[R-390] More C709 info

n4buq at knology.net n4buq at knology.net
Thu May 4 09:36:05 EDT 2006


Well, sadly, replacing C709 didn't help one bit.  I'm about to give it up.

I checked R704 (lifted one side of the coil) and it's about 160 ohms.  I'm 
not sure what else could be wrong.  Someone suggested the coil might have a 
shorted turn.  Possible.  It shows no sign of physical damage, though.

The only thing I haven't been able to check are the bypass capacitors.  I 
suppose one (or more) of them could be leaking.  I'm dreading replacing any 
of those, though.  Not sure how many "brands" of PTOs did this, but this 
PTO has a "bank" of about six 0.005 caps encased in a waxy substance that 
holds them together into a single "block" of individual capacitors.  
Replacing one would be nearly impossible unless one leaves the old 
capacitor in place and runs the new one in a different position.

I'm either going to leave it alone (the radio DOES work), or completely 
strip it and replace everything I can still get parts for.  I did this to 
the second crystal oscillator in my other R390A.  It helped just a bit, but 
not all that much so I'm hesitant.

I'm still curious what other folks are seeing at the "raw" output of the 
PTO.  I dragged down the ME-26D/U and I'm seeing about one volt at the 
output connector.  I'm getting about 2.8V ptp on the 'scope, but since 
neither one is calibrated (and the 'scope is acting up anyway), it's pretty 
much anyone's guess as to the actual value, but suffice it to say, they 
both are both showing a pretty low voltage so neither one may be that far 
off.

Sorry for the long post, but it really bugs me not to be able to isolate a 
problem and fix it (provided there really IS a problem and this voltage 
output isn't all that abnormal).

Thanks,

Barry - N4BUQ

On Wed, 3 May 2006 20:56:43 -0500, "Barry" <n4buq at knology.net> wrote :

> I'm not trying to peak it for a certain frequency.  I realize it needs to 
be
> as flat as is possible; however, I want the PTO's output to be of 
sufficient
> level so as not to have to compensate for that somewhere else in the
> receiver.
> 
> As Tim said, my general voltage level is quite a bit lower than it 
probably
> should be anywhere in the tuning range.  Lifting the capacitor and seeing
> the voltage level rise like that is, to me, an indication of significant
> leakage.  I'll know when I try the good capacitor.  If I'm correct, I 
should
> see a slight rise in voltage level due to resonance.
> 
> Thanks for the comments!
> 
> Barry - N4BUQ
> 
> ----- Original Message -----
> From: <Flowertime01 at wmconnect.com>
> To: <R-390 at mailman.qth.net>
> Sent: Wednesday, May 03, 2006 6:54 PM
> Subject: Re: [R-390] More C709 info
> 
> 
> > Barry,
> >
> > Why are you still trying to peak a VFO in the middle of its frequency
> range,
> > when you know you want a flat output across the range?
> >
> > Why do you want more mixer noise at 500KC in every band than you want at
> say
> > 100KC and 900KC?
> >
> > What part of flat like a pancake are you missing?
> >
> > Roger.
> >
> >
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