From: Garey Barrell 
Subject: Re: [drakelist] Re:  T-4XB idling current revisited



W5EI{at}cs.com 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
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