Hi Don: I missed K7FD's post, so I don't know what he described. Let's work with your observations.
Your transmitter's meter is indirectly showing you plate current. If you have a "pegged meter" and no drive, that's not good. Of course, you knew this already. ;-) "The relay does not stay open" means the relay must then be closing or pulling in (it 'clicks' when you power on the transmitter). Is this so? If so, then something is keying it. If apparently nothing is keying it, and nothing else is attached to the rig, turn the power off. Check vox gain and anti-vox controls. Turn vox gain completely CCW. See if transmitter still keys itself. If so, pull the 6EV7 tube. Try replacing this tube and see if the transmitter still "phantom keys." The T-4X series needs a bias voltage of -45 to -65 volts from the AC-3 or AC-4. You say your supply is putting out a bias of -20 to -55 volts.. This sounds to me that your bias voltage may not be negative enough, which could be why you have gobs of idle current. As the grids approach 0 volts, the tubes will begin to conduct. To check the bias supply, I would do the following: 1) Isolate the power supply from the rig and unplug the power supply from AC mains. 2) Check R5, R6 and R7. If R5 has gone low, so will your voltage. 3) Break the connection at the cathode of D7 and check it. Could be this is weak and that's the reason someone installed the larger caps. 4) If you found no problems at this point, check both sections of C5. I wouldn't worry too much about the exact value here. The larger capacitance will knock down more ripple and stiffen the supply. But, that doesn't mean you don't have a problem with them. Sometimes capacitors check fine in a static or low volts DC check, then breakdown at high voltage ( in this case, -65 volts) or under load. 4) If the supply checks good, set R6 to its maximum, then reconnect the supply to the transmitter and AC mains. >From this point on, you're making voltage checks so remember to stay safe. 5) With a load connected to the transmitter RF output connector, turn the transmitter on. Leave xmtr gain low. The transmitter should not be keyed and you should not have any plate current indicated. If it is keyed or you have indicated current, turn the transmitter off. 6) Verify the bias voltage. Check between pins 9 and 6 (or pin 9 and chassis) of the power supply Cinch-Jones connector on the rear apron of the transmitter. When you power on the transmitter, you should see the maximum bias volts of -65 volts or so. If not, a tube or component failure in the transmitter may be bringing the supply low. The bias supply goes to several circuits in the radio, not just the PA. Check the 6EV7 tube first (controls VOX and relay driver). These go flaky all the time. V1 (half of it is the AGC amp) would be another one to check. Good luck. Let the list know how you made out. 73, Russ WB8ZCC ----- Original Message ----- From: <[EMAIL PROTECTED]> To: <[EMAIL PROTECTED]> Sent: Friday, February 27, 2004 12:30 PM Subject: [drakelist] T-4XB idling current revisited > When powered up, my T-4XB is showing high (pegging the meter) idling bias > current in a similiar manner as that described by K7FD. The relay does stay > open. Bias voltage from the AC-4 is a little low at -20 to -55. The schematic > for the PS calls for a two stage electrolytic cap at C5 with values of 20 mfd > and 150 volts in each stage. My AC-4 has two stages of 40 mfd and 150 volts at > C5. This cap appears to be an original. The rest of the components are the > same as in the schematic. I'd like to replace C5, and would like to know > whether it would be better to go with the 20 mfd value called for in the schematic, > or use the 40 mfd. value. I have a pair of 22 mfd. 160 volt caps as well as a > pair of 47 mfd 250 volt caps, so I could go either way. > > Thanks, > > Don McNallen > W5EI > ---------------------------------------------------------------------- Submissions: [EMAIL PROTECTED] Subscribe: [EMAIL PROTECTED] - subscribe drakelist in body Unsubscribe: [EMAIL PROTECTED] - unsubscribe drakelist in body Hopelessly Lost: [EMAIL PROTECTED] - help in body of message Brought to you courtesy of TLCHost.net http://www.tlchost.net/ ----------------------------------------------------------------------
