Wednesday, January 13, 2010
Drake T4Xb On the Air Again
The T4Xb is on the air again. The radio was purchased along wtih a R4B, MS4 and AC4 about 2 years ago. The R4 was working when received but failed after a few months. Last week I found a loose rivet on the RF Gain Pot that was keeping it 3/4 muted. After repairing the loose connection it was working well again and I deceided to unpack the T4 and give it a try. The rig fired up and seemed to be working for about 15 min. and then the +250 V filter Cap in the AC4 'smoked'. I replaced the twist lock dual cap with two computer electrolytics under the chassis and tested it again. Now working sort of but only at low power levels and the plate meter was pegging at very low output like 5-7 watts. The tubes were putting out 80 watts with the bias control cranked all the way and the meter was 'hard pegged'. One of the guru's on the Drake list suggested looking at the meter shunt resistor which is located on the vertical board behind the three power resistors. This resistor was found to be open and the 2.2 ohm resistor was replaced and this brought the power up to 110 watts with a more normal meter reading. The meter was still reading a good bit low though so back to the Drake Forum and it was suggested to put a 5 ohme resistor in the B- line and measure the voltage drop to set the idle current. I had a 6 ohm and calculated that .075 A should give about .45V (Note the AC-3 had a test point here with a 5 ohm res but it was deleated in the AC4). Then I removed R39 (on the slide switch) and replaced it with a small 0-200 ohm pot. Adj the pot until the meter reads .075A then measure the pot and replace with closest value resistor. Mine changed from 122 ohms to 24 ohms, not sure why but now the meter is reading correctly at the idle current point at least. Another try on a Cantenna and by increasing the loading a bit more managed 120W output. Still a bit low but the plate meter is correct at least and the output is in the ballpark at least. Alignement or new driver or PA tubes is probably the next step but for now 120W is fine for my purpose. Lots of fun to get these old things on the air again!
After working FB for a couple of weeks I did buy some used tubes on eBay for the Finals and they were weaker than what I had. I left them in for a few days and one AM when I turned it one it started smoking after a min or two and I am still troubleshooting it again. Should have left it alone. The Filament fuse opened and so did R3 the 100 ohm 5W resistor in the AC4 250V B+ line. I looked for where the smoke was coming from but just cant see it so I think it is a wire somewhere. I hooked up the filaments and replaced the resistor with a 'test' one and the 250V line is still nearly shorted somewhere.
With much help from the 'Drake List' folks the following problems were solved. One of the Finals was shorted and took out a screen grid resistor R30 68 ohms. This is where the smoke 'came out'. With the original finals back in the radio was not switching VFO's properly with the R4B and it was drawing idle current all the time. Tube V2 was also found to be shorted Cathode to grid and was pulling down the 'transmit control' line. After replacing V2 the rig started performing correctly. Lots of work trying to get a few more watts out of the rig was a bigger 'learning' experience than I expected but it was a good way to spend the 'Blizzard of 2010' here in Maryland.
Thanks again to all who helped me troubleshoot this!