The AM Forum

Here's today's progress on repairing the 3CX3000 amp. I do not know where the parasitic supressor may have gone  or if there ever was one in this amplifier. I made this one. It is a 10 Ohm resistor with 2T of 1/4" brass strap. I could not get copper. Also the plate choke there, it is 1.25" diameter. I had a heck of a time making that. The next section of plate choke, which will be in series with this one and connect it to the high voltage, is not installed yet. 

The way it turns out, I flattened the supressor's 2 turn coil a little to make it fit. There is about 1/4" between one turn of that coil and the top few turns of the plate choke. The winding of the plate choke is almost exactly 90 degrees to the winding of the supressor's coil.

Is anything obviously wrong with this?

That non-ind resistor and two-turn choke cannot go to ground.  It needs to be totally insulated from ground, and in series with the plate choke.

73DG

Yup, mount it right on top of the RFC and the copper wire would attach to the end that's mounted to the top of the RFC. The other end of the parasitic  choke goes to the plate.

Maybe you have the DC blocking cap on the other side of that insulating material? I guess you could run a strap over to that terminal? Not sure just how your setting it up?

Yes the DC blocking cap is on the other side of that green PCB material panel. I's a 1/4" thick glass epoxy panel.

So right now as above the pic shows:
1.) The supressor connected to the plate and to the screw that passes through the greenish insulating panel and into one end of the DC blocking capacitor.
2.) The copper wire that is the plate end of the choke, just lying there on the plate.

It will be redone so the supressor assy. goes from the plate to the hot end (top) of the plate choke, and from there a wire or strap will go to the screw of the DC blocking cap.  The real problem is space.

The reason the RF choke is mounted on the aluminum bar is because if it were mounted on top of the black bypass cap there, it would be too close to the fan housing (1/4") and I am afraid it might arc (4800VDC+RF+ tuning up).

The original plate choke may have been different, may or may not have had a bar type mount, and it was missing. There is precious little room above the tube. 1" between the anode teat and the cabinet top. The chimney extends 3/4" above the tube anode radiator. This means the supressor can't fit right above the tube and that is why it's against the glass wall there.

This reminds me, there might be a 3CX2500 chmney somewhere here from a decade ago. It is short, 2" or so. It might have been given to another ham though. You know how that goes. A white cigar box also having a RCA 1960 in it IIRC.. hmm..

Ok please keep the comments coming. I will work on this tomorrow after church and show what I did, then if it's approved I can get ready for test. I need to understand how to check in-circuit for plate choke resonance with the MFJ analyzer. Do I leave the DC blocking cap disconnected for that ?

I think the suppressor inductor could be wound with a smaller ID closer to the resistor O.D.