wb6dgn wrote: > "There is a process I use to easily see which stage is > bad. Reply if you need more help." > > Maybe you'd consider posting it here? Always looking for better ways to do > things. > Tom DGN
Verify you have drive to the PA by looking to see how much is coming out of it or any filters that follow. Then take the watt meter and put it on the antenna connector and terminate the meter with a suitable load. I use a Bird 43 with matching 100 watt dummy load. I usually start with a 10 or 25 watt slug no matter the size of the PA. If the correct drive is coming from the exciter, make sure the correct voltages are present on the stages of the PA. PNP RF transistors are used in the MICOR VHF PA, so put your thinking cap on upside down. If you have a hand-held radio, tune it to the frequency of the transmitter, and remove the rubber duck and set it off to the side - out of arms reach. Key the defective radio and touch the base of the first transistor in the PA with the metal blade of the Motorola MICOR tuning tool. Watch the S-Meter on the hand-held or listen for quieting. On my Yaesu FT530, I normally get about 1/2 scale S-Meter reading on the base of the first stage. Keeping the radio keyed, touch the collector - the S-Meter reading should be considerably stronger and quieter. If not, you might have found your problem. Anyway, continue doing this down through the stages until you find the bad one - moving the hand-held radio back away as need to add loss. Once you find the stage that has no apparent gain, take a small capacitor (a few hundred pF) and trim the leads so you can use it to bridge across the base to the collector. The leads only need to be long enough to be able to go across the transistor to couple energy at the base to the collector. If the stage is bad, you will see the PA make *some* power - there will be some indication on the watt meter. The transistor, when bridged shows output power, is likely bad and needs replaced. This method can generically to troubleshoot other makes and models of PA's. DISCLAIMER: I have worked on some ill engineered equipment that will go spurious and blow up if you touch one of the RF transistor conductors with some metallic object. This has never happened to me with a MICOR or MASTR II PA, but has with Spectrum and some other junk. Do This At Your Own Risk!! In the MICOR, the first or second stages are usually the culprit, unless something has taken out the final stages, which should only be replaced as a whole, with matched units, if one or more are found to be bad. Kevin Custer
