Please help me fix this HP8601A

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neazoi

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This HP8601A (manual here https://bama.edebris.com/manuals/hp/8601a/ ) stays fixed at the maximum frequency on each range, no matter If I move the frequency knob.
The VCO tuning works, when tested with external voltage from the bench power supply.
I suspect the DC amplifier after the discriminator.
I attach all the tests I did in the image (please zoom in).
Greens check ok. Reds do not. Check the voltages. The output ramp voltage to the VCO stays fixed.

Any help towards the right direction, so as not to have to desolder every component to check?
 

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Moving parts are a likely suspect in electronics. The frequency potentiometer contains moving parts. It's an adjustable voltage divider. I imagine the center terminal should have changing volt levels when you dial it.

However if the ground loses contact at its pertinent pin, then there's no more voltage divider effect. The center pin is exposed to maximum voltage. The VCO is stuck at maximum frequency.

A potentiometer is easy to check, or to unsolder if need be.
 

    neazoi

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I know but I hav checked these things already as you see in the diagram.
Anyway, problem found. It is the hybrid IC in another module, the (FLL) loop amplifier. Thankfully people have already found solutions to that. A modern MMIC that can privide around 30db of gain does the trick. HP states that gain should be flat, but this is a part of an FLL not ALC, so a bit of non-flaness won't harm I think. But anyway, multi-GHz MMICs should be pretty flat at VHF and down.
 

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