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Title: DX-100 Grid current displaying "negative" Post by: wb3jih on August 08, 2020, 09:28:27 PM Hi all -
Was hoping to get some help on a DX-100 that I am helping to get back on line. We are making good progress, and So far seem to have good output, good modulation, and the audio sounds quite good on the monitoring receiver. All functions seem to be working, but have hit an odd problem. When I turn the meter switch to "Grid", the meter deflects to the left -- as like a negative reading. For any experts on the rig -- is this something that might be obvious? I could probably figure it out EVENTUALLY -- but maybe someone could offer me a shortcut? Or at least some recommendations for places to begin looking... Thanks in advance! Jeff Title: Re: DX-100 Grid current displaying "negative" Post by: Pete, WA2CWA on August 09, 2020, 12:25:48 AM Check the 5.4 ohm meter grid shunt resistor. If OK, might also check clamp adjustment.
Title: Re: DX-100 Grid current displaying "negative" Post by: kb3ouk on August 09, 2020, 07:56:52 AM Here's something even more obvious that could cause that and easily be overlooked: make sure the wires from the meter to the grid shunt don't flip polarity anywhere. Remember, these things were built from a kit and its very possible it was put together with mistakes.
Title: Re: DX-100 Grid current displaying "negative" Post by: WD5JKO on August 09, 2020, 10:04:30 AM Try pulling out the 6146 RF tubes. Reverse grid current could be due to leakage within the tubes themselves. Could be a cathode oxide flake touching the control grid. Jim Wd5JKO Title: Re: DX-100 Grid current displaying "negative" Post by: AJ1G on August 23, 2020, 05:45:42 AM i recently brought a DX-100 up in operation here, and on initial power up, saw negative final grid current meter deflection. It is only present, however, when the drive level is backed off and/or the driver stage is off tune, When drive level is properly set, the grid current sits at zero on standby. One thing i did notice is that if I place the mode switch to CW with the plate switch to OFF, in order to generate a spot signal, the grid current will be significantly higher than when in transmit with the plate switch ON. When set up to transmit with the final grid current set at 5 ma, and final plate (actually cathode) current set at 200 ma, the grid current when spotting in standby will be up over 7 ma, which I believe is pushing the 6146 grids a bit on the high side. i therefore make it a habit to back the drive level off a bit when spotting.
I suspect the negative grid current under reduced drive conditions has something to do with the final amplifier's protective clamp circuit. Title: Re: DX-100 Grid current displaying "negative" Post by: DMOD on August 23, 2020, 10:33:51 PM Hi all - Was hoping to get some help on a DX-100 that I am helping to get back on line. We are making good progress, and So far seem to have good output, good modulation, and the audio sounds quite good on the monitoring receiver. All functions seem to be working, but have hit an odd problem. When I turn the meter switch to "Grid", the meter deflects to the left -- as like a negative reading. For any experts on the rig -- is this something that might be obvious? I could probably figure it out EVENTUALLY -- but maybe someone could offer me a shortcut? Or at least some recommendations for places to begin looking... Thanks in advance! Jeff Not much information regarding voltages present in the rig, or how much power and modulation percentage you are getting. What is your 6146 screen voltage? I would set the screen voltage to about 165 volts initially with the clamp tube adjustment pot. (500k ohms) until you get things sorted out and then set for about 175-180 Volts. Have you tested either the 6146 or 6AQ5 tubes as yet? Phil |