[VHFcontesting] Microwave modules transverter question

[David Olean]

Hello Tim,
    I had to repair my 432 MMT unit as the driver amp, a plastic transistor 
on the main board decided to go flakey on me. It was the last stage before 
the 432 energy goes into the PA die cast box.  It caused the same problem 
that you see...very low output on TX.  The receiver worked just fine. All my 
MMTs are long gone and the manuals went with them, so I can't give you more 
info. Tuning the IF to 27 MHz could cause a drop in power when using 28 MHz, 
but I would not expect more than maybe 2 or 3 dB max drop. You are talking 
about 10 dB or more. I would suspect a plastic transistor decided to go 
belly up. (It could be other bad parts too) Best way is to check for rf in 
the various stages after the mixer. The 404 MHz LO must be fine as you can 
receive OK.  The TX stages are along the back of the board away from the 
side with all the connections. You can see the 432 drive signal where it 
goes into the PA die cast box thru a small low capacitance feed thru 
bushing.  You can test the level with attenuators and a power meter or a 
spectrum analyzer.  Some lucky souls may have a scope that goes that high in 
frequency. That makes checking it rather painless.  Of course the PA could 
be hosed too. Seeing that it connects to the outside world, it is a likely 
culprit. If you see significant drive going into the PA box, then the PA 
could be dead. How about current draqin on TX? You should see idling current 
from the PA stage. If it is very low, that could mean trouble in the PA. 
High current means trouble too. I seem to recall that 12 vdc at 700-900 ma 
is about normal. I think 250 ma or so on receive was normal too. If you send 
CW or talk on SSB, the idling current should jump well up to over 2 amps on 
peaks. If you see a small increase on voice peaks, then I would suspect a 
lack of droive to the PA. All this will hopefully help you zero in on the 
problem.


Dave K1WHS

----- Original Message ----- 
From: "Tim Coad" <timcoad at gmail.com>
To: <VHFcontesting at contesting.com>
Sent: Thursday, July 17, 2008 3:05 AM
Subject: [VHFcontesting] Microwave modules transverter question


>I have 2 different Microwave Modules transverters... a 28-222 and a 28-432
> and they both have the same problem- Almost no output power.
>
> When I got the 28-222 it was working fine, but then the output power 
> started
> getting flaky and after a bit the power out no longer registered on the
> power meter, but it was still transmitting at a very low power. It 
> receives
> fine.
>
> I just picked up the 432 transverter, and it has the same problem. I can
> hear it fine on other 432 rig, but it does not register any pwr on the
> meter. I do have the jumper set to take 5mw drive.
> Has anyone had the same problem?  Ideas?
>
> Actually there's one more compliaction with this one, which may or may not
> be related to the low power out. Someone modified it to have a 27Mhz IF
> instead. If I drive it with 28 Mhz it transmits at 433MHz.
> Could transmitting 1Mhz high cause it to have a great reduction in output
> pwr? Anyone have an idea of the output bandwidth range? I do not have
> capability to drive it with 27Mhz to test it.  (Also..how do I retune it
> back to 28MHz?)
>
> Thanks
>
> Tim - NU6S
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