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Re: [AV Media Matters] Wire Recorder

Reviving any old tube hardware can be done but must be done cautiously, as
capacitors may have been unused so long, that the dielectric of the
electrolytics must "reform".  Also, old insulation may have cracked in
storage, and now be allowing a wire to ground out.

My standard procedure involves the use of an AC VARIAC autotransformer to
gradually bring up the AC line voltage, after allowing the unit
to cook at a
lowered voltage in steps, until the capacitors reform the oxides that form
dielectrics.  If you do not have access to a variable transformer such as
this, I also have used a lamp socket wired in series with the hot
side of the
AC line, and have placed a 60 watt bulb in series with the AC
line, to drop
the line voltge.  Various wattage bulbs would give you varying
voltage drops.
You gently apply voltage through a fused outlet, and if the fuse
holds, let
the lowered voltage have an effect for some period of time.  How
long may be
needed depends on the condition of the caps, and the resulting hum voltage
impressed on the DC bus.  You may have to estimate and by trial
see what the
hum will do by longer reforming.  The objective is to lower the
hum to normal
tube levels.

Ideally, you should divorce the rest of the circuit from the power supply
parts by unsoldering the main DC line out of the power supply. Do
the power
supply first by itself.  You must reform electrolytics in the
audio amplifiers
eventually anyway, but fewer pyrotechnics are possible if you sectionalize
your efforts.  You may also have to replace non polar wax
coupling capacitors
between plate and following grid tube sections.

Of course, before doing any of this, a close physical inspection should be
done for any obvious problems like bulging capacitors, chemicals
leaking out,
broken seals on ends of electrolytics where the rubber seal is
located, etc.
Also, using an ohm meter, you should check that no direct short
circuits exist
on the DC and filament buses.

A shop which does antique radio restoration should be able to
help you with a
process for rejuvenation.  Also, there is a book out on the hobby market
covering some of these points.  Check under Radio restoration as keywords.
In Tempe AZ there is a firm that stocks NOS parts, tubes and
these books.  I
believe they are called "Antique Radio Supply".  There is a web
site I have
heard, but I do not have the URL.

Most of the older circuits used higher impedances internally to
the tubes than
the modern circuits you want to couple into.  However, by use of matching
transformers, or even by careful capacitive coupling, you should
be able to
isolate the High Voltage tube DC circuits and couple audio out to
wherever you
need it.  The old line out standard, and broadcast standards, were 600 ohm
audio lines.  Some newer, especially consumer amplifiers, are 1000 ohms or
higher; but, at the fidelity of wire recordings, you probably
aren't worried
about modest impedance mismatches doing anything to the quality.  I really
prefer the isolation of transformer coupling when connecting between tube
circuits and newer low voltage solid state gear.

Good luck and let us know how you come out.

Stuart Rohre
ARL Univ. of TX, ESL Recording Lab

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