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[TenTec] Century/21 Model 574 receiving what's not there?

To: <tentec@contesting.com>
Subject: [TenTec] Century/21 Model 574 receiving what's not there?
From: tosee@ekit.com (tosee@ekit.com)
Date: Wed Aug 20 22:27:59 2003
WARNING!! I'm prolix when I use a keyboard!

I'm a pretty new ham who started as a General because I primarily 
want to work QRP CW on HF.  I was looking for an old rig that I could 
pretty much plug 'n play on HF with a key, an antenna, an antenna 
tuner and an SWR meter.

I picked up a Century/21 Model 574, serial number 574-0253, at a swap 
meet yesterday in the hope of acquiring a rig that I could use for CW 
QRP by just keeping the Drive control turned way down.  The seller 
(who wasn't the previous owner; a silent key was) said it powered up 
and the digital readout presented sensible numbers, but he hadn't 
checked it out further.

I opened the top and bottom covers and it looked really good inside--
no mods or repairs that I could see.  The aux XTAL for 28.5 to 29.0 
MHz was in the socket, and the seller had the owner's manual.

When I got it home, I turned the Drive control all the way down and 
kludged an excuse for an antenna connection into the SO-259.  (As a 
newbie, I don't yet own a decent antenna or a single 50 ohm coax 
cable with PL-259 connectors.)  

When I turned it on and started listening, my first thought was that 
someone had put an internal crystal calibrator into the receiver 
circuit.  I could hear what sounded like unmodulated carriers on all 
5 bands at various points throughout the dial and could also hear a 
(perhaps) 5 Hz beep at some points on the dial.  I can zero-beat all 
these things, but sometimes there were 2 "signals" pretty close to 
each other, separated by not a lot of Hz (I don't exactly have 
perfect pitch).

The idea of a crystal calibrator soon fell by the wayside because 
the "signals" weren't zero-beating on multiples of 25 KHz or 100 
KHz.  Likewise the 5 Hz pulses weren't on any frequency that made 
sense to me--for example, one place I could hear the 5 Hz was 14242 
KHz or so.

I can hear real CW signals and listen to SSB QSO's, but the zero-
beatable 5 Hz beeps and things that sound like unmodulated carriers 
are quite audible with no antenna connected, the antenna connector 
shorted or unconnected, and the RF "Gain" control set all the way 
down (CCW).

I wonder if the 5 Hz stuff is coming from the Digital Readout board.  
If so, how does it show up as 5.0 - 5.5 MHz IF?

The "signals" I'm describing are not making their way from some 
transmitter on the ham bands.  I have 2 general coverage shorwave 
portables with BFO's and don't hear these "signals" on either of my 
portables; just on the Century/21.  (Only 1 receiver is turned on at 
a time just to make sure that one receiver isn't picking up signals 
from another of my radios.)

Any ideas on how to get rid of these pseudo-signals?  My first 
thought, since I don't have a bench full of test equipment, is to 
start replacing electrolytic capacitors all over the place since the 
unit is around 25 years old.  Other suggestions would be most welcome.

Tnx & 73,
Irving M. Prais

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