On Tue, 2 Mar 1999 19:56:47 -0500 "Jeff Weinberg W8CQ" <w8cq@indy.net>
writes:
>Hello Amp Guru's,
>
>I have a question for the multitude. I have a perfectly functioning
>TL-922A. Recently, I have noticed a strange anomaly. Under keying
>condition, that is keyed up with no modulation, I am seeing about 270
>mA of plate current (normal),
High. 200ma in SSB is per the manual.
but am showing approximately 10 mA of
>grid current. I cannot figure out why I am showing the small amount
>of grid current.
Check the bias zener and C-26 across it. The cap has a history of
failure.
>
>I have changed tubes between three different amps and three different
>sets of 3-500Z's and it happens with all tubes. I am sure the problem
>is in the amp. In addition, I have checked the shunt resistors in the
>grid circuit, and they all check good.
What shunt resistors?? In an un -voodooed TL-922 there are no resistors,
just a pair of grid chokes plus 220pf caps per pin. Those original
chokes are too big, current and low resistance wise. I suggest replacing
with something in the 750uh to 1.5mH 160ma and 25 Ohms DC resistance
range...as used in the SB-220. If there are some strange parts in the
amp, it might be best to throw them away and follow the original design.
While you have the half million screws removed( Do Jap designers get paid
by the screw? The THP and Sigma amps are other examples of a screw
fetish), replace the VHF choke, L-2, with a 15-20 Ohm 20W real wirewound
( not cement) resistor. It will do wonders for reliability.
Another common TL-922 problem ( Line voltage dependent) is excessive
filament voltage. Adding resistance in the filament xfmr primary to
obtain around 4.8VAC RMS across each tube will result in long tube life.
It will minimally affect the cut-off bias and relay voltage.
Rich's relay mod is excellent when using a modern fast switching exciter.
>Also, the meter is perfectly functioning in the HV or RF position.
>The grid current aside, all is functioning properly.
>
>Anyone have any ideas what may be bad or where to continue looking
>(Rich or Carl)?
Hope I hit most of the pertinent points Jeff. After a week away I have
over 200 msgs to answer.
73 Carl KM1H
>
>Thanks,
>
>Jeff Weinberg W8CQ
>w8cq@indy.net
>
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