[R-390] A story of two grounds.
Philip Atchley
[email protected]
Mon, 21 Oct 2002 21:14:21 -0000
Hello all.
Sometime back I re-capped and serviced this '67EAC R-390A that I'm presently
using and it performs VERY well. I chose to go the "under chassis" route
for the 'lytics rather than rebuild the cans.
One thing that I noticed was that this set had some 120Hz noise in the Local
audio that, while not loud was noticable with the volume turned all the way
down. NOT a hum, it was more like the line related switching noise one
hears from Triac controlled lamps etc. So today I decided to troubleshoot
it (I HAVE to get a helper to move this thing around 8^). I suspected a
ground loop.
To make a long story short. When I mounted the capacitors under the chassis
I mounted two solder lugs under existing screws, cleaning under them to make
sure they made good contact. This so I could make a VERY neat installation
of the capacitors as the lugs provided a convenient mounting point for the
ground end of the 'lytics. I've done this before with good luck. Previous
sets had the Solid state rectifiers while this one has intact tube
rectifiers. Anyway, on a hunch I replaced the ground lugs with two
insulated terminal strips (Radio Shack) in which I had cut off all but one
insulated lug. I then connected the negative end of the capacitors to these
lugs and ran a ground wire back to the two appropriate Capacitor Octal
sockets giving the capacitors their original ground points.
Upon testing, Voila! That did the trick. IF I put my ear on the speaker I
can 'just discern' the noise. In normal operation certainly not noticable
like it was.
MORAL OF THE STORY: I'm sure Art Collins spent a lot of money researching
and designing the grounds in our beloved R-390A's, don't try to second guess
him 8^)
One additional Note: The ground lug where these caps are originally
grounded IS NOT all that far from where one of my ground lugs was located.
73 de Phil KO6BB
Chasing NDB's (Non-Directional Beacons)with .....
LF Modified R-390A,'67 EAC (26 valves).
Homebrew 'Hybrid' Longwave CW receiver.
20 Meter Long Alpha Delta Sloper Antenna.
50dB Gain Low Noise Tunable Preamp.
Heathkit HD-11 Q-Multiplier.
Merced, Central California.
37.18N 120.29W CM97sh.