[EICO] Eico 720 putting out an awful note
shoppa_eico at trailing-edge.com
shoppa_eico at trailing-edge.com
Sat Aug 20 21:04:04 EDT 2005
OK, I cleaned up my recently-acquired Eico 720. New electrolytics
in the PS, new plate choke for the final, "new" 6146 (replacing the
6146B the previous owner put in the final and which may have caused
the old plate choke to burn up.)
I hooked up a 75W light bulb as a dummy load and put in a known good
40M crystal. On the scope it looks great - it peaks at the right
harmonic on 40M, 20M, 15M and 10M, and I can load up into the light
bulb just fine, with the dip in plate current when the bulb is brightest.
This is great!
But when I listen to the transmitter on a nearby receiver, it sounds
awful. Some chirp when I key up but that's not the worst part: an
awful 60Hz hum and a lot of what sounds like 60Hz FM'ing of the carrier.
The electrolytics in it are new and are a little larger than the original
(47uF vs 40uF) in capacity. Rectifier seems to be original with the
radio (5U4 by GE with 1961 date code). The other tubes seem to be
original too (6CL6 for the oscillator, 6AQ5's for the multiplier/buffer
and the clamper). Seems to be plenty of drive on the final, on 40M
it drives it just fine with the drive level set to the lowest, and it
takes a bit more drive on the higher bands but that's to be expected.
I was a little surprised that the 6CL6 was putting out 70V of RF, but
maybe that's normal?
The only electrolytics seem to be the two in the PS. The PS choke
seems to be just fine. If anything, the voltages in the unit seem to
be on the high side by 10 or 20 percent, but maybe that's just higher
line voltage (120 vs 110).
I will try putting in a known good signal source into the VFO input
and see if it sounds as bad.
At least I thought my 40M crystal was good: is it possible that an
old/damaged crystal could lead to awful 60Hz hum/FM on the carrier?
I don't think so.
One thing I never understood was how the clamper circuit works.
I read my ARRL handbooks from the 50's and 60's but I don't really
understand how changing the screen voltage on the final is supposed
to affect class C action. Maybe I gotta brush up some more.
Not sure if it's related, but the computer (about 15 feet away) crashed
while I was tuning up into the dummy load. Maybe I'm supposed to put the
cover on the radio, huh?
Tim.
More information about the EICO
mailing list