[R-390] Gain drift

[Larry H]

Dave, the inputs should not be sensitive to movement - sounds like it might
have a shield grounding problem on the input coaxes.  And, the unbalanced
input should not need to be grounded.  There should be very little noise
while turning the MC between bands.  Noise indicates poor contact in the
band switch.  Tube shields are needed on all the sensitive tubes in the RF
and IF sections.

Sounds like you're making progress.

Regards, Larry

On Mon, Oct 15, 2018 at 6:46 PM dog <agfa at hughes.net> wrote:

> Today I took out the RF section. Used some alcohol to clean the switches
> with an acid brush bristles cut short, there was some grease on the
> wipers I cleaned off. Checked the operation of each contact, seems
> normal, spring action seems good nothing is messed up. Used alcohol to
> take the wiper around the whole circle both switches to try to clean the
> contacts. The alignment seems OK, but there is an amount of backlash in
> the wiper, it centers the wiper better turning one way opposed to the
> other way, exactly like the other one I have. Pulled all the xtals and
> hit the contacts with a copper brush, new de-ox on the xtals and a bit
> on the switch wipers and a few places around the contacts, not much.
> Pulled and inspected every solder joint in the osc section, contemplated
> replacing the 5000pf bypasses but didn't. Solder joints all seem good.
> Back together, there seems to be somewhat less noise, but still the
> drifting gain on startup. Cooled it off for a while and pulled the input
> RF amp. Turned it back on and didn't see the gain drift issue but much
> less ACV on the diode load. I'll try it once more without the RF amp.
> And then maybe another RF amp tube.
>
> I notice that with no inputs on the ant inputs, the RX is very sensitive
> to movement around the input ports. Like there's a lot of common mode
> currents making noise in the input. If I put a load on the balanced port
> it's much quieter. Of course if I sock a 50uV signal into the balanced
> input I get Carrier reading with AGC.
>
> Did I read somewhere that if I use the balanced port I should short the
> unbalanced ports to gnd?
>
> Do all these RXs make noise when changing the bands? I almost always get
> a big crackle when I change bands. But now as I rock the bandswitch on a
> particular band there's lots less to no noise. The 27M and the other one
> that seemed noisy before seem more like the rest now, stable. No issues
> tuning the output caps unlike yesterday.
>
> Almost lost the Oldham coupler spring putting it back together today.
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