[R-390] VFO Output Level

[Flowertime01 at wmconnect.com]

Barry,

I acknowledge your concern that the VFO in hand has low output. Thinking that 
dealing with the problem now rather than later after complete failure is very 
reasonable. Considering the thing could hang at the current condition for the 
rest of your life and just living with it is equally unacceptable. I see your 
need to get the thing fixed.

As Tim pointed out any one of those resistors or caps could be the problem 
source. In fact there may be more than one problem. Also suspect the chassis 
wire harness and connectors. Any connector point (tube socket pin) could be a 
resistive problem. Any solder joint could also be a problem. So its not just the 
parts that need inspection. You need to look at solder joint and wire.

I was thinking of the ballast tube and BFO tube in the filament string. A 
small drop in filament voltage from a change of resistance in the filament string 
could be a cause for low tube emission. Any tube changed into the VFO would 
act the same (low output) because the problem could be elsewhere in the 
filament string circuit.

You may not be up against a sudden failure. Your VFO could hang like it is 
for a long time. OK but not good enough. Tim's idea to check those resistors is 
sound advice. 

I would pick an easy one to get to as a candidate for adjustment. You can get 
a larger output voltage by changing the grid, screen or plate resistor. But 
first I would do a whole rebuild.

The schematic shows a set of four 5,000 caps as by pass caps on the screen 
and plate. Shoot my self in the foot here and suggest that their value and lead 
length are not critical. The figure in the TM is not the same as the as built 
circuit. (new models). The block of caps (Zenith TV) was just an easy assembly 
procedure. Some early production has the caps all set in the same location 
side by side. 

One common construction practice is to place all grounds to the exact same 
point under a tube circuit. So likely all the caps have short leads to the same 
ground point for the caps on the cap block package. Then the other lead is 
what ever was needed to get the circuit built. Bunching the cap block near ground 
provided some extra working room on the other side of the caps. These caps 
are in the decoupling circuits. As long as they were in the box under the VFO 
tube and decoupled the VFO signal off the DC lines before it gets coupled else 
where in the receiver it is OK.

You were looking for a new length of coax. One of the Fellows pointed out, 
that due to it's short length (under 24 inches) that the impedance of the coax 
did not exactly mater. Nevertheless you found and went with some good coax.

Like wise, decoupling caps and lead length at the VFO frequency is not as 
critical. While RF is a science you can still do a lot of cooking in the kitchen 
without resorting to a cook book. Use good caps and good resistor and just 
rebuild the circuit under the tube socket.

A shoot gun replacement of:
R701 150K Grid resistor.
R702 56k Screen resistor
R703 2.2K Plate resistor
and the four by pass caps.
C705, C714, C708, and C707 all 5,000 pf with some .0047

Some disk caps of better quality than a Radio Shack Value Pack would be good.
The voltage rating is not critical. All these parts are out of the frequency 
determining circuit. But they sure do impact signal size. I would round up the 
parts and plan a couple evenings work to just rebuild the circuit under the 
tube socket.

If you think the variable cap C706 is leaking to much signal to ground, you 
could run a cap in series with it. Any thing bigger than the 1.5 - 8 PF would 
leave you with an effective 1.5 - 8 PF. A 100 PF mica would do as a new 
isolation if you think the variable glass cap is leaking. You may just unmount and 
remount that item to ensure you have a good ground bond for it.

Time consuming and educational. Then again this is part of one of your 
recreational activities. Time invested and return on time have no tangible metric. 
Go for it and enjoy your self along the way.

Roger AI4NI