>I repaired the amp and had another failure. Logic tells me there's a good
>chance the failures are related,
sound reasoning methinks.
>and I'd appreciate your thoughts as I am
>perplexed.
>
>To recapitulate, the first failure occured the first day I used the amp
>after it sat idle for six months. Previously my friend, the amp's owner,
>had used it at his station, so the amp had never been previously run at my
>QTH. It worked, seemingly alright, for a few QSO's on CW. Then, while the
>amp was on but in standby mode, and I was copying a fellow, there was a
>flash, a bang, and the amp lights went out.
Deja vu.
>One fuse was blown,and the a.c.
>breaker tripped (20 A, 240 V). Inside the amp, the 160 V 33 uf electrolytic
>which filters the 110 V control voltage half-wave rectifier had exploded.
Failure of this capacitor is usually hastened by 160m RF leaking into the
power supply compartment past the too-tiny HV bypass cap of 2000pF.
>The metal foil appeaded to have shorted at least one of the control and
>filiment transformer's 120 v lugs to the case. That was what I thought blew
>the fuse and tripped the breaker. One end of the bias zener was cover by
>foil, and I assumed 120 V had gone through it - although the other end
>should have basically been floating if the amp was in the idle mode. In any
>case, this failure happened with the amp sitting in standby mode where 100 V
>should have been applied to the grids to cut them off. I replaced the zener
>and capacitor tonite, and the amp seemed to perform as before when slowly
>and carefully tested.
>
>The idle plate current was running about 180 ma indicated which is what it
>showed before. I transmitted a few minutes, CW, and when I unkeyed, a relay
>chattered in the amp, and the amp's transmit light flashed off and on
>several times. I noticed the rig was sitting peacefully in receive. Then,
>the amp seemed to settle down, but I noticed about 100 ma of plate current
>when there should have been none. I found two things upon inspection.
>First, the very same capacitor which was replaced - although with a 250 V
>rather than 160 V variety was very hot to the touch, and the case had
>expanded slightly. Second, after much searching, one of the grid chokes (to
>ground) was open - and I could see where the wire had failed.
My guess is the intermittent parasitic oscillation at c. 120MHz caused
the glitch. If you own a dipmeter, it might be interesting to measure
the dip freq at the rear of the DC-blocker C at the rear of the Tune-C.
The dip is usually deep enough to suck-out the dipmeter with close
coupling, which indicates the resonant circuit that makes the dip
probably does not have a low Q.
> I also
>noticed this choke did not match the other one, nor did the solder, so this
>is not the first time I suspect that the choke had failed.
>
Repeated choke failures can be caused by either encore parasites or by a
shorted tube. Also, the half-wave rectifier should be replaced with a
full wave bridge and the 80V-rms winding rewired accordingly. Changing
the 922 from V-cutoff bias to R-cutoff bias may save the unfused filament
transformer from getting crispy-crittered if and when a tube eventually
shorts from the bursts in grid current that smokes the grid choke. [for
nitty-gritty, see 922 article].
>Does this sound to you like that parasitic problem you describe?
Either that or maybe, just maybe, your 922 is very seriously posessed by
The Devil !
>Do you or
>does anyone sell the vhf parasitic killer you describe on your webpage. I
>am much puzzled. Thanks!
>
Yours truly does. Are you willing to loose c. 2% of the power out at
29MHz by using lower-Q VHF parasite suppressors?
cheers, John
>
>| The 10w zener in a 922 may short during a parasite. An article about the
>| 922 is available on my Web site.
>|
>| >Does anyone know of a source for power zeners? I lost the bias zener in
>an
>| >old Kenwood TL-922. It's 7.2 volts at unknow watts, but it carries both
>| >grid and plate current which could total near an amp, so it's likely at
>| >least a 10 watt device - and it looks it. Kenwood says it's
>discontinued,
>| >and it's replacement, a "ZX7.5" is $49.70, and I'm hoping to find a less
>| >expensive solution. It failed when a 160 volt electrolytic blew up and
>| >spewed foil around the base of the control transformer, and I think the
>| >zener got a dose of 110 v to ground - the amp was sitting in idle mode at
>| >the time. Thanks for any suggestions.
>| >
>| >73, John, ke5c@arrl.net
>| >
>| >_______________________________________________
>| >Amps mailing list
>| >Amps@contesting.com
>| >http://lists.contesting.com/mailman/listinfo/amps
>| >
>|
>|
>| - R. L. Measures, a.k.a. Rich..., 805.386.3734,AG6K,
>| www.vcnet.com/measures.
>| end
>|
>
>
- R. L. Measures, a.k.a. Rich..., 805.386.3734,AG6K,
www.vcnet.com/measures.
end
|