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Author Topic: KWM-2A Frequency Xmit/Rcv Offset in SSB  (Read 247 times)

W2JJV

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KWM-2A Frequency Xmit/Rcv Offset in SSB
« on: March 01, 2022, 04:10:16 PM »

Since adding a digital display DD-103 to my Collins KWM-2A i noticed a consistent 280 cycle offset between transmit and receive modes in LSB .  Any advice as to whether it is in the Collins rig or the ESP digital display?  Not sure where to start.

73, David
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W1VT

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Re: KWM-2A Frequency Xmit/Rcv Offset in SSB
« Reply #1 on: March 01, 2022, 04:24:00 PM »

If you have the program WSJT-X running and a reasonably stable general coverage receiver you may be able to listen to the oscillators and have the program track the frequency shifts.
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VK6HP

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Re: KWM-2A Frequency Xmit/Rcv Offset in SSB
« Reply #2 on: March 02, 2022, 12:18:00 AM »

David,

I don't have a DD-103 or KWM-2 myself but I can suggest a few checks.  First, make absolutely sure there is no T/R shift with the counter entirely disconnected. The "Vietnam" mod amongst others is specifically designed to fix a well-known KWM-2 T/R shift due mainly, as I recall, to PTO voltage regulation issues.

Second, check the shift with various stages of attachment and powering up, starting with just the DD-103 power on/off, then PTO (VFO) attached or not.  Finally, you could try a high impedance connection to the counter via proximity pickup as suggested in the manual.  There are various KWM-2 fixes to align T/R frequencies but if it's the counter causing the issue, I'd fix that problem first.

I'm still looking at whether to add a display to my S-line twins but, as I think I mentioned before, I prefer a system that counts all three oscillators (PTO, HFO, BFO).

73, Peter.
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W2JJV

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Re: KWM-2A Frequency Xmit/Rcv Offset in SSB
« Reply #3 on: March 02, 2022, 11:14:47 AM »

Thanks both for good advice.  Not sure if the "Vietnam Mod" was made on this 2A or not but certainly the potential exists that this could be the problem and the cure.  Will continue to investigate before getting into the rig's wiring.  It's a real tribute to these rig's serviceability and longevity that there is so many resources available so many years later.

73 and tnx agn,
David
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N6AF

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Re: KWM-2A Frequency Xmit/Rcv Offset in SSB
« Reply #4 on: March 04, 2022, 08:25:29 PM »

I had the same issue with my 1963 KWM-2 (recently acquired) although the RX to TX shift wasn't that much.  I ended up doing a Zener mod similar to the 'Vietnam mod'.  There are several versions of stabilizing that.  The first step I suggest is monitoring the supply line that runs to the PTO and see how many millivolts it shifts when going from transmit to receive.  Not to complicate things too much, but one method to confirm that the frequency sensitivity would indeed be corrected if you stabilized that line, you could run the rig on a variac and then 'push' that PTO voltage back close to what it was in RX mode, when in TX mode.  If you indeed see the PTO get back on freq then you know you'll have killed the gremlin by Zener stabilizing that line.  I use my SDRplay module as a super high resolution spectrum analyzer to quantify the frequency shifting.  I got my KWM-2 shift down to less than about 15 Hz once I got that Zener in there.  Good luck!
73 N6AF Chuck
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