I am going thru a pair of Drake L7's I picked up. Had to get a schematic and look on the net for info on this one. It's "A little weird"
First thing I noticed was this thin plate thing attached from the top of the plate choke to the center of the plate choke. What's that about? I have seen single section plate chokes, multi section, some gapped winding's in plate chokes, all to get rid of resonance. I have even heard of a plate choke made around the early 90's that was resonant on 11 meters only, so when the amp was ran on 11 meters, it would go BOOM, but that was fixed and addressed when the Ham 12 meter band was authorized as sometimes that same choke would make the amp go boom when the poor Ham was on 12 meters LOL. However, I have never seen a cap, which that weird plate turned out to be, attached between the windings of the plate choke. I'm still not totally sure of its purpose, but I would bet it has something to do with the 160 meter band that was added to this amp. Its younger brother, the Drake L4 did not have 160 meters. That cap across the Plate Choke is on the schematic, C41, a 47pf capacitor. Also, from the net, that cap is a known problem with this amp. It blows up, just like mine did. It was recommended from the net to replace it with a 50pf doorknob cap, so I replaced it with a 5kv 50pf doorknob.
Next is there is another much bigger thin plate thingy connected to the Plate Tune Variable Capacitor. According to the schematic and also me following the wiring to verify, this big cap is C42 which is a 307pf weird fixed capacitor. Though one end is directly connected to the Plate Tune cap, the other end goes straight to the Band Switch and when the Band Switch is in the 160 meter band, it grounds the other end of that cap and hence activates it, but again on 160 meters only. Nothing weird about that. Most full Ham Band amps add extra capacitance like that as you go up the bands. However, another weird cap that I have never seen before.
3rd and last weird thing I see, at least on the top/output side is that the Tank Coil, you know the coil in-between the Tune and Load caps, is that the Tune Cap is tapped about the center of the 10 meter coil. I have never seen this before either. Every Tune cap I have seen was connected right at the start of the tune coil and output of the plate blocking caps. The Load, at least in a standard Pi circuit, was always connected to the other end of the tank coil. I have never seen a Tune cap connected at a mid point in the tank coil. More to come on the L7
8 сен 2024
