it 
takes a while to focus on the symptoms.

40 meters is working.   Adding the other bands is harder than I 
expected because there are wrong crystal modes and wrong 
mixing products that aren't that far off.   Basically you have to set it 
all up with a GDO before turning on the power.   Guess how I 
learned that?

Getting rid of the gassy tubes seems to have mostly fixed the filter 
passband.   Guess one of the bad tubes was loading the filter.  

I'm actually using two filters -- one in the usual place, right behind 
the mixer, and the other at the end of the IF, right before the 
detectors.   Haven't checked the skirts yet or the ultimate rejection 
but they should be good enough!

The passband was four bumps, symmetrical, with the center two 
maybe 6 db higher than the sides.   Now it's nearly flat -- just four 
tiny bumps.

It's the easiest receiver to tune on an SSB signal of any I've used.   
I think that's because the product detector is so good that you 
don't have extra voice harmonics to get right. 

The crystal oscillator is still a problem.   Jim can you describe the 
standard circuit you mentioned?   I'm going to try changing that; 
there are too many 'issues' with what I'm doing.

When I have a CO that works well, I'll set about optimizing the 
output level -- sensitivity vs. spurious signals.

The new push-pull VFO has been no problem at all.  It drifted 
steadily downward for a few hundred CPS and the second try at 
compensation -- a 3 mmf command set N750 ceramic cap on the 
6J6 socket -- pretty nearly eliminated that.   I'll probably rewire that 
socket -- there are so many parts there that access to make 
changes is quite difficult and I really should shorten the cap leads 
to get more and faster compensation.

There are always surprises -- usually the crystal oscillators are 
trivial and the VFO is a problem.

Walt
KJ4KV