Tom, We have several of these 225 watt units in operation 440 MHz. The correct PA is the 20 watt unit. It looks identical to the 75 watt chassis, just has fewer circuit boards on the inside. Yes it is overkill for this much heatsink for 20 watts output power, but one needs to remember that these radios were not only designed to, but were very capable of running key down 24/7.
Before I got all excited about having the wrong PA being used as a driver, I would pull the cover off of the solid state PA unit and see what it has for parts on the inside. You may have a 75 watt unit that has been modified by either removing or bypassing the high power driver and PA boards. The 20 watt chassis will have two circuit boards on the right side of the PA chassis (the end with the input cable) and the left side of the chassis (the end with the output cable) should be empty of circuit boards and transistors. I do not have my manual handy, but most of the Micor UHF station manuals should have the part number for the 20 watt PA chassis. If the unit actually has a 75 watt PA that has been used to drive the tube, about the only damage I would expect would be to the grid circuit. The use of 75 watts of drive may have shortened the life of the tube due to over driving the input. These PA decks will move to 440 MHz with some work and retuning of the tube cavity plates on each side of the PA deck. What you are looking when tuning the PA cavity is max power out at minimum plate current. If you are not familiar with High Power transmitters, please be careful of the 1500 volt plate supply. It will knock you on your A** or worse. Make sure all of the interlocks work and you have a good way to bleed the high voltage off before getting to carried away working on the PA deck or power supply. Good Luck, Joe - WA7JAW --- In [email protected], TGundo 2003 <tgundo2...@...> wrote: > > Thanks for the Replies so far- > > I have found a little info on the unit. It appears to be a 225-250W 450-470 > PURC amp. It is supposedly 20-25W in, but I have not found what PA would be > the correct IPA. In the cabinet I found that the amp was bypassed, the unit > was using the 75W pa as the final. I found the motorola manual numbers for > this unit, so now begins the search for one. > > Hopefully the Tube in this puppy is still ok. If anyone has any more info on > this unit PLEASE feel free to pass it along. > > Thanks! > Tom > W9SRV >
