[R-390] re: Call for measurements - 100dB carrier reading

Thu Aug 30 21:43:58 EDT 2007

Hi

To make the AGC swing the oscillator, the agc has to get to the  
oscillator. There are two basic routes. One is via the B+ and the  
other is by the oscillator load.

FIrst thing to check is to see if you have field change 7 on the  
screen grid of the oscillator. Your crystal oscillator may also be a  
VHF vfo. All kinds of strange stuff will happen when that's going on.

Next thing to check would be to see if the regulated B+ is stable.  
Your regulator tube may have reached end of life.

If you are swapping tubes, check the first mixer tube. If you have  
something other than a 6C4 there, that may be the problem.

Check the grounding on the coax at J221, and take a look at c327. If  
the tank is significantly off tune odd things can happen. T207 should  
have a "real" peak.

If nothing has popped out so far, next step it to dig into the  
voltages on the first mixer. It may be getting to much AGC, or it may  
have to large a cathode resistor

Hope that helps ...

Bob

On Aug 30, 2007, at 9:00 PM, Jim M. wrote:

> Sorry if I was critical, it is interesting but I would really like  
> a solution to another more esoteric problem as long as the gray  
> matter is networked here - that is the "pulling" of the 17 Mhz 1st  
> crystal oscillator by strong signals with AGC turned on.  As the  
> AGC varies, the osc. pulls 50-80 Hz causing a chirp or whoop on CW  
> signals.  Turn AGC off and it doesn't do it.  Noodle that one.  I  
> am out of ideas - doesn't happen on my other 390a.  Have changed  
> tubes, swapped crystals, nothing.  N4BE
>
>
> ----- Original Message ----- From: "David Wise"  
> <David_Wise at Phoenix.com>
> To: <r-390 at mailman.qth.net>
> Sent: Thursday, August 30, 2007 6:18 PM
> Subject: RE: [R-390] re: Call for measurements - 100dB carrier reading
>
>
> A 10 ohm 1-turn pot is 10 ohms per turn.
> A 100-ohm 10-turn pot is 10 ohms per turn.
>
> Okay, it's not "no better", but it's not 10 times better.
> If the 1-turn is 270 degrees, the 10-turn is only 33% better.
> It spends its life in the bottom turn, the other
> nine might as well be fixed.
>
> If you've tried both and seen a drastic difference, it
> must be attributed to the wire size and build quality,
> not the number of turns.
>
> I started this discussion not to supplant the
> time-honored ten-turn pot (I already conceded that
> it relieves the symptom), but to get into the
> design problem that makes it necessary.  It's
> like a crooked wall with a jack bracing it.
> I'd like to know how it could have been built
> straight in the first place.
>
> I like mental candy bars,
> Dave Wise
>
>> -----Original Message-----
>> From: r-390-bounces at mailman.qth.net
>> [mailto:r-390-bounces at mailman.qth.net]On Behalf Of Jim M.
>> Sent: Thursday, August 30, 2007 2:41 PM
>> To: r-390 at mailman.qth.net
>> Subject: Re: [R-390] re: Call for measurements - 100dB carrier  
>> reading
>>
>>
>> Also, a 100-ohm 10-turn
>> is no better than a 10-ohm 1-turn.
>> ------------------------------------
>> ???
>> A 10 turn pot (such as the one I identified in previous
>> email) will allow
>> much finer adjustment of zero, and I believe would be more
>> stable.  I have
>> used that replacement in several 390a's now and the meter
>> zero is always
>> dead on zero regardless, once I set it.  The 10-turn pot
>> provides good
>> vernier control as compared to a single turn.  It's wattage
>> is more than
>> adequate.  And it mounts easily.  And it is sealed.  And the
>> shunt resistor
>> stays across it.  AND THE METER NEVER DRIFTS.  Me thinks you
>> guys are making
>> the problem much harder than it needs to be in terms of the
>> meter zero
>> adjustment.  Not to criticize, but I think this subject has
>> turned into a
>> "mental candy bar" as they say in engineering, sometimes
>> called "analysis
>> paralysis."
>>
>>
>> ----- Original Message ----- From: "David Wise"  
>> <David_Wise at Phoenix.com>
>> To: <r-390 at mailman.qth.net>
>> Sent: Thursday, August 30, 2007 1:23 PM
>> Subject: RE: [R-390] re: Call for measurements - 100dB carrier  
>> reading
>>
>>
>> Bob Camp already asserted that wattage was not
>> an issue.  I'll fill in the detail.  13mA * 67.5mV
>> is less than 1mW.  Going beyond worst-case,
>> 13mA**2 * 100 ohms is still only 17mW.  Unless
>> you were being deliberately perverse or have
>> access to nanotechnology, you couldn't make a pot
>> that can't take this.  They picked a big one so
>> techs wouldn't need a magnifying glass, that's all.
>>
>> A 10-turn cermet would be fine electrically, but
>> aren't they pcb-mount trimmers or something?
>> There would be mechanical challenges and
>> it would look odd.  Also, a 100-ohm 10-turn
>> is no better than a 10-ohm 1-turn.
>>
>> I haven't read Chuck's ER article.  If he was
>> just changing R537, he was perpetuating the
>> same old foolishness.  The lower the shunt,
>> the more likely that tube variations will
>> tip you past the point where zero is not
>> achievable.  Lame!  That's what drove me
>> to look for something better.
>>
>> Regards,
>> Dave Wise
>>
>> > -----Original Message-----
>> > From: r-390-bounces at mailman.qth.net
>> > [mailto:r-390-bounces at mailman.qth.net]On Behalf Of Perry Sandeen
>> > Sent: Wednesday, August 29, 2007 10:14 PM
>> > To: r-390 at mailman.qth.net
>> > Subject: [R-390] re: Call for measurements - 100dB carrier reading
>> >
>> >
>> > Gents,
>> >
>> > over the years the 100 ohm pot was replaced 10 turn pots but
>> > still using a 2 watt unit.
>> >
>> > IIRC, Chuck Felton in his ER article on R390 upgrades shunted
>> > the post with a low value 1/2 watt
>> > resistor.
>> >
>> > The question becomes: Could one use one of the multi-turn
>> > cermet type pots instead? They are cheap
>> > and plentiful.  Multi-turn 2W pots are in the $20 range now.
>> > I do concede the point that is won't
>> > look "original".
>> >
>> > Had anyone tried this?
>> >
>> > Regards,
>> >
>> > Perrier
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