Hi Bert (and everyone following this thread),
Thanks for your helpful response and suggestions earlier.
I wanted to provide an update: the issue was actually affecting both FID
channels (not isolated to one side). After swapping RF1 and RF2 between
channels, the dropout problem was still on the highband—confirming
it wasn't specific to either RF board.
The root cause turned out to be the channel switching box with part number 0191422700.
Symptoms pointed strongly to a heat-related electronic fault:
On a cold startup, everything was stable initially.
After ~3 minutes of running, the FID dropouts began (consistent timing across tests).
This delay strongly suggests thermal buildup causing a component to fail.
I measured voltages by probing the back of the Molex connectors (under operating conditions):
J1: +15.04 V, -14.84 V, +7.44 V, +10.93 V, -10.92 V
J3: +5.28 V, +3.46 V
These seem reasonable. And, the replacement board eliminated the issue
immediately—no dropouts, even after extended warm runs—so the
fault was clearly isolated to 0191422700.
Before pinpointing this, I had already swapped the following (none of which
resolved it):
REF Gen board
DDR1 board
RF1 and RF2 (swapped between channels, then replaced with known-good backups)
I also verified that the VFS was functioning correctly—no issues there.
Replacing the 0191422700 fixed everything. If anyone else sees similar
intermittent FID dropouts that start after a few minutes of warmup (especially
if swapping RF boards moves the symptom), definitely consider checking/replacing.
Best regards,
Shawn Wagner
________________________________
> From: main_at_ammrl.groups.io on behalf of Spin-Doc
> Sent: Wednesday, March 11, 2026 11:59 AM
> To: main_at_ammrl.groups.io
> Subject: Re: [AMMRL] Signal Blanking on High Band
Dear Shawn,
Do you have a 400MR with VFS synthesizer (19“ rack unit) or synthesizer
board (on the right of the reference generator board)?
Anyway, in both cases you can try to swap the synthesizer channels 1 & 2 by
swapping both RJ45 controller and BNC cables, followed by „su“.
An oscilloscope could also be handy for troubleshooting (expecting to see a cw
sine wave at desired frequency plus 20 MHz IF (so 420 MHz on high-band).
Other things to check would be
- swap rf1 vs. rf2 controller boards (blanking signal generated there)
- check main power supply voltages, especially 3.3, 5 and 11 V (behind right
side panel, white molex plugs at the bottom). Watch out for browned pins
(tighten contact & clean both contacts, don’t use automotive contact
spray!)…
Best regards
Bert
-> Sent via mobile <-
Spin-Doc
Magnetic resonance equipment, service, training & more
Dr. rer. nat. Bert Heise
Owner, applications scientist
Kiefernweg 13
58239 Schwerte
Germany
Web: http://www.spin-doc.net
Mail: bert.heise_at_spin-doc.net
Phone: +49-171-5135315
Representing MestreLab software, Stelar relaxometers & Barthel HF amplifiers
Am 11.03.2026 um 18:24 schrieb Shawn Wagner via groups.io:
Hi AMMRL Community,
Before I go hunting for the problem, I wanted to check if anyone has seen this
before. I have an unusual issue on a Varian 400MR (DirectDrive).
The signal on the high-band channel (channel 1, 1H/19F) blanks for ~0.125 s
every ~1.6 s. During the blanked period the receiver output goes completely
flat. This does not occur on channel 2.
When tuning the high-band channel you can see the same behavior. If the blanking
occurs during the tune sweep, the signal disappears and only noise is visible.
Things I have checked so far:
*
Swapped the XMTR and ATTEN/SW boards → no change
*
The problem occurs even with no transmit pulse
*
Verified the probe using a network analyzer
*
The issue is only on channel 1
One additional observation: if I turn the transmitter pulse off and set the
receiver gain to 0, the blanked portion of the FID (~0.125 s) actually shows
more noise than when it is acquiring normally.
Has anyone seen this type of periodic receiver blanking on a 400MR? If someone
recognizes the symptom and can save me from rooting through the console
internals, I would greatly appreciate it.
Thanks,
Shawn
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Received on Fri Mar 13 2026 - 08:43:25 MST