[EMAIL PROTECTED] wrote:
Got C5 replaced, and then checked the AC-4 bias voltage. It ranges high now, from -50 to -90 volts. I tried the PS supply with the bias pot turned all the way down. Noted some improvement as the meter no longer pegs, but stops at 250 mA--still way too high. Within a few seconds of power-up, the transmitter keys in TUNE mode. Have the vox control on the side of the chassis turned all the way CCW. I guess the thing to do would be to subsitute a good 6EV7 as previously suggested (will have to order a new one), and maybe replace all the diodes in the AC-4.
Don -
Sounds like you're on the right track. A couple of recommendations.
1. Check the meter shunt resistor. It is a 2.7 ohm, 1/2W (sometimes 3.3 ohm) mounted on the small board just in front of the relay. You'll probably find that it is out of tolerance, high, causing your meter to also read high. I replace these with a 1 W flameproof. Once it is replaced, you may also have to replace the ~100-200 ohm resistor mounted on the meter slide switch up between the Tune and Load controls. This is a select-at-test resistor, selected to make the meter read correctly at 0.07 A. The meters are not very linear, and the bias setting current is much more important than the absolute accuracy at 340 MA.
2. Check the resistance of the two cathode resistors of the finals. These should be 15 ohm, 1W. They will also likely be high. Any variation between the two can cause one tube to take more of the plate current than the other. I use Ohmite OY ceramic composition here. This is a 2W resistor but about the size of the 1W carbon composition. Some have used carbon film resistors here. Some say the film spiral shape forms enough of an inductance to burn up, but I have never seen this actually happen. Carbon comps, (the originals,) are OK, but should be measured and matched. The actual values may be within 10%, (5% is better,) but the "difference" should be 2-5% max.
3. Check the 68 ohm screen resistors. These are not as critical, but if they are more than 5% high, I'd replace them.
All of the above occur "normally" with age, but are almost certain if the finals have been overloaded by insufficient bias voltage.
The "keying" may be the 6EV7. That is certainly the first suspect! You can measure the voltage at Pin 7 of the 6EV7. In Standby, it should read several volts negative, depending upon your meter's input impedance. If it reads very near 0, or even slightly positive, pull the tube out. If the voltage goes several volts negative with the tube out, it's a gassy 6EV7. If the voltage is still near 0 with the tube out, it's NOT the tube and will probably require further investigation. There are several VERY high resistance resistors in that circuit that tend to go high in value. Also some leakage paths that can load down the circuit. It's probably the tube! If you have trouble finding a 6EV7, you can also use a 6FQ7.
The good news is, when you get all this done, the T-4XB will probably be good for another 30 years.
73, Garey - K4OAH Atlanta ---------------------------------------------------------------------- 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/ ----------------------------------------------------------------------
