Symptom....? vfo jumps freq slightly and instantly.? A light touch to vfo brings it back. This is not a slight slow change over time. Its a slight jump.? Very noticeable if using a freq counter hooked to pin 11 on back panel socket.?

I traced this to the 300PF mica cap that is across the VFO plates.? Replace with a 1 percent 300PF mica cap.? ?These mica caps cause the same instability and jumping in Collins gear.

W7wrx

That's good to know.??
The relay next to the 6aq5 will will cause instability in my SR2000 over time .

Tim

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I do understand it has nothing to do with your issue.? Just mentioning it could be as simple as...?
And again good find.?

Good morning Clark Et al,

That Mica is a new one for me.? This is the first I've heard of one failing that way.? Will enter it in my log of can bees.? Thanks all for the interesting conversation.

Historically, the most common reasons for VFO drift or frequency change are listed in sequence.

1.? The dog bone caps C121, 123, and 124 pealing.
2.? K2 pin 12 contact.? It carries no current depending on contact wipe to maintain contact.? It must be cleaned annually, or sharp changes can occur.
3.? The 6AQ5A cathode is used to switch the radio from transmit to receive and back.? It is run at the heat margins, exposing K2 to excessive temperature.? A heat dissipating shield should/must be used on the tube.
4.? Incorrect back lash adjustment when restoring the VFO:? Should be 2 teeth.
5.? The VFO should experience little or no drift at the band center and up to 300 cycles at the band edges within the first 10 minutes.? Those are design parameters assuming QC was careful.? After 10 minutes, you could tune an SSB signal, go on vacation and upon return participate without retuning.? ?
6.? There was a change submitted to mitigate the progressive natural 300 cycle drift to the band edges by mechanical VFO and PTOs but rejected because tubed radios were near EOL and digital VFOs would relegate mechanical VFO/PTOs to history.

Kindest Regards Jim K9AXN??

Thanks, JIm!? I will clean K2 just for the heck of it.? The radio seems stable now.? The Mica caps have a silver migration problem. They were all perfectly fine 10 to 15 years ago. But now, they should be changed.? ?Very, very common on Collins rigs now as well.??

It does have a little lash on the VFO knob. I will look into adjustment.? Its not bad for a tube radio and traditional VFO with a cap.? ?If you run the radio on SSB for an hour really making a lot of transmissions, the entire rig heats up of course and that can make my rig go slightly up.? Once it cools back off it returns back to where it was.? I think this is likely normal.? Also understand I am staring at counter that is showing 7 digits.

I was unable to find carrier oscillator xtals for the second SR2000.? I am in process on building my own Solid-state Oscillator.? It will have three channels.? LSB, USB and CW.? As you know USB is kicked over for CW.? This means I need a third channel.? The Board is programmed to exact frequency and is still controlled by the Mode switch.? I have to use a Low pass filter as the output is square Wave. One of my Elmers helped me with the design of the filter.??

Thanks for the history lesson!

C