WDØHCO wrote in message
...
Hello Again Thermo-man and everyone else. Well your on the right track.

There is a slight voltage variation of about 1/2 volt on the screen of
V3 that
can be detected when a strong signal is present, and a change of a couple
hundredths of a volt on the cathode.

There should be no voltage changes. Oscillators are very fragile things -
doesn't take much to get an amp to oscillate but keeping it on an exact
freq
is an art. Most military receivers have buffer amps after various
oscillators to keep things steady. This is why they have so many tubes !
Any
little change will affect frequency. SOMETHING is causing this voltage
change. What could it be ??

I don't know, but I tried jumping a different value resistor at this
circuit thereby greatly changing the voltage on the screen and there was no
freq shift.


I tried other values for C56 and find that the shift is less with less C
and
larger with more C.

Big clue! What your doing is adjusting the amount of coupling from the VFO
RF into the cathode circuit of V3A mixer tube. Well DUH!

But what most folks forget is that a cap is a two way RF door. Part of the
Main sig is coming back to the VFO the other way...

It travels down to the Colpitts circuit of C59 and C61 (both 1000 puff).
The
idea is to attenuate the main sig to ground (C61) while coupling the VFO
RF
to C56. If C59 or C61 values have changed then the main sig will couple
into
the VFO tube Cathode circuit (V3B) and cause all sorts of problems. It
could
also couple into the control grid -and- VFO LC circuit if C58 has changed
value (100 puff). You can also check resistors R34 and R32 but I really
don't think that's the problem.

I thought that was a good idea, so checked it out. Those caps are
ok, so no luck there.

I think next I will remove C56 entirely and see if the VFO is still
shifting.

This will prove that it's not a power supply problem. Your not coupled to
anything so the VFO should not shift frequency.

Removing C56 absolutely clears up the freq shifting alright.


NOW FOR SOME FUN...

You can couple the VFO to other parts of the Mixer tube. Heath has it
coupled to the Cathode of V3A but you don't have to have it that way....

What happens if you run a wire from C56 and plug it into the Plate ? Well
since an equal number of electrons are emitted from the Cathode as they
are
arriving at the Plate - I would expect no change in VFO
performance/problem.

What happens if you run a wire from C56 and plug it into the Grid Leak
circuit (Pin 3 6EA8 V3A)? Now your more isolated from the main sig but
your
VFO drive level is too high! You can compensate by using less cap in C56.

What happens if you run a wire from C56 and plug it into the Control Grid
circuit (Pin 2 6EA8 V3A)? Now your VFO is mixed with the main sig but at a
much lower level. Again, your VFO drive level is too high! You can
compensate by using less cap in C56. T2 bandpass limits the amount of VFO
passing back up to the V2A Het Mixer. The main sig level is too low to
bother the VFO unlike the Cathode injection point. This might be a good
alternative solution to your VFO problem.

So now your can change the VFO mixer level.

What if you could adjust the BFO and 1st HET oscillator levels too ? Well
you would have one heck of a receiver customized for present band
conditions, DX or local QSO's and even individual tubes! Lets face it -
all
factory circuits are a compromise of performance, tube values and
conditions
and operator likes and dislikes.

Actually this is a pretty good idea for a project if you don't mind
butching
up a set. And since you can find 16's for $25 to $50 bucks you wouldn't be
out by much if you blew it up.... hmmm when's the next hamfest???

-B


So I can't find anything wrong, its just bad design and it needs a
buffer. I live pretty close to W1AW and it is the main source of this rigs'
problem. None of my other rigs are bothered by strong signals except maybe
for the old S-38. So I built up a little source follower circuit with a
MPF-102 on a terminal strip, played with circuit values a bit and it stops
the freq shift just fine, BUT the output of the follower is too low. The
mixer does not give full output. So I guess I build an amp for that circuit,
or maybe put a tube cathode follower in there or throw the radio out the
window....................the saga continues...........