Cobra 2000 Temperature Sensitive

[HavaV10]

Hello all. I have a good working Cobra 2000 1984 Taiwan model. Opened up clarifier. It's drifting due to air temperature. I use this radio out on my patio and in the morning I'll turn it on and it's cool outside the freq counter, say using channel 38 27.385.3 if I let it warm up for about 45 minutes it will stabilize and be rock solid at 27.385.0 with both the fine and course at 12 oclock. Does this on all modes LSB/AM/USB. During the day the sun will shine on the cover some warm it up some and it will drift almost 1KC off to 27.384.2 So my question is what causes this I have another 2000 and it doesn't do this. Maybe a bad PLL, VCO block or Crystal? Would the 10.240 crystal near the PLL cause this? Radio has been recapped. Just need a idea for what component to replace and try to fix this issue. Thanks for any suggestions.

 

[brandon7861]

Remembering Mr. Carlsons video on how non-NP0 caps are very temperature sensitive, I would see if any of the ceramic caps in the crystal and clarifier circuits are missing the NP0 mark. Maybe someone swapped one.

With that much temperature drift, I would think touching the cap would warm it enough to immediately notice. Just go around touching them all and watch for a change.

As for which ones, 800Hz of drift has to be more than a few pF change. I think we are dealing with a drift of maybe 20-40pF so that (in my mind) rules out sub-100pF values. I would focus on the larger values around the base and emitter of the oscillators rather than the lower pF values at the crystals. So by TR30, touch C117 and C220 and then over by TR32 touch C123 and C122.

I am not saying it has to be one of those, they're just good place to start. Touch everything with a little heat and see what makes it move.

Edit: it is possible touching them introduces parasitics that make it move too, so maybe use hot air or try to pay attention to the drift rate after your finger touching it has stopped moving.

 

[VMC]

Can of freeze spray is your friend.
As Brandon said, ceramic caps (all of them) can be an issue as they age, as they are hygroscopic over time. I'd hit caps and resistors first and see which one makes it go off frequency.