[R-390] Another R-390A back among the living, and de-nuclearized....

[Pete Lancashire]

congrats and thanks for another rescue.

I hope to get both my 390 and 390A in a condition to where at least I can
pass them on to someone who will fully restore them, and make a few $'s for
my son.

-pete

On Sun, Jun 10, 2018 at 2:40 PM Bob Weiss <bobweiss1967 at gmail.com> wrote:

> New to the group, and just finishing up the repair/restoration of a
> Stewart/Warner R-390A (s/n 2211) which I received as several boxes and
> bags of parts, and have managed to put back together into a fully
> functional receiver. Thanks to all who maintain this list, and who
> contributed to the Y2K manual(s), which were a huge help during the
> process.
>
> In addition to replacing all the paper and electrolytic caps, I
> installed a GFCI-compatible AC line filter, solid-stated the power
> supply, added inrush limiting, and swapped out the selenium rectifier
> for silicon. Unit works great, and will go through one final iteration
> of the complete alignment before installing into a rack in the operating
> location.
>
> Another upgrade was the replacement of the infamous radioactive panel
> meters. A check with a Geiger counter (Ludlow, with 2" pancake tube)
> showed nearly 10,000 CPM at the (completely intact) meter face, and much
> of that was gamma, verified by shielding the probe with a sheet of
> aluminum with little effect.  Figuring that having a couple unnecessary
> gamma sources a few feet from my head when sitting at my bench certainly
> wasn't helpful in any way, I decided to go for the ALARA approach, and
> seek replacement meters.
>
> I lucked into ma nice matched pair of DeJur VU meters on eBay, which
> were exact mechanical replacements for the DeJur meters that were
> originally installed. The meter faces were marked as "Electro Ind.
> Corp.", who apparently made magnetic tape recording gear at one point.
>
> Replacing the line level meter was a simple matter of connecting the new
> and old meters  to an audio generator, and checking the sensitivity of
> the new meter against the original. The new meter was quite a bit more
> sensitive, but adding a resistor of around 950 ohms in series produced a
> reading that perfectly tracked the original across the whole range.
>
> Of course, the carrier level meter was a bit trickier to replace. I
> began by carefully opening the other VU meter, and removing the internal
> copper oxide rectifier bridge and resistor, ending up with a DC meter
> with around 1.5k resistance, and 250 uA full scale sensitivity. I also
> removed the scale card from the meter, and scanned it into MS paint 3D,
> where I reworked the scale into an S-meter, rather than the simple 0-100
> scale of the original 390 meter.
>
> Digging around online, I came across KH6GRTs design for an opamp circuit
> for adapting common meters to the R-390 bridge circuit. I decided to
> update his design somewhat, eliminating the long-obsolete LM308N in
> favor of an Analog Devices OP07, and building the entire circuit using
> SMD components to save space. The board fits nicely right on the back of
> the meter, with plenty of clearance from the cam gears behind it.
> Powered from the dial lamp supply, it was tested against the original
> meter and adjusted to 1 mA full scale using an external current source.
> It tracked nicely against the original across the scale.
>
> The replacement meters with white faces and black/red lettering look
> much nicer and are easier to read that the black-faced originals, IMO.
>
> Pictures of the installed meters and the amplifier PCB available at:
>
>
> https://www.flickr.com/photos/140826987@N07/
>
> If anyone on this list is interested, I had a couple extra PCBs made,
> and would be willing to sell them at $20 each (fully assembled, just add
> meter and one resistor) to anyone who wants one. Done by OSHPark, they
> have a dark purple soldermask that hides well inside the radio, without
> drawing much attention to the somewhat heretical sand-state parts
> attached to it.
>
> As to the original pair of meters, they are fully functional, and
> available free to anyone on the list who wants them, and is willing to
> be a careful steward of them going forward and keep them out of a
> landfill or other improper disposal. The only catch is that I am not
> willing to mail them, as radioactive materials are banned according to 2
> separate PO clerks, and I'm not willing to see what happens if I mailed
> one and it happened to trip a radiation detector somewhere in shipping.
> They would need to be picked up in Northern NJ. Contact me off-list if
> interested in adopting them, before I consign them to hazmat disposal.
>
> 73,
>
> Bob Weiss N2IXK
>
>
>
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