LeCroy 9310C DSO--input attenuator issue? - Page 1
Forum section: electronics / testgear

author: N2IXK
author_rank: Frequent Contributor
timestamp: March 08, 2015, 03:12:39 pm
content: Just rescued a LeCroy 9310C from the dumpster. It was really dirty, but besides some pretty bad phosphor burns on the CRT, it cleaned up really well. A tweak of the CRT cutoff control greatly improved brightness/contrast and the scope seems to be fully functional (even the floppy and printer work, and the RTC still had a valid date!).The issue I am having appears to be the Channel 1 input attenuator. It functions properly on all settings BELOW 0.2V/div, but at all settings 0.2V/div and above, the displayed gain is EXACTLY 10X what it should be. (1V/div actually gives 100 mV/div, etc).I am ASSuming there is a relay that isn't properly switching in a 10x attenuation stage somewhere in the front end. I can hear a relay clicking when going from 0.1V/div to 0.2V/div, but perhaps the contacts aren't making properly?Looked through the forum archives and the Yahoo LeCroy group for references to this issue and couldn't find any. Has anyone dealt with this before? Haven't torn the scope apart yet other than popping the top cover to blow out the dust. It appears that total disassembly is needed to get at the input attenuators.

author: N2IXK
author_rank: Frequent Contributor
timestamp: March 14, 2015, 10:47:17 pm
content: I finally dug into this thing, and my suspicions about a relay switched X10 attenuator were exactly right. Testing the relay shows intermittent operation and high resistance of both of the NC contacts, while the NO contacts appear to be working properly. This is a TO-5 can hermetic DPDT RF relay made by Teledyne (who own LeCroy). Manufacturer p/n is 732-12. Found a US seller on eBay who offers NOS units for $25 each. Mouser has in stock for $37. Will order one and hopefully that fixes things.Getting the old relay out was a real PITA, as it was mounted flat against the board, and the TO5 can itself was soldered down to the PCB for shielding in a couple places, as well as being soldered to an identical relay right next to it. Got it out without beating up the PCB too badly though, and all the holes cleaned up and ready for the new one to arrive.

author: radhaz
author_rank: Regular Contributor
timestamp: March 14, 2015, 10:55:25 pm
content: Interesting relay. I've not seen one like that before.

author: SeanB
author_rank: Super Contributor
timestamp: March 15, 2015, 05:14:50 am
content: Lovely relays, I was enamoured of the EBDC26/28 series of avionics relays, in a rectangular can. Only relay I ever came across that was wound with enamelled nichrome resistance wire instead of enamelled copper. Such a PITA to replace on the boards though.

author: N2IXK
author_rank: Frequent Contributor
timestamp: March 19, 2015, 08:14:01 pm
content: Put the new relay in, and it seems to have fixed the problem with channel 1. Now gives proper gain on all settings.Unfortunately, after playing around with it for a couple days, channel 2 is showing the same behavior intermittently. Looks like the corresponding Ch2 relay needs replacing, as well. Its a real PITA job, with a small pin pitch (with an SMD resistor nestled in the middle of the pin circle!). Biggest pain is the fact that they tack soldered the relay can to the groundplane of the board. needs a LOT of heat to get it to pop loose. I actually tore a bit of the groundplane loose, but was able to repair with a bit of copper foil tape. And the relays aren't exactly cheap, either. There are a total of 4 of them between the 2 input channels, all of the same age.I did take the one I removed from CH1, and hooked it up in a circuit to exercise the coil on and off. and switch some LEDs to monitor the contacts. After a few minutes of being banged on and off at ~2Hz while switching ~20mA of DC, the intermittent operation seems to be cured. Perhaps some kind of oxide film builds up on the contacts and gets broken up with use? Tin whisker type phenomenon?While I had it all apart, I upgraded the DRAM to the max supported (64M), using NOS 72 pin EDO SIMMS from eBay. Bet LeCroy charged a lot more for the 64M option than the $10 I paid.

author: SeanB
author_rank: Super Contributor
timestamp: March 19, 2015, 08:25:04 pm
content: Running the relay with current flow with operation will cure it short term, but the contact surfaces inside are now worn, so it will go intermittent again. Best is a new one. Yes, they are a PITA to change, especially on an old board with fragile traces. I learnt a lot about trace repair there.

author: N2IXK
author_rank: Frequent Contributor
timestamp: March 19, 2015, 09:46:23 pm
content: Kinda figured that. Certainly wouldn't want to subject the PCB to the rework trauma to install anything less reliable than a brand new one.I actually have some NOS Bristol TO-5 relays (1967 date code) that would be a drop-in replacement, but they have 28VDC coils...