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

[Bob Weiss]

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