Since the PA in these radios is class C they are always active and they
rely on the loss of input signal to drop the amplifier. Sounds like you may
have some oscillations in your PA after the exiter drops generating the
spurious output. I have seen this many times before. especially on the
PA's that have the ceramic substrates instead of a PWB. They tend to crack
causing grounding issues and impedance problems. Even the regular PWB type
can do strange things, your problem can also stem from running the PA out of
its design freq range. If you are, you might look into possible mods to
bring it in range. The service manual details the parts list items that are
different for the different splits. Usually the PWB is the same and various
caps and inductors may be different.
These types of problems can be hard to find because so many things can
cause them, and they are usually intermittant in nature. Bad transistors,
or bad decoupling caps on the power feeds to the transistors, etc. any of
these It might be as simple as removing the board and cleaning and
applying new heat sink grease and making sure the hardware is all snug.
Another thing to check would be the SMT type caps that are sometimes tacked
right on th transistor. those are prone to cracking and you usually cant
see this until you heat one side of the cap with an iron. If it is broken
it will easily fall apart, leaving its other half attached to the still
soldered side. If you have an RF milivolt meter, sometimes you can use that
to help isolate which transistor cicuit is causing the problems. Just be
sure to hook the PA to a dummy load before you do any troubleshooting.
Hope that helps,
Wade - KR7K
----- Original Message -----
From: "maxjam_99" <[EMAIL PROTECTED]>
To: <[email protected]>
Sent: Sunday, January 16, 2005 8:00 AM
Subject: [Repeater-Builder] Mitrek VHF wide band noise on TX
I have a vhf mitrek that has been split for repeater operation. It
has work flawlessly until now. When the TX is unkeyed the transmitter
will stay keyed for approx 10 seconds. During this time it generates
a wide signal +-1Mhz centered on the TX frequency with full power out.
Once 10 seconds or so it will unkey and cease tx'ing. If you key the
TX during the time it is generating this wide signal then it will TX a
proper clean signal, unkey and start over.
Any thoughts?
Thanks...
Sean / N�PBA
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