Garey Barrell <[EMAIL PROTECTED]> made an utterance to the drakelist gang ---------------------------------------------------------------------- Steve -
The VOX "hanging" is often the tube (6EV7) itself. I skipped over that when you said you had swapped the tube, _assuming_ (we all know what that stands for,) that you had replaced with a new or known good tube. The fault occurs with a "gassy" tube when it is turned ON "hard" as in relay closure, as the gas molecules build up a charge on the grid, holding the tube ON. As someone else said, the VOX and keying circuit in the T-4X series is VERY high impedance, and it doesn't take much leakage current to upset the balance. If you look at the resistor values, (44M ohms in the sidetone switch circuit,) you can see why....! Any of the T-4X transmitters can be keyed by touching your finger across the key contacts.
R95 is a 3.3M resistor across D10 that set the VOX Delay in early T-4X units. It was replaced in late 1968 by a 15M pot mounted on the rear panel. I have seen a few instances where the pot had developed a leakage path from the rotor to the chassis, causing the VOX to "hang".
I don't know if anyone warned you about spraying "contact cleaner" (even DeoxiT) around the KEY jack, V9 and V10 tube sockets. ANY spray cleaner (other than Freon, now outlawed) leaves a film of lubricant on insulation that attracts and holds contaminates that can form leakage paths _between_ contacts. The best thing to use now is 91% or better alcohol to clean switch wafers, tube sockets, key jacks etc. DeoxiT _liquid_ should be applied ONLY to contact surfaces. If all you have is DeoxiT D5 spray, spray a little in the cap and apply the liquid to contact surfaces only with a toothpick or component lead. I use DeoxiT D100L, which comes in a poly bottle with a hypodermic style needle. Pots and slide switches get a shot INSIDE of spray CaiLube MCL cleaner/lubricant.
73, Garey - K4OAH Atlanta Drake 2-B, 4-B & C-Line Service Supplement CDs <www.k4oah.com> n0tu/Steve wrote:
"n0tu/Steve" <[EMAIL PROTECTED]> made an utterance to the drakelist gang ---------------------------------------------------------------------- So I Deoxit the pots, tube sockets, relay and everything that moves on the T4X's underside EXCEPT the plastic gears and PTO gears! Also blew out the cobwebs and wiped down the copper chassis. Looks like a new radio minus the few corrosion spots. Powered up and no change! BUT when reinstalled the old 6EV7 the VOX worked again!!?? But still not to spec? I should say the 6EV7 I swapped with was an unknown as in used. So I've got to order a NEW 6EV7 for starters and then do some more troublshooting. BUT in the short term my T4X is back on the operating bench! Bias seems ok? ...haven't checked for 'ripple' yet? Tks for your hits/support. Steve
---------------------------------------------------------------------- Submissions: [email protected] Unsubscribe: [EMAIL PROTECTED] - unsubscribe drakelist in body Hopelessly Lost: [EMAIL PROTECTED] - help in body of message Zerobeat Web Page: www.zerobeat.net - sponsored by www.tlchost.net ----------------------------------------------------------------------
