The AM Forum

HI FROM TIM... MY APACHE IS ACTING UP....GRID CURRENT DROPS DURING AM TRANSMISIONS,DROPPING AS I MODULATE,AND PLATE CURRENT RISES...TOOBS APPEAR OK,SO AM I LOOKING AT POWER SUPPLY PROBLEMS,OR SOMETHING ELSE...MAYBE THE TOOBS ARE BAD,BUT APPEAR OK....REPLACED THE HV,AND LV WITH NEW TOOBS,PROBLEM STILL THERE...SWR IS FB...ANY IDEAS?....TIM....sk..

Tim, check out the VFO tube, or driver tube, and associated components.  Replace the tubes first, then check out resistors.

  Hmmm... sounds like  either the driver tube is crapping out, or one of the finals is.

Hi Tim:

Have you looked at the modulation on a 'scope?  What does the pattern look like
when the rig is acting up this way?

Ken
W5KFS

...YA...I'LL CHECK THE DRIVER...6146s,ARE OK FINE...MODULATION ON THE 'SCOPE LOOKS FINE..JUST ONE OF THOSE THINGS SENT TO TRY US..6AU6 FER THE VFO IS OK AS WELL....I'LL CONTINE..TIM...SK..

Tim,

I would suggest changing the 350V LV power supply filter capacitors. I've seen this happen on my Apache as well as another Apache I worked on. Recently my Apache's grid current was dropping in AM transmit. After replacing the LV filter caps, it's a triple 20ufd can located on the rear of the power supply, the grid current problem was fixed. This is the single silver can located next to the rear apron, right next to the 2 contact screw connections on the rear of the Apache's chassis. If the carrier in CW is clean, you do not have to replace the 2 tall black 125ufd @ 450VDC caps. I went ahead and changed all of the electrolytics except for the 125ufd HV caps and my Apache works much better than before.

Output that drops with modulation indicates poor grid drive or the OB2's under the final stage are firing on voice peaks. The OB2's are not used in AM or CW. They are, however, used on sideband. If you do not have the SB-10 sideband adaptor and do not plan on using sideband with your Apache, you can remove both OB2's under the final stage.

Hope this helps.

73's,

Mike
W5RKL

Another thing I would suggest is to set the clamp tube control. The Apache's plate current should be zero with the drive control turned completely counterclockwise. If it's not then the clamp tube adjustment located on the front left side of the chassis, is not properly adjusted. You adjust this pot with full output and then turn the drive control completely counteclockwise. Adjust this control until the plate current just rest at zero with the transmitter keyed.

73's,

Mike
W5RKL