This is a perspective that I'm very familiar with and it is a popular
one on the Swan users list. I think it has to do with the preferred
end-result:
1) A restoration
or
2) A "fix"
My first HF rig was a Swan 500c that was given to me broken about a year
ago. My only previous experience with vintage radios of any kind was an
old console AM radio that I repaired in my teens (Has anyone tried to
plug in that piece of furniture? I'll bet I can make it work).. I
learned a lot about HF Transceivers and Swan transceivers in particular,
but one of the things I found above all else is that you could divide
the users into the above two groups. The second being the "users" group.
You could ask a question of the group like: "Hey, how much drift should
these rigs have when they are properly warmed up and in good working
condition?" and you'd get a response like, "Mine sits on the shelf! It
doesn't drift at all! (hur, hur)".. A typical response of a restorer.
Once restored, the rig goes into perfect museum mode. I'm not trying to
knock those people, it's their hobby, but it's different than mine. My
objective was to make a daily driver, not a museum piece. It is simply a
matter of fact that _sometimes_ (non-zero), when those oil & paper
capacitors fail, they short. When that happens in a radio, you're
getting plate voltages headed into parts of the rig that are not
designed to have plate voltages. Better safe than sorry.
I found that there are basically two problems with replacing old caps
with modern caps:
1) The modern caps are WAY smaller than the old caps, requiring a bit of
ingenuity at times to figure out the best way to place them
2) The old caps sometimes leaked current even when new, and the circuit
was designed around that fact. You replace a cap with a modern one, and
all of a sudden the circuit doesn't work anymore. Solution is to add a
bleeder resistor across the cap, to allow a small amount of current leak
through like the original did.
When I replaced the original 6LQ6 finals with 31LQ6's (6LQ6 is $70/each,
31LQ6 is $15/each) I was met with either horror [from the restorers] or
requests for information on how it is done [from the users]. The power
supply in my rig is a chopped up mess, but is more functional than the
original ever was. The original "multi-cap can" electrolytic is now just
there for looks, the real caps are installed on the board below.
About the only caps that I found to still be in good working condition
in this rig (~1968) were the ceramic disc caps. Those were all still
working great, even the HV ones. A lot of the resistors had failed,
though.. Some of those resistor divider circuits that were designed to
have 100k resistors now had 30k resistors. I didn't know that resistors
did that, but I do now. :)
-Adam
On 6/30/2011 12:53 AM, JohnK wrote:
Generally, old electrolytic caps that have just been allowed to sit
can be re-formed and henceforth operate properly. This allows the
vintage component to remain in the vintage set. [Second-last resort is
to hide a new type inside the old case. Last resort is to 'destroy'
the vintage item by a butchery replacement. [Always keep all the old
components for the next owner].
There are military re-former equipments. One has a table of
capacitance vs allowed leakage current vs voltage. Google for more
info - but beware the Audiophools.
John Kaesehagen.
[PS. Reforming is just the [slow] application of a [preferably
guarded] voltage to the capacitor - exactly as was done during
manufacture. ]
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