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[AMPS] SB220 Grid current

To: <amps@contesting.com>
Subject: [AMPS] SB220 Grid current
From: measures@vc.net (Rich Measures)
Date: Mon, 12 May 97 00:28:01 -0700
>
>On Sun, 11 May 1997 19:05:17 EDT ko4a3@juno.com (H. J. Kohl) writes:
>>Help,
>>      The grid current meter on my SB220 is all the way to the right 
>>with key down, even on SSB mode of the exciter.  On dekey the meter 
>>pegs the left side.  Is this a tube problem or meter circuit problem.  
>>  Suggestions please.
>>
>>      Thanks,
>>      John 
>>      KO4A@juno.com
>
>The SSB position will worsen whatever the situation John, run the tests
>in the CW position with no drive when keying the relay.
>A few things to check:
>
>1. the Zener diode
>2. the .82 Ohm grid metering resistor
However, if you find out that this resistor is open or damaged, keep in 
mind that normal dissipation is about 0.04W and the resistor is rated at 
2w, so it is unlikely that the resistor was defective.  IMO, it takes an 
anomaly to blow this resistor.  [see 9/90-10/90 QST, "Parasitics 
Revisited"]
>3. One tube at a time

>Before doing anything put a back to pack pair of diodes across the meter
>terminals. Any 1N4nnn series will be fine. At least the meter will be
>saved if something nasty happens. 
 Good advice, methinks. 
>If those 3 basic steps check out OK, get back to me and I'll work along
>with you. 
>
>73....Carl  KM1H
 Another precaution for stock SB-220 and stock TL-922 owners:
If a 3-500Z develops a (bent) filament to (grounded) grid short, the 
(unfused) filament/110v supply transformer will burn up quickly unless 
the amplifier is switched off.  The reason for this is that the internal 
110v supply is connected directly to the cathode (fil. winding CT)  to 
bias the tubes to cutoff during standby. If a filament shorts to one of 
the grids, the +110v supply shorts to ground.  .No fuse = crispy-critter. 
 
 The fix is to NOT apply +110v to the fil. CT during standby--i.e., 
remove the existing wire from the bias relay contacts to the +110v 
supply.  The tubes will bias themselves to cutoff just fine when the 
cathode bias relay contacts open, thank you.  Purists may want to connect 
a 100k ohm 0.5w resistor in parallel with the relay contacts---but the 
tubes will bias themselves to cutoff either way.   During standby, the 
potential across the 100k resistor will be about 23v. ... ...  If a tube 
subsequently shorts, the grid-I meter will peg backwards---but no damage 
will occur.  
Rich---

R. L. Measures, 805-386-3734, AG6K   


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