Hi all,
Hopefully someone out there has had experience with an AMT pulse amplifier failure in blanking (CW) mode. The amp is Model# 3900B-15, 6-245 MHz _at_ 300W broadband on ChB & 200-500 MHz _at_ 100W high band on ChA. My question is does this pulse amplfier share electronic circuitry for CW blanking of ChA and ChB? Both appear to have sporadic failure which started recently and at I believe the same time.
1H decoupling appears to have sporadic failure when when doing an array of 13C observe with proton 'nny' decoupling (13C sensitivity sample of 60% C6D6/40% dioxane). Here all array elements use the same parameters and various degrees of 1H decoupling are observed with a d1 delay of 15 seconds. An array of 1H coupled 13C observe looks fine.
13C decoupling also appears to have sporadic failure when doing an array of 1H observe with blanking 13C decoupling (13Cpwx90 inverse standard 1% Iodomethane / 1% Trimethylphosphate / 0.2% CR(AcAc)3 / CDCL3). In this experiment the 13C is enriched so the methyl protons provide a 1H spectrum triplet in which the 13C satellites form the outer peaks of the triplet and these satellites go through a null _at_ a 13Cpwx90 and invert at a 13Cpwx180. While this null and inversion is observed you see one array element with good 13C decoupling followed by 1-3 elements with no 13C decoupling. The Varian Inova has a second pulse amplifier but it is dual broadband. If I do a soft command (rfchannel='1324') to use this second pulse amplifier to do the decoupling pulse on 13C I get a beautiful array of 13C progressive decoupling with no misses.
Unfortuneately I can't do a soft command to do 1H decoupling on a different channel since only one of the 4 available channels is highband. Also the soft command changes all channel hardware, not just the amps. Thought of changing the wiring to the 2 amps at the console amp and router board but the the ribbon has both ChA and ChB so can't change one without changing the other and changing highband is not an option. So can't do a wiring change to the amps to achieve the same results as the (rfchannel='1324') and isolate the problem to the highband/lowband amp. I have switched all 4 XMTR and XMTR CTRL boards to prove none were defective and have switched the two 620 and two 320 synthesizers all with no effect to try to isolate this problem to the high/low band amp. Amps are high price _at_ $24,000 so can't aford to guess wrong on this. Anyone with any experience or insight along these lines please reply!
Thanks much.
Regards,
Bruce Deker
P. Bruce Deker, Ph.D.
NMR Facility Manager
Univ. of Vermont, Cook A-214
Burlington, VT 05405
(802) 656-3125
p.deker_at_uvm.edu
Received on Wed Dec 15 2010 - 07:00:17 MST