DEMI 70 CM TRANSVERTER PROBLEM

  • 1
  • Problem
  • Updated 2 years ago
  • Solved
First let me say I already have one of the transverters from Ukraine for 2m and have no problems in either TX or RX with my FLEX 6300.  Have had it for about 6 months.   Just received a brand new, waited for 6 months, DEMI L432-28 70cm transverter.  And, I have no RX signal and no indication of TX signal.  The panadapter is showing nothing with a very faint reading below -120.  However, the rig does key up when I go into TX and I am getting about 15 watts out on the meter but no indication of modulation on the panadapter.   I have set up the DEMI with a  RF Freq of 440.000, which it was customized to do, a Max Power of 0.0 dbm, also customized for that power rating, and a RX Gain of 20.0db.  I have tried increasing the RX gain to 40.0db with no effect.  I must be doing something wrong which is very simple.  I use the same dual band vertical for the 2m Ukraine transverter with pretty good results.  What am I missing?
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Mark Gottlieb

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Posted 3 years ago

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Mack McCormick, Elmer

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Please send a screen shot of your transverter setup. Also, make sure you're xverter out goes to DEMI in and DEMI out goes to RX A unless you are set for common in out.

Mine works perfectly. Also, which Flex model?

Mack
W4AX
Alpha Tester
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Mark Gottlieb

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Thanks for the quick comeback.  I have a FLEX 6300 and the transverter is set up for common in out.
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Mark Gottlieb

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Here is a screenshot with the transverter on.
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George Molnar, KF2T, Elmer

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That transverter works from 432 to 434 MHz. You're 8 MHz past the operating point. Also, when using a transverter through the XVTR port, you will see no indication (meter or panadapter) of transmit.
(Edited)
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Mark Gottlieb

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That would normally be correct but this one was specifically modified to operate on FM only 440-450 by DEMI.
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George Molnar, KF2T, Elmer

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Thanks for the info. What is the specified IF, then? Do you get any signal (RX only) if you plug into the RX-A port?
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Mark Gottlieb

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I don't have an RX-A port on the FLEX or the DEMI this is set-up for common in out.  I did, however, lower the RX Gain from 20db to -1.0 and am seeing this, now, on the Panadapter.  70cm is used a lot around here and I am seeing no signals from 440-450.
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Mark Gottlieb

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Mark Gottlieb

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I had an idea to put the XVTR frequency settings to 432.000 but there is still no change.  I did try to operate simplex on 435.000 but there is no modulation of my voice on the panadaptor as I am talking.  No indication of a modulated signal but the meter is showing 20 watts going out to the antenna.  As I said, I utilize the 2m Ukrainian transverter frequently with no issues on the same dual band antenna.   The SWR on the DEMI is showing 1.2:1 when in TX.
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Mack McCormick, Elmer

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Correct mic input? Gain? Cables faulty? Sounds like no audio getting to radio.

Mack
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Mark Gottlieb

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I wish it was that.  Same headset I use for HF and am getting nominal readings when I have used it today on the FLEX.
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Bob Wright, N7ZO

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Hi Mark,

When DEMI optimizes the transverter for 440-450, I think all they do is re-adjust the RF filters.  The LO frequency stays the same, which is 404 MHz (the fourth harmonic of the 101 MHz crystal).  So the proper RF frequency on the XVTR setup panel is still 432 MHz  (28 + 404 = 432 Mhz).

73, Bob, N7ZO
(Edited)
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Mark Gottlieb

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Yes, you are correct on all points after about 2 hours of experimentation, I concur.  I do know for a fact they just reconfigured the filters which is what I ordered.
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Roy Laufer

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I have the very first Demi 70cm rewired for 70cm Repeater use on the high end of the 70cm band (I had to send the first modification back, when it failed miserably). These setting should help - 

My XVTR settings are:

RF Freq 440.00MHz
IF Freq 36MHz
Max Power -6.0 dB

(Your mileage may vary.)

I hope that this helps.

73,
Roy AC2GS
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EA4GLI - 8P9EH - Salvador

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Make sure you are switching it on to use the internal reference. The on off switch can turn on the device both switching up or switching down. One is to use the internal reference the other is to use the Apollo board and an external reference.
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Samuel Strongin

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I emailed this link to Steve's personal email at DEMI
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Mark Gottlieb

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Transverter issue has been solved!  Thanks to all.
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John - WA7UAR

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Any update Mark about what was done to finally resolve the issue?
(Edited)
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Mack McCormick, Elmer

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Please let us know how you fixed it so we all learn.
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Mark Gottlieb

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Official Response
Better late than never...the problem originated, and was solved, through the FLEX XVTR settings.  I had 412.0 as the LO setting and it should have been 404.0 mHz.  Also, I had the dbm setting at 0.0 dbm and it should have been 15dbm.  Once those two settings were adjusted, everything has been working fine since.   The DEMI is a nice piece of engineering.  I am routinely hitting repeaters about 40 miles away with an antenna in the attic and only a 25 watt output.

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