[R-390] Rush Limbaugh sure sounds good on '67 EAC ;-)

[Forrest Myers]

Don,
 Yes, I did use the 70/25 vole line transformer you gave me. Before hooking
it up, I did some calculations assuming an 8 ohm speaker. I figured that
600/8 = 25. The square root of that is 8.66. That should be the turns ratio
for a 600 to 8 ohm transformer, 8.66/1.  Also calculated for 4 ohm speaker
and came up with a turns ratio of 12.24/1.
 Then took my ohm meter and figured out which side of the transformer had
the primary wires and which had the secondary. The primary side had three
wires. I don't remember the colors but I figured the black one , marked "C",
was the common wire. The other two wires on the primary side were marked 25
and 70. The other side of the transformer had at least four wires coming out
of it, maybe five. There was a black wire there also, marked with the letter
C. The other taps were labeled with a  number followed by a "W". I don't
remember the numbers but I think they were 5, 10 and such.
 I hooked up the primary of the transformer to my audio signal generator and
set it for 1000cps (that's 1000hz for the younger crowd). To start with, I
hooked the 70v and the common wire across the audio generator output.
Measured the AC voltage going into the transformer and noted that value.
Then started measuring across the various taps on the secondary side for the
proper voltage according to a turns ration (or voltage ratio) of 8.66/1. I
didn't find a good match so re-connected the primary using the 25v and C
wires. This time, I got a match on the secondary side using the common lead
and the second wire away from it. Found that the common lead and the wire
next it gave correct results for a 4 ohm speaker.
 Since the mystery speaker I was using was assumed to be 8 ohms, I wired up
the transformer accordingly. On the primary side, I used the common wire and
the 25v wire to connect to the local audio output of the R-390A. On the
secondary side, I hooked the common wire and, skipping one, the third wire
from the end to the speaker terminals. Fired it up and it sounded pretty
good. Did some experimenting by changing both primary and secondary taps
while listening to the R-390A and found the primary taps were correct. Found
that the secondary side needed to be wired to the common wire and the second
wire, not the third. It appears that my mystery speaker was a four ohm
speaker instead of an 8 ohm one. It sounded OK as originally wired up but
was a little louder on the other secondary tap.
 This is a fairly long winded explanation and probably confuses more than it
helps.
 I'm currently beefing up my workshop infrastructure to be able to support
working on a radio as large and heavy as the R-390. I've worked on the audio
unit and swapped out the blocking capacitors in the IF unit. However, I have
to get a bigger and stronger workbench before I can remove the front panel
and get into serious work. Serious work like re-capping the RF unit and
cleaning up the gear train.
 Am really enjoying working on the radio though.

Cheers and 73

Forrest Myers
AG4ND