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

[David Wise]

>Low resistance pots once had a reputation as being "low reliability"  
>parts. Many places put them on a "do not use" list or made them

Thanks, Bob Camp, for your cogent bit of history.
That's a compelling reason for them to go carbon.

It's still a sloppy, naive implementation.  I've
figured out how they should have done it; that was
my "simple mod" from a few posts ago.  But to preserve
the original full-scale reading requires we change
R549.  The next 20% value, 68K, overshoots the mark,
so I'm looking for an acceptable combination of R537
and R549.  I regret having to do this, because
originally I thought we wouldn't have to remove
the IF deck.  That would have been irresistable.

Bob brings up a point that had not occurred to me.
In any circuit variant that does not shunt the pot with a
fixed resistor, if the pot goes open, the entire V504
cathode current, around 12mA, goes through the meter,
subjecting it to more than 10x overcurrent.  Unfortunately,
it's not possible to protect against this with the stock
meter, because the voltage during the fault condition
is well below any semiconductor diode turnon.  So I guess
the message is, don't ditch R537.

If you change R523 to 10 ohms, the original 22-ohm R537
will limit a fault to 3x, which the unusually large
wire in the 17-ohm* stock meter should withstand, while
reducing the worst-case available reference voltage from
100mV to 69mV which is still plenty.

* In the past I have said it was 18.  I actually got around
to measuring mine the other day, and it's 17.0 .  Sorry!

Barry N4BUQ mentions that his 20-ohm pot is "a bit tricky".
Is that because of the turn-to-turn jumps?  I always thought
that was the real reason Collins went carbon, but not having
played with an R-390 I have no justification for this.
I don't know the wire size in Barry's pot or the 10-ohmer
I found.  As Barry says, we shouldn't get uptight over it.
The zero is not exact; it drifts with line voltage and
shack temperature, probably more than one wire's worth.

Would anybody with an R-390 care to comment?

Regards,
Dave Wise

> -----Original Message-----
> From: r-390-bounces at mailman.qth.net
> [mailto:r-390-bounces at mailman.qth.net]On Behalf Of Bob Camp
> Sent: Thursday, August 30, 2007 4:41 AM
> To: Perry Sandeen
> Cc: r-390 at mailman.qth.net
> Subject: Re: [R-390] re: Call for measurements - 100dB carrier reading
> 
> 
> Hi
> 
> Just about *any* pot will work just fine. Another approach is to put  
> a very low value pot in series with a fixed resistor.  There's not a  
> lot of power involved here. The only risk is that if the circuit  
> opens up you may loose the meter.
> 
> Bob
> 
> 
> On Aug 30, 2007, at 1:13 AM, Perry Sandeen wrote:
> 
> > 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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