[R-390] Re: little things

[Roy Morgan]

At 09:43 PM 5/12/2005, W. Li wrote:
>Over the years, I have made some little additions to
>my trio of R-390A's that may be of use to you guys.

Bravo!
This is the kind of post that helps both the new folks and the 
experienced-encrusted graybeards.  Thanks for the summary of Tips.

>  Antenna input
>I used Chuck Rippel's suggestions

I plan to experiment with little toroid baluns - small enough to fit inside 
the Twinax connector. A reducer of some sort and a piece of coax fitted 
with a female BNC cable mount connector will finish the job.

>   F101 and old two conductor pwr cord
>... a new Corcom EMI power receptacle...

This is a fine mod.  The EMI situations we have are far less demanding than 
the miltary had.  Most modern EMI filters don't trip ground fault 
interruptors.  The three wire grounded line cord is a VERY good idea.

>TB-103
>Jumpered 6 and 8 to bypass R101 (6800 ohm) to increase output. Works.

For those like me who don't remember the details: TB-103-6 is local audio 
out (600 ohms), TB-103-7 is ground, TB-103-8 is phones out, in parallel 
with the front panel phones jack.  So this mod changes the output impedance 
of the phones jack and rear terminal from about 7.4k to 600 ohms.

>I-177 5814A accessory (to test 5814's)

Of course one remedy is to acquire some more tube testers.. heheh

>C603 and C-606
>To reform these capacitors I used Roy's circuit, (B+ supply with vairac): 
>... an enclosed case... 3PDT toggle either powers the
>Variac up or discharges the electrolytic thru a 1000
>ohm bleeder.

This is a fine test fixture.. very handy.  Embellishments could be:
  - An octal socket for the plug-in caps,
  - Series current limiter resistors for each section
  - On-discharge switches for each section
  - External cap connections
  - A panel voltmeter, or terminals to hook up your VTVM or DMM
  - A switch to select which cap is being measured.

>...the Sangamo's & Cornell-Dubliers reformed
>nicely with only 0.3mA leakage at 250v.

I use a 100 k series resistor and set the variable supply so it drops about 
100 volts. This gives a reforming current of one millamp.  If the variable 
supply is able to reach 550 volts, you can reform most any electrolytic to 
1 to 5 ma with little danger to anything.

>Audio deck
>Nolan... four washers as a "mini-stand-off"...some
>heat to leak out

Cooler is better, for sure.

>220VAC muffin fans running at 110
>One of these days I plan to mount these fans outside
>the sides

Again, my crude fan plate for the R-390/URR is shown at:
<http://home.comcast.net/~roysmorgan/ba/FanPlate.html>
Some changes may be needed for the R-390A.

Roy

- Roy Morgan, K1LKY since 1959 - Keep 'em Glowing!
7130 Panorama Drive, Derwood MD 20855
Home: 301-330-8828 Cell 301-928-7794
Work: Voice: 301-975-3254,  Fax: 301-948-6213
roy.morgan at nist.gov --