[ARC5] [BoatAnchors] Rethinking Re-Capping

[john]

Hi David

Many good observations here. I'm going to keep this filed as excellent 
info for future troubleshooting sessions. Thank you for taking the time 
to document your restoration practices.

73
John K5MO

On 12/2/2017 5:07 PM, David Stinson via BoatAnchors wrote:
> Rethinking Re-capping.
> 
> Concerning the restoration of
> historically-significant radios: my views and
> goals have evolved over the years.  My goal-
> personal fulfillment from answering the challenge
> of these sets aside- is to give the radios the
> best chance to be preserved beyond my time as
> their temporary care-taker.  To do so, I believe
> the radios must play, and should do so with the
> minimum disturbance of the original assembly
> possible.
> 
> There was a time when I would say "replace very
> electrolytic and every paper and every Micamold"
> as a matter of course.  I no longer do that.  Been
> working on a set of three RAX receivers that have
> been awaiting their turn in my barn and storage,
> for at least 12-15 years.  All have been playing
> 12-24 hours now without failure.  One needed no
> cap replacements at all.  The second needed one
> and the third needed two.  I changed no resistors.
> Did repair some bad trimmer caps in two receivers,
> but not counting them for this post; different
> issue.  To find those that actually need te be
> changed requires a reliable schematic, a good
> meter, a variable B+ supply, patience and a finger
> tip.
> 
> In a tube type receiver such as the TCS, SCR-274N
> or RAX, there are several paths that B+ travels to
> ground.  Most of them go through tubes.  When the
> tubes have no filament voltage, the tube plate or
> screen pin is a "dead end;" an open circuit.  No
> current should flow through the tube's plate or
> screen circuits.
> But there are usually other paths- like
> screen-voltage dividers- that provide a path for
> some current flow.
> 
> If you spend some time with the schematic, you can
> trace all the B+ distribution busses, looking for
> paths to ground.  For instance- in the -274N
> receiver, two 7000-Ohm resistors are connected
> between B+ and ground as a screen-voltage divider.
> With the tube filaments off, the receiver out of
> the rack and with no local control box installed,
> the only path for B+ to ground is through those
> two resistors, unless there are leaky bypass caps.
> So, with no filament voltage, if we connect our
> variable V+ supply and increase to 100V, we should
> observe no more than  100/14,000= 7 milliamps of
> current flowing.  If we have more, we have leaky
> capacitors somewhere.
> 
> So how do we find the leaky cap(s)?  Did this test
> on one of the RAX receivers.  After allowing
> "reforming" time for the electrolytic caps (and no
> filament voltage), at 100V on the buss, it was
> drawing 20mA.  100V was dropping somewhere and at
> 20mA, the leak was turning that current into 2
> Watts of heat.  That doesn't sound like much, but
> it builds-up quickly.  After a few minutes, a
> finger-tip on one of the .05 uFd screen bypass
> caps felt the heat and the ZM-11 proved it was the
> villain.  After replacing that cap, the current on
> the B+ buss dropped to 6 mA; just 2 mA above that
> expected and close enough to operate.  Left the
> 100V on the buss for four hours and no increase in
> the current.  That's not a promise no other cap
> will fail- they are 70 years old.  But if the goal
> is to do the least possible and given I'll be
> running these at reduced B+, the risk of damage is
> small and acceptable.  If one can't "feel the
> heat," a study of the B+ distribution on the
> schematic will reveal places one can lift a wire
> or part to isolate the branch with the bad actor.
> 
> In many radios (not in RAX but in others), there
> are coupling caps that go to grids which can be
> leaky.  Apply B+ with no filament voltage as
> before.  Measure the grid of the tube fed by the
> capacitor coupling.  If you have a positive
> voltage on that grid with no filament voltage,
> change the coupling cap.  NOTE: Use a VOM for
> this, not a VTVM.  The very-high input impedance
> of the VTVM may be "spoofed" by an electro-static
> charge on the grid side of the capacitor plate.  A
> good VOM will load it just enough to give you a
> "real" reading.  Exception:  Leaky cathode
> bypasses, especially the one at the Audio PA, need
> to be checked individually, since there's no
> current path to find them without filament
> voltage.  Quick check:  Calculate the current that
> should flow through the cathode resistor if you
> put +10V on the tube cathode.  i.e. if the cathode
> resistor is 330 Ohms and you connect +10V to
> ground to the cathode, you should see 30 mA of
> current flow.  If (after a reasonable "reform"
> time) it's pulling more than 30 mA, your cathode
> bypass is leaking.
> 
> But what about OPEN bypass or coupling caps?  With
> the danger of frying things from bad bypasses and
> coupling caps past, one can power the set and
> check performance.  If you have open bypasses or
> coupling caps, *the circuits will tell you.*
> Oscillation in IF amps or Converters usually
> equals open bypasses.  A screwdriver touched at
> tube bases will often change the "motorboating"
> and tell you which stage is oscillating, or use a
> scope.  If the oscillation changes with tuning,
> the Mixer/Converter stage is oscillating (this
> happened with one of the RAX receivers- open
> cathode bypass on the Converter).  If the audio is
> distorted, look for open bypasses in the 1st Audio
> and Audio PA.  If you have no gain through a
> stage, check any coupling caps that might be
> involved.   But we shouldn't assume in these cases
> that capacitors are the culprit; resistance and
> voltage chart checks are your friends.  For
> instance- I've revived three TCS receivers lately
> and I find wonky resistors to be an even bigger
> problem than capacitors.
> 
> With these techniques, I have three RAX receivers
> on the bench playing nicely for hours.  I've
> changed three capacitors, no resistors and one
> tube.  Did also repair some trimmers as noted
> before.
> 
> GL OM ES 73 DE Dave AB5S
> 
> 
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-- 
III