[R-390] More C709 info

[n4buq at knology.net]

> I dont recall if you said or not - did you try replacing the PTO tube?  
The 
> tube in my SW PTO went bad a few months ago, sensitivity dropped but it 
> would still tune.  Checked fine in the tube tester.  Decided to replace 
it 
> anyway, and that fixed it!  So even if the tube tester says its "OK" try 
a 
> new one anyway.
> Jim
> 

I tried three different tubes, all with the same results.  I have a sleeve 
of NOS 5749's and I may try one of those, but I'm thinking the tube may be 
okay if all three of the others produce very nearly identical results in 
the PTO.

I did check the PTO output from my other R390A ('56 Motorola).  The scope 
shows 8V p-p at its maximum point.  It isn't flat, though.  There is a 
definite bell shape to the output that is not centered at 3MHz.  It tapers 
down to about 4V p-p at the endpoints.

I suspect that when I built this one (my first), my "vast" knowledge of the 
R390A was not quite, well,,, so "vast".  I probably tuned it for a maximum 
output and moved on.  I might be able to do better now (although the radio 
does work just fine so I might be sorting the pepper from the fly 
excrement).

One thing I "discovered" last night.  A tube extender won't fit in the 
stock PTO, but if you remove the little tube shield clamp bracket (2 small 
screws that go in threaded holes so no nuts fall off on the inside, at 
least not on this PTO), the extender does fit.  I checked all voltages and 
they are spot on (probably the only place in this radio where all voltages 
check exactly what they should).

Thanks for all the help, guys.  I have some good suggestions to try and see 
where the problem is on this PTO.  Maybe I'll track it down yet.

Regards,

Barry - N4BUQ