The AM Forum

Hello, its me again....
I have been having decent success in stabilizing my Valiant VFO and have worked a number of qso's on cw so far with pretty decent reports.
The VFO still is a little unstable, occassional quick freq. jumps, tone shifts when switching between ZERO and VFO...still its much better than it was.

I changes some out of tolerance resistors and made some VFO voltage measurements, see attached.

All in pretty good shape but Terminal 8 should read zero and I am getting -7.5vdc on it....

This leads into the VFO to a coil in there...

Is this indicating a component I can zero in on to change (sorry for the bad pun)

Any thoughts greatly appreciated
73
John-N2LK

Tell us what Terminal 8 is connected to and then we can use the schematic.


(http://www.mbzponton.org/n2awa/radio_Johnson_Viking_Valiant_schematic.jpg)

OK Pete, see highlighted schematic where Terminal 8 goes across C22 (300pf postage stamp cap) then into VFO compartment to L17.
From there it leads to Pin 5 (Plate) of XV1 6AU6 tube.

Resistance measurement is 90K at Terminal 8 (manuals says 100K +/- 20%). Voltage measurement there should be 0 in the table but I am reading a -7.5 vdc. No key down, rig set in CW mode, etc. per manual for measuring socket voltages.

Resistance at pin 5 (plate) of 6AU6 is 14.74K, appears within tolerance there.

I am wondering if C22 needs replacement??

Any thoughts appreciated
73
John-N2LK

If C22 was bad or leaky, you would read a positive voltage.

OK, Thanks!

What position is the OSCILLATOR switch in? (C1, C2, VFO, Zero)

The 6AU6 is the VFO but not the crystal oscillator. The schematic shows the switch in the full CCW postion (C1).

It matters because Terminal 8 will likely have a negative voltage on it if the switch is in VFO or Zero because that circuit is connected to the -265V bias supply through a 47k (R42) and 1M (R7) resistor.

Hi Clark-
I measured again and with Bias supply voltage at -263.9vdc and OSC in VFO position I read -20vdc on Terminal 8.
When I flip to ZERO it drops on Term 8 to -11vdc....in C1 or C2 position I read miliamps but unsteady, multi-meter is "hunting" for a readout.

I guess then I am ok in this dept then. I have replaced today the 2 - 500pf postage stamp caps in the VFO and will do the 1000pf caps tomorrow.
All resistors in VFO seem in tolerance, so hopefully the cap change (with all else I have done) will put the slight freq. jumps/changes to bed..

Thank you
73
John-N2LK

Mine does that. Make sure your tube pins are clean and making a good connection. Are you hearing chirping?

Its funny, I swapped out the 4 silver mica caps in the VFO yesterday and listening to the VFO on my receiver, sounds so smooth now
All bands, was so easy to calibrate 160 and 40 meters last nite.

So I tune up on 80M this AM and hear some chirp on 80m while actually transmitting. This was not an issue till now..I had the chirp settled.
The VFO signal in Zero and VFO sounds GREAT, steady, smooth...but in transmit I have some little crud..still have to check other bands.

Must be downstream from VFO, I am not going back into that VFO again...done with that...keyer circuit I am suspicious of now.
All fun..
73
John-N2LK

   Hey John,


         U may want try this..

   From the WA3DSP write up on the "Ranger", Same
thing as far as the VFO is concerned.

http://www.crompton.com/hamradio/JohnsonRanger/RangerModsSchematic1_2-5-2013.pdf