old Cobra 148 GTL DX EXPORT MK I PC879

[Marc_uk]

the VR (is it VR6 ?) that sets TX frequency only - doesn't do anything,
i got this on Ebay and don't want to send it away, it's been years, but i'm no stranger to PCB work.

apart from that it works fine - except the drift - any ideas on that as well would help too.
Thanks

[Bozo]

Hey @Marc_uk

If VR6 isn’t doing anything, it could be:

-A faulty or dirty potentiometer. Try cleaning it with contact cleaner and rotating it a few times.
-A problem in the circuit. Check for bad solder joints or damaged components near the PLL and VCO.

Drift is common in older radios and could be due to:
-Aging capacitors. Replace electrolytic caps, especially in the PLL, VCO, and oscillator sections.
-Voltage regulation. Verify your regulator (e.g., 7808) is stable.
-Crystal aging. The 10.240 MHz crystal might need replacement or re-soldering.
-Temperature sensitivity. Consider insulating the oscillator section if needed.

Since you’re comfortable with PCB work, a good place to start is checking solder joints for cracks or cold connections.

[Marc_uk]

Heyy Many Thanks Bozo, that's great info and it's gong to be a great help. that's exactly what i will do. seems there's a bit of a dry joint somewhere too, just tapping the set makes the s metet light change brightness a little, i think i'd better buy a small soldering iron too, don't want to lift the tracks on this old girl, maybea cheap ESR meter too. I need to give the set some De-oxit anyhow as an LED segment comes and goes as it pleases too.
I had someone (who is highly regarded) remove the EPROM recently, so i will double or triple check around the MB8719 area too, maybe he didn't check the operation of the VR,
guess i had better dig my old PC879AB docs out too.
i think caps and xtals might be needing replacing if the other stuff doesn't help.
Thanks, you've given me a direction and plan

[Marc_uk]

just an update - it seems that there are people who modify the smaller 'fine' control on the kc shift / slider so it works on TX also instead of just RX only,
although the outer control slides on TX and RX perfectly already. This dumb 'modification' **disables* the VR6 TX freq adjustment on the PCB and you lose functionality.

I learned that a diode, a 1N4148 or - similar needs to be put back in where the TX FREQ VR is on the PCB and then the blue wire needs putting back to 8v RX.
An alignment is needed afterwards.

it seems like a stupid 'mod' to enable and duplicate an already existing front panel control and lose functionality,the fine control to just do the exact same
as the outer already control does that, but 'unlocking' the clarifier - you lose the ability to temporaily tune in to a slightly off-freq user whilst he's talking (he might be drifting !)
- without having the ability to stay on your frequency along with everyone else. ludicrous.

if it goes horribly wrong, i shall have a scrap Uniden PC879AB board ! wish me luck.