>
>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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