[ARC5] Heater wiring - 12V vs. 24v

[Mike Morrow]

>...the "test" tuning knob under the lid is missing.

That is the lock-tuned receiver tuning key (A.R.C. 9554).  It was used
to tune a receiver to the desired frequency, and then turned around and
screwed over the spline connection.  The R-23 and R-24 were NOT designed
to be lock-tuned, and were not provided with this key under the cover.
Only the R-25, -26, and -27/ARC-5 were.

>To conclude:  The filament wiring for this set assumes a 12 volt supply
>confirmed by measuring the resistance of the filament circuit when cold
>and by measurement of filament current when running.  

The easiest, sure-fire way that I use to determine if a mystery set's
filaments are wired in series (for 24 vdc) is to pull out the tubes and
measure the resistance from the LV+ dynamotor terminal.  With all tubes,
out, it will be the only of the three terminals with a very high resistance
to ground, regardless of wiring.  Then insert the RF tube.  If low
resistance in now detected, the wiring is 12 vdc.  But if resistance is
still high, it's 24 vdc.  Insert the MIXER tube (12K8) and measure the
resulting low resistance to ground to confirm the series 24 vdc filament
wiring.

Mike / KK5F