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
