Hey guys,
I've been working on this little PAL 200MDX amplifier. I really like it but it's got some issues which I'm slowly resolving. The first is the inverter circuit was fried. I updated to a modern style circuit using an IC and some mosfets and it works like a charm now with more stability than original. I also replaced the main power relay as the contacts on that were spent and it would refuse to key up sometimes.
Right now my main concern is with the input SWR. When the amplifier is off or in standby mode the input SWR is acceptable, about 1.5. With the amplifier in transmit mode the input SWR skyrockets to above 3:1. I see no input adjustment on this thing. The only internal adjustment is a small coil which is between the drivers and the output but this doesn't help the SWR. It's mentioned in the manual as a way of helping make the input drive wattage higher. Despite this the amp does put out about 40 watts in low and about 80 in high but I don't like what it's doing to my radio and I imagine it can do a bit better than that.
The high SWR seems to be due to the connection that takes the input RF to the cathode of the driver tube. This connection is made using some of that really tiny coax. Could this be the issue? I'm thinking of replacing it with regular RG-8 or something. There's an inductor 27uh and a .01 capacitor to ground. Could changing the values of these help? Should I have to though or is this an indication of another issue?
Here is the schematic,
http://www.cbtricks.com/Amp/pal/200_mdx/graphics/pal_200mdx_sch.pdf
I've been working on this little PAL 200MDX amplifier. I really like it but it's got some issues which I'm slowly resolving. The first is the inverter circuit was fried. I updated to a modern style circuit using an IC and some mosfets and it works like a charm now with more stability than original. I also replaced the main power relay as the contacts on that were spent and it would refuse to key up sometimes.
Right now my main concern is with the input SWR. When the amplifier is off or in standby mode the input SWR is acceptable, about 1.5. With the amplifier in transmit mode the input SWR skyrockets to above 3:1. I see no input adjustment on this thing. The only internal adjustment is a small coil which is between the drivers and the output but this doesn't help the SWR. It's mentioned in the manual as a way of helping make the input drive wattage higher. Despite this the amp does put out about 40 watts in low and about 80 in high but I don't like what it's doing to my radio and I imagine it can do a bit better than that.
The high SWR seems to be due to the connection that takes the input RF to the cathode of the driver tube. This connection is made using some of that really tiny coax. Could this be the issue? I'm thinking of replacing it with regular RG-8 or something. There's an inductor 27uh and a .01 capacitor to ground. Could changing the values of these help? Should I have to though or is this an indication of another issue?
Here is the schematic,
http://www.cbtricks.com/Amp/pal/200_mdx/graphics/pal_200mdx_sch.pdf