[VHFcontesting] more on MMT 432

[MICHAEL SAPP]

Tim & Folks:

I realized no one answered your IF retune question.....


>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?)

Refer to the appropriate schematics available at http://www.qsl.net/wa2wim/eme/eme.htm   which are in a zip folder.

Both the rx and tx sides of the 28Mhz IF section have separate RF coupling transformers. One or two turns of the respective  inductor slug should be all that is necessary to repeak them to 28 Mhz from 27 Mhz.  An MFJ-259B analyzer outputs around 5mw, so you can use the analyzer to determine curent resonance and then tune for lowest SWR on the IF side back to 28 Mhz. If you use that approach leave the DC power off to the transverter. If you can't get a low SWR  on the IF tx input then suspect something amiss with the tx IF input attenuator or IF transformer. (Note: bypass the 1K resistor for low-level attenuator input operation for this test) Same for the rx IF (but no attenuator).  Mike wa3tts

P.S. I used this method to repair an old MMT 1296 rx converter---it had an IF input transformer wiring fault that was immediately obvoius with the MFJ-259B test.   Actually I ended up ripping out the 2M IF preamp and installed a small diplexer (p87, 1987 Mid-Atlantic Conf Proceedings, P. Drexler) and turned the rx converter into a bi-directional tx/rx LO-Mixer for a 1296 transverter project.