[R-390] New Project Follow-up

[Cecil Acuff]

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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