[R-390] New Project Follow-up
JMILLER1706 at cfl.rr.com
JMILLER1706 at cfl.rr.com
Thu Mar 8 10:43:43 EST 2007
I have noted that the resistors tend to drift high with age. If they
are within 10% (the silver band indicates 10% tolerance) then you are
probably OK. If significantly higher, or if they have a scorched look,
they should be replaced. Nice thing about tube circuits is that you
can check most resistors in circuit with a good digital ohmmeter
without having to unsolder them. In my experience, look out for any
resistor carrying power such as those screen or plate circuits. You
tend to see a lot of 2200 ohm half watt resistors in plate circuits in
the IF and elsewhere - check them for drift or charred look. Look at
cathode resistors too (most are typically less than 600 ohms). I would
replace with 1-watters if they need to be replaced. If screen
resistors increase in value, the stage gain will tend to decrease (if I
recall my tube theory correctly). Being 10% high is probably not too
bad, but beyond that is pushing it in my opinion. Otherwise, you can
let the failures "
find themselves". Don't be surprised to see fixed mica caps and
trimmer caps go bad - so far I have had to replace several of them in
RF cans and oscillators. And just by wholesale replacing
questionable .005 uF screen and B+ and AGC bypass caps throughout the
radio, or tightening up tube sockets or chassis screws that hold down
ground lugs, I have increased performance considerably. The constant
heat-cold on/off cycles over the lifetime of the radio will actually
cause screws to loosen! And any cap. in the radio is a candidate for
age-induced failure - any. Good luck Jim N4BE
----- Original Message -----
From: Cecil Acuff <chacuff at cableone.net>
Date: Thursday, March 8, 2007 9:43 am
Subject: Re: [R-390] New Project Follow-up
To: Bill Hawkins <bill at iaxs.net>, r-390 at mailman.qth.net
> I agree with Bill that you should reduce the probability of
> failure by
> changing parts as indicated with parts with much lower probability
> of
> failure.
>
> Carole has already proven that degraded performance is a result of
> degraded
> components in his radio. Not really defined as a failure in the
> normal
> sense but certainly degrading.
>
> Especially since his project is a survivor from St. Julians Creek,
> more work
> will be required. Many of the carbon comp resistors will be out
> of spec.
> due to humidity exposure....a result of the outdoor storage of
> those
> radio's. Those radio's are great projects and well worth the
> effort to
> bring back. Many will say it's not possible but that has been
> proven wrong
> more than a few times by many here.
>
> More work? Yes. Anybody can fix up one that has lived a charmed
> life.
> Carole I would suggest you spend the time and check all the
> resistors you
> can while in circuit. I would focus especially on the high value
> resistors
> as they seem to drift the most. Others have mentioned a specific
> value that
> tends to be off quite a bit...my memory fails me on those
> notes...seems it
> was for some 1K ohm resistors but I don't remember for sure.
> (somebody
> chime in here)
>
> I don't advocate wholesale replacement of resistors but I do paper
> caps.
> (only in the "A") Too much history of problems with the paper
> caps in the
> "A" series....do yourself a favor, especially since it's a "Creek"
> radio and
> change them. It's not that hard....you can pick up a kit with all
> the caps
> needed from a couple of different sources on the list. Walter
> Wilson is one
> of those.
>
> The probability of causing a component failure in an adjacent
> component is
> very dependent on your removal and replacement technique. I
> advocate high
> heat and very short durations. Use a high wattage iron and get in
> and out
> quickly. A vacuum solder sucking device whether it be the spring
> loaded
> type or a vacuum desoldering station is a good thing to have and
> use.
> Solder wick is usually too slow and requires too much heat
> soak...save it
> for the PC board work. In point to point work like these old
> radios I use a
> Hakko iron on a temp. controlled station with a wide (1/4") chisel
> type tip
> and run it at 600 to 700 degrees. Low wattage irons require that
> you stay
> on the joint for extended periods of time to get everything to
> melt, heat
> soaking all the components in the area increasing the chance for
> further
> damage. Keep a wet sponge in the iron stand and keep the tip
> clean and
> always wet it with fresh solder before hitting the joint to
> facilitate good
> heat transfer quickly. Develop a good technique and you'll have
> little
> effect. I also use dental tools on those joints to pry the leads
> open to
> allow removal. Works great!
>
> Hope that helps...
>
> Cecil Acuff
>
>
>
>
>
> ----- Original Message -----
> From: "Bill Hawkins" <bill at iaxs.net>
> To: <r-390 at mailman.qth.net>
> Sent: Wednesday, March 07, 2007 9:22 PM
> Subject: RE: [R-390] New Project Follow-up
>
>
> > Well, see, the problem is that probability is everywhere. That's
> > why Murphy achieved immortality. There are people who get around
> > probability by changing everything - but they don't because there
> > is a probability that new parts will fail.
> >
> > There are parts that will cause cascading damage if they fail, like
> > the blocking cap for the mechanical filters (no problem in the 390).
> > Reduce the probability of a catastrophe by substituting the
> probability> of failure of an old part with that of a new part.
> >
> > Recognize that changing a part, with the heat of a soldering iron,
> > raises the probability of adjacent parts failing. Usually, the best
> > thing to do is to not replace a part until it fails. This is
> eloquently> expressed as, "If it ain't broke, don't fix it."
> >
> > OTOH, getting at some parts in an R-390 class receiver requires
> a lot
> > of work. Deal with it. That's part of the charm of using the
> peak of
> > vacuum tube technology. You get to know how your radio works.
> >
> > I know this doesn't help, but that's life. You can drive
> yourself up
> > a wall by demanding certainty.
> >
> > Regards,
> > Bill Hawkins
> >
> >
> > -----Original Message-----
> > Carole White-Connor said:
> >
> > Does anyone have a list of frequently troublesome resistors (like
> > Chuck's list of troublesome caps)?
> >
> > I ask for a reason. I came across an article where Chuck notes that
> > R-614, a 560-ohm resistor in the AF section, frequently drifts,
> > sometimes to 700-800 ohms. Mine tested 1,000 ohms! I changed it out
> > and it made a tremendous difference in the audio. I'm wondering if
> > there are other known drifters.
> >
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