[R-390] On 3TF7's...

[Roy Morgan]

At 05:01 PM 10/4/2004, Drew Papanek wrote:

>The 3TF7's voltage range is 8.6 to 16.6 volts; at 122 VAC line the 3TF7 
>would see  about  16v. ...13 ohms in series the 3TF7 would see about 12v 
>at 122 VAC line.   If the line were to then drop to 110v the 3TF7 would 
>see 8.6v; at any lower line voltage it would drop out of regulation.

Drew,

Thanks for your sensible idea.  I would add that the resistor will limit 
the inrush current to the ballast and the tubes, and presumably lengthen 
their lives.

The ballast would "drop"out of regulation fairly quickly on the low voltage 
side, not just quit like solly state or even hollow state regulators can. 
On the upper side of the voltage range, the degradation in performance is 
less sudden.  There used to be a web link for a graph of the ballast 
characteristics with explanation of how they work, but I cannot find it 
today.. the page was ballasts.htm  If anyone finds that, please let me 
know. (I do have a copy of it here.)

>National issued a service bulletin instructing NC-300/303  users to insert
>resistance in series with the radio's unreliable 4H4C ballastube; R-390x 
>owners are hardly alone in dealing with ballasfailures.

Hammarlund issued a bulletin to use a 6V6 instead of the 4H4C in the HRO-60 
and other radios. One reported reason was the increasing difficulty in 
finding the ballast tube.

>  The whole issue of half wave rectification to run the tube filaments has 
> as you mention been thrashed out at great length in the past. One idea 
> that has not been tossed around is to lift the far end of the regulated 
> filament string and then full wave rectify the AC....

While re-reading the article in HSN* on the VLF mod to the R-390A, I 
discovered a seldom-referred to Ballast replacement mod.  (The 12 volt 
tubes with shorted ballast, and the 42 ohm 5 watt resistor mods are also 
mentioned.)

It is basically a triac acting as both rectifier and voltage adjust 
device.  It requires connection to only the two pins used by the ballast 
tube, and ground.  The ac filament supply is rectified, filtered by a 3000 
uF cap and sent on to the two regulated filaments.  A 14 -15 volt zener and 
pot allow for adjusting the output voltage.

*  "The R-390A on Longwave -- Cheaply"
 From Craig-Healy comes this article he originally wrote for "LOWDOWN"

Published in the predecessor to Hollow State News:
The R390 USERS GROUP
A Newsletter for URR Users
Vol 1 No. 2    a tradition since March

(My copy of this issue is not dated, but it mentions to expect the next 
issue in  September 1983.  The publisher, T.J. Skip Arey, WB2C (G??), says 
they had 70 members at that time.)

Roy

- Roy Morgan, K1LKY since 1959 - Keep 'em Glowing!
7130 Panorama Drive, Derwood MD 20855
Home: 301-330-8828 Work: Voice: 301-975-3254,  Fax: 301-948-6213
roy.morgan at nist.gov --