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Hints and Tips
R-390A and R-390
 | Some Orange Drop capacitors have a black line (right side) indicating the
side connected to the outside foil. I'm told that ALL orange drops
have the outside foil attached to the lead on the right-hand side as you
read the lettering. For bypass capacitors, the outside foil end should
be attached to ground. For coupling capacitors, the outside foil
should be connected to the plate of the prior stage rather than the grid of
the following stage. |
| Put an "aged" 5749W tube with good transconductance and low
noise in the PTO. New tubes are not recommended here. |
| Test all the 12AU7/5814A tubes in a tube tester for transconductance.
Put the best one in the detector, and the worst one in the line audio stage
(unless you use the line output). |
| To check the AGC oil filled capacitor (C551 on the R-390A), let the
receiver warm up for about an hour. You can use a hair dryer to warm
up C551 if you're impatient. Set the FUNCTION switch to CAL mode with
the BFO off and the AGC on Fast, and tune to one of the 100 Hz markers,
peaking the carrier meter reading. Note the carrier meter
reading. Now switch the AGC to Slow and recheck the carrier meter
reading. If the capacitor is good, you should see little or no drop in
the carrier meter level. If the level drops, the capacitor is leaking
enough to affect AGC action to a degree. |
| Use Phil Wood waterproof bike grease for lubricating the detent wheel,
camshaft edge surfaces, veeder root gears, brass shaft bearings, slug
rack bearing slides, and all mating surfaces where the slug rack fingers
slide against metal. There is an incredible difference between this
and other "similar" products. If you want a smooth gear
train, this stuff is a "must have" item. I found mine at a
local bike shop, but they also have a website.
Thanks to Matt Parkinson, KE6UOS, for this great tip. |
R-390A only
| Pull the 6C4 tubes, and test all 6C4 tubes for noise in socket V204.
Or even better, inject a 455 KC signal at E211, and check each tube in
socket V204 for best S/S+N ratio. Put the best tube in V202, the
second best in V203, and the worst in V204. Throw out any tubes that
are significantly worse than your "best" performer. Final
sensitivity can vary considerably depending on the noise of the 6C4
tubes. (Don't try this on an R-390, as some of the filaments are in
series, and breaking the filament chain to voltage regulator tubes makes B+
go through the roof) |
| Do the same for all 5749W tubes in the IF deck, testing in socket V504 and
putting the better (lower noise) tubes in the earlier stages. |
| Always remove the can around Z503 before making any adjustments.
This coil often has come loose, and spinning the coil core will spin the
coil and break the wires. After you are sure it's firmly in place
while you adjust the coil, but the can cover back in place and make your
adjustments. |
| If you have to remove a turn from your Cosmos PTO: Take an aspirin
tablet and place it under the litz wire. Get a ball of solder on a hot
soldering iron and use it to press the litz wire into the aspirin tablet.
The wire will be stripped and tinned all in one operation. |
| On an old R-390A IF deck, replace C553 with a 600V Orange Drop prior to
first power-on. Failure of this capacitor will destroy mechanical
filters. |
| RF slugs and IF slugs are NOT the same. Sometimes they are hard to
tell apart. Some IF slugs have more of a greenish tint to them,
compared with the RF slugs. The IF slugs have a shiny, shellac looking
surface appearance, as compared to a dull surface on the RF slugs. |
| Any bad mechanical filter can wreak havoc with the AGC. The filter
does NOT have to be switched in to cause problems. |
| If you have the IERC or other black (inside and outside) tube shields, use
them. But if you have the shiny aluminum ones, the tubes will run
cooler without the shiny shields. But HSN suggests leaving the tubes
on V201, V206, V505, and V701 to prevent leakages paths. |
| You should be able to find date codes on C603, C606, C103, and S106. |
| Q-reducing resistors are used inside the T501, T502, and T503 IF
cans. These give a flatter frequency response. Some rigs may
have had these resistors "clipped" inside the cans, which gives
higher Q. I always reconnect these resistors if they've been
clipped. Older IF transformers have 47K resistors for R511, R512,
R553, and R554, and have 82K for R522. Later models have 39K and
68K. These values are matched to the coils so don't alter them. |
R-390 only
| Always have all decks connected and all tubes installed when the power is
on. Some heater filaments are in series in this receiver, and breaking
the filament chain will cause the B+ to be unregulated. Your 180 volt
B+ will quickly climb to over 300 volts. |
| If your R-390 seems to block and distort on strong signals, check that the
ANT TRIM shaft is not getting grounded. This shaft is connected to the
ANT TRIM capacitor and is at AGC voltage potential. If it finds a path
to ground, your 1st RF amplifier stage will have zero bias voltage and will
overload. |
| The IF deck wafer switches should be clean with no oxidation. Only
one-tenth of an ohm resistance can cause the Q of the tuned circuits to
change by 5 to 10% (see the final engineering report). That's why they
used both front and back of the same wafer paralleled to accomplish the same
function. I suggest treating the wafer switches with ProGold or
Stabilant. Apply DeOxit first if there is any visible oxidation on the
wafer switch. |
| Never remove the RF deck without the green gear in place, unless you just
need practice doing the mechanical alignment of the gear train and cams. |
| Check the 47 ohm resistors under the 6082 tube sockets. If any have
drifted above about 52 ohms, replace them all with 5 watt replacements. |
| The slugs in the R-390 are NOT all the same. They are of different
ferrite material and are of different diameters in later models. |
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