[Collins] 51J-4 1st IF problem

[Dr. Gerald N. Johnson]

On Sun, 2008-09-28 at 13:40 -0400, Mark Croney wrote:
> I am working on a 51J-4 problem and I thought I would share this with
> the group and see if anyone can offer some insights.
> 
>  
> 
> The symptom is that there is an intermittent rise and drop in signal
> strength.  While listening to what should be a steady signal, the
> signal strength meter can jump to different signal strengths
> intermittently.  
> 
>  
> 
> I also notice that V301, 1st IF Amplifier, can be wiggled in its
> socket and make similar changes in signal strength occur.  When I push
> V301 all the way into its socket, there is a strong oscillation that
> occurs.  I have to pull the tube up from the fully seated position for
> the receiver to function normally.  I’ve swapped tubes with no change
> in performance.  I’ve also used some DeoxIT on the tube socket with no
> change.  I am suspecting some of the mica coupling capacitors
> underneath that are house within the module where the mechanical
> filters are mounted. 

The mica capacitor closest to that tube tunes the input coil to the
mechanical filters. Unless wiggling the tube is moving the leads to that
capacitor and causing it's value to change that shouldn't be a problem.
It should go from connected to disconnected, nothing in between. I'm
thinking a solder problem on a socket pin or since the tube will
oscillate at times, the .01 ceramic screen bypass. Changing the circuit
function by how long the tube leads are causes me to think VHF parasitic
oscillation. And pulling the tube partly out of the socket lowers that
oscillation frequency to the point the bypassing is adequate to stop it.
Very slight changes in AGC voltage could change the tube gain to put it
in or out of oscillation. And when oscillation the grid current
developed can modify the applied AGC voltage to change the S-meter
indication. The S-meter simply uses a gain controlled IF stage as a
single tube VTVM showing the AGC voltage developed.

More than one Collins radio has needed some RF or IF stage parasitic
suppression (even the tone oscillator in 32S had that problem). Once I
got in and didn't find the .01 bypass broken or open, I'd add 100 ohms
series resistance to the control grid lead (coax out of the module) and
maybe 100 ohms between the screen pin on the tube socket and the DC
supply and screen bypass.

I'd also make sure the IF tube was truly a 6BA6 and not some miracle
super high replacement that would be more prone to oscillation when the
circuit and layout was built for the lower gain of the 6BA6.

But I'd be most suspicious of the filter switch. Switches and tube
sockets give 100 or 1000 times more trouble than mica or ceramic
capacitors, but not as much trouble as tubes. Tubes are in sockets
because they are a wear item that need replacement. Nearly everything
else in the radio is soldered because replacement isn't needed often.

A quarter drop of DeoxIT from the dropper bottle on the filter switch
contacts followed by several operations of the switch can make a
dramatic improvement in consistent signal strength. Sometimes in other
Collins gear of similar vintages, vigorous operation of a rotary switch
that has gone open circuit will restore operation without having to
apply DeoxIT.
> 
>  
> 
> Has anyone had a similar experience?  Also, If I do have to obtain
> access to the inside of the mechanical filter mechanism to access
> these capacitors, what’s the best way to do that?  
> 
http://www.jptronics.org/radios/Collins/354A1/354a1.pdf is the manual
for the kit to put the module in a 51J-3. It should give some hints on
how to get access. My 51J-4 manual gives no hints, not even V301
resistance or voltage measurements.
> 
> Thanks,
> 
> Mark
> 
> W4ZF
> 

-- 
73, Jerry, K0CQ, Technical Advisor to the CRA
All content copyright Dr. Gerald N. Johnson, electrical engineer