[EICO] Eico 720 putting out an awful note

[James M. Walker]

Hi,
Well first I have to ask is the VFO/XTAL switch in the proper position?
then a comment, the rectifier is a GZ34, although a 5U$ will work, I
use the 5U4 in mine.

What size choke did you put in to replace the burned out one? Check the
6AQ5 clamp tube if possible replace it and try again to see if the 60
Hz note goes away.
can't think of anything else, I have to assume you have the jumper in
for cw only operation!
Jim
WB2FCN
http://eshop1.chem.buffalo.edu 


shoppa_eico at trailing-edge.com wrote:
> 
> 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.
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