They may have done something like I do with the plate current meter. I don't ever put high voltage on meters, that is very dangerous. I put the plate current meter shunt in the center tap of the high voltage transformer, and run low voltage wires to the current meter. The bleeder resistor and caps go to the center tap itself, so the meter does not read the bleeder current or the surge or the capacitor charging. The shunt goes to ground. With a meter that has an internal shunt, one wire goes to ground, the other wire goes to the center tap, bleeder resistor(s), and the filter cap.
Only plate current will show up on the meter, since its the only current returning to the high voltage transformer center tap. Hi voltage on meters is crazy, especially modulated voltage, as peaks can go way high, and arc over quite a distance. You would need to mount meters on insulated bits, behind glass so it does not arc to your finger as you tap the meter to see if its sticking. There is no reason to have the meter on the high voltage on the modulator. You can do the same thing on the modulator power supply as well. If you use the same supply, you need to isolate the ground returns from RF and MOD decks to separate shunts. Brett N2DTS > -----Original Message----- > From: [EMAIL PROTECTED] > [mailto:[EMAIL PROTECTED] Behalf Of patrick jankowiak > Sent: Sunday, March 28, 2004 1:08 AM > To: amradio@mailman.qth.net > Subject: [AMRadio] KW restoration and need a bit of help on this one > > > Well I have about finished up the KW rig. Seeing the HV meter rise to > 3700V and having nothing explode was very rewarding. The HV is > adjustable by two variacs and I will run much less, probably 2500V. > > (http://208.190.133.201/tuckerkw/tucker_transmitter.html) > > I have a strange issue however. I hope the wisdom of the group will > help me here. > > 1. I note that the modulator plate meter is in series with the B+ lead > to the mod xfmr, and has the high voltage wiring to prove it. I can > crank the HV up to 3000V and play with the modulator bias and see the > meter work. This point is made to contrast with the next point. > > 2. The final current plate meter is more mysterious in its operation. > It has only the regular 600V wire, and the leads disappear into a part > of the wiring harness I have not investigated as of yet. The meter > does not move, and to be honest I have not yet a safe way to observe > the plate current of the final. > > 3. I did reduce the bias enough to see some screen current, (bias=100V > G2=500V Ig2=50mA Ep=2500V), in an attempt to see if I could get a bit > of color on the plate by burning up maybe 300W or so there. No such > luck with the color, I do not dare to fiddle too much with this > without a working plate current meter. I know that I have HV on the > plate because I have used the final to slightly load the B+ and > demonstrate that HV current is being drawn. The final's filament CT is > grounded at all times. the unit is keyed by the final screen and the > HV. Setting the screen volts and bias to zero results in a faster > bleed-down of the HV when unkeying the rig than when bias is kept on > the final, so I know it is drawing some current. (HV filter=40uF) > > 4. Ok, so now we would expect that the plate meter must be wired into > the negative side of the B+ supply, and this would account for the low > voltage wiring etc. Unfortunately this would not be the case, as the > final's filament center tap is grounded, as is the B+ supply's > negative side. The design does not seem to follow any of the usual > schemes presented in various volumes of the ARRL handbook or the Radio > Handbook. > > 5. Both sides of the plate current meter have a very low resistance to > ground (about 1 ohm, not too easy to measure). > > 6. so.. any ideas how this might be wired up? It's very strange and > does not behave as expected. My only other avenue is to further > investigate (un-lace) the wiring harness and see where the leads go. > > 7. Another topic: Do mercury vapor rectifiers require a certain > minimum current to remain in the conducting state? I only see them > glow during the power surge when I key the unit and the filters > charge. During the test described in "point 3" above, the screen > current would suddenly jump from 0 to 40-50mA accompanied by a drop in > high voltage from 3000 to 2500, and I adjusted the final bias. > Adjusting it back to reduce the indication of current resulted in it > sudenly shutting off to 0mA. Thus is strange behavior. It is doubtful > that it is self oscillation as the plate and grid circuits are not > tuned alike, and also i moved the frequencies well away from each > other and repeat this reliably. > > Any comments and wild speculations will be welcomed! > > Anyway I got tired and quit for the night. I must remain alert when > working on this dangerous equipment. > > Patrick > _______________________________________________ > AMRadio mailing list > AMRadio@mailman.qth.net > http://mailman.qth.net/mailman/listinfo/amradio
