[R-390] Meter zero and AGC

[Drew Papanek]

Kees wrote:

<snipped>
>If you move the green/blue wire to the center tap and remove the
>wire short from the center tap to one end, you have essentially
>made the pot a voltage divider for the meter and the cathode
>resistance stays fixed at about 19 ohms (parallel 22 ohm and 100
>ohm). Seems to work fine and the pot is much easier to adjust.
>
>Haven't found a down side yet.

Making the pot a voltage divider for meter would change meter sensitivity as 
zero setting is varied.  Maybe that wouldn't really matter - to most of us 
an "S" meter reading is just a relative indication anyway.

Some list members have suggested keeping the original circuit configuration 
but changing 22 ohm resistor to 8.2 ohms. I use that setup and carrier meter 
zeroing is easy and stable (clean that pot).

I too have opened up the carrier level meter to reset tension on the jewels, 
thereby eliminating stiction.  I found (on my example anyway) that the four 
fasteners holding assembly together are tubular nuts and hollow screws with 
no "persuasion" necessary for disassembly.  Jewel tension is easily adjusted 
after loosening a small jam nut.  I cleaned inside of glass with lacquer 
thinner - perhaps the dull accumulation there is due to rubber gasket 
outgassing over the years.  Caution: meter scale and pointer are usually 
coated with radioactive radium paint.  Take care not to inhale dust. Perform 
operation outdoors, wipe up dust with damp cloth and dispose of 
appropriately.  Scraping the paint and using some to decorate cookies whilst 
snorting the rest for a new kind of high is right out.

On AGC time constant switching voltage spike and "The Moment of Silence": 
some have used a 24 volt zener to limit AGC line voltage and have reported 
it to be effective.  The 2 diode fast attack / slow decay AGC (Dallas 
Lankford) modification is also a fairly good remedy.

Drew




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