[R-390] Solid State Ballast 'quirks'?

[Roy Morgan]

At 07:35 PM 6/2/2004 -0400, Bob Camp wrote:
>  ... If I was going to bet on the cause it would be a loose connection 
> early on the IF strip or at the tail end of the RF chassis. The key clue 
> being that you still have static when the signal goes away.

Troubleshooters:

Bob's comment is right on, but reveals a possible other mis-adjustment. If 
the IF strip is contributing much at all to the set's noise (with the RF 
section not working), its gain is probably set too high. See the procedure 
to set the IF gain on Chuck's website at http://www.r390a.com/ and 
specifically at:
Under "Technical":  "Setting the IF Gain for Maximum Performance"
http://www.r390a.com/html/gain.html
Note: "Terminate the antenna input" means put a resistor across the two 
balanced input pins of about 100 to 200 ohms.

The basic concept of this adjustment is that the radio's internal noise 
should come almost entirely from the first RF stage for best 
performance.  It's common for folks to crank the IF gain adjust pot up so 
they get louder signals. This upsets the carefully designed balance of gain 
in the various stages of the radio and leads to reduced performance - 
particularly in small signal detection capability and in overload  and 
cross modulation characteristics.  Those who used to service these sets in 
military situations where small signal performance was very important 
report that new RF front end tubes are noisier than those with some hours 
on them.  So if you are tweaking for best noise, use a broken-in 6DC6.

>  ..  It's good to have a VTVM to do the measuring with. ... for $30 if 
> you are in a hurry or quite a bit less if you shop around.

See this link for a Heath V-7A priced at $15.  (I still have the one I 
built in about 1959, and it runs just fine!):  http://www.af4k.com/miscpart.htm

If you want a genuine TS-505 as specified in the R-390 manuals, here is one:

>TS-505 VTVM ("http://www.w2ec.com/ts505.jpg"), includes cover and probe.
>$25 plus shipping from NY 13732
>
>Ray W2EC "w2ec at bmjsports.com"

These VTVM's have a meter face that is a bit harder to read than the Heath, 
but they are quite indestructible.  By rights, no R-390 repair person 
should be without one for authenticity. It has been rumored that very 
difficult to find faults in R-390 receivers have cured themselves 
permanently when the TS-505 was turned on and brought to bear.

Note that this meter also has an AC/RF probe that is useful from 30 cps to 
10 mc and below 40 volts RMS.  The diode used is a 1N70 germanium diode, 
not the hard to find and expensive vacuum tube ones found in the HP 410 
meter probes.  If you blow it out, a modern PIN diode will likely work just 
fine.

By the way, the same fellow has a URM-25D that has been overhauled:
>URM-25D Signal Generator ("http://www.w2ec.com/urm25d.jpg"), perfect
>operational condition, with manual copy and HSN reprint of Dallas
>Langford notes. All caps have been replaced per the HSN notes.
>Also includes accessories as shown: ("http://www.w2ec.com/urm25dacc.jpg").
>$125 plus shipping from NY 13732


Happy troubleshooting,
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
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