LeCroy 9345AM fan doesn't work any more (fan itself is fine) - Page 1
Forum section: electronics / repair

author: exit_failure
author_rank: Regular Contributor
timestamp: June 27, 2018, 08:40:05 am
content: I tried to switch the fan on my LeCroy 9354AM 'scope for a more silent one but when I pulled out the connector and plugged in the new one, I accidentally plugged it in the wrong way round. Since then even the old one doesn't work any more. I wouldn't have expected that plugging in a DC motor the wrong way would damage the 'scope but apparently it did or something else went wrong. The scope itself still seems to work fine and both fans work normally on another 12V power supply.The service manual for the 'scope can be found here:https://archive.org/details/LeCroy_9354A-M-L-TM_Service_Manual_CompleteFirst, I measured the voltage at the fan connector, which is 15.2V(which is the same voltage I measure at the power supply connector). Then I measured the current with a 1k and then a 150 Ohm resistor across the connector. Both times i didn't get anything. I proceeded to measure the the +/-12V and +/-8V voltages at TP203 and they all are well within the specs defined on page 7-37 of the service manual.The voltage across the original fan is only about 1.8V when I connect it. After letting it settle, I get about 900Ohm if I measure across the connector without the fan installed.Next, I measured the power supply at it's connector while being plugged in. They all measured a bit higher than the defined max (or lower than the defined min for the negative voltages) in the service manual (page 7-31). The deviation was about 200mV from max/min but this most likely is due to the CRT, processor board and front panel not being connected (which the manual says should be present for measuring the power supply).After that I measured the voltages across those two tantalum caps directly adjacent to the fan connector. They showed the exact same 15.2V as the fan across them and did not drop down when I connected then fan. They aren't dead shorts either.The orange component (arrow), which I don't really know what it is (resistor?, measures 2.8 Ohm) is on the +5V rail and should not be relevant for my problem, should it?To be honest, I'm a bit at a loss here. Where would you suggest, I continue troubleshooting the 'scope?

author: exit_failure
author_rank: Regular Contributor
timestamp: June 27, 2018, 08:52:40 am
content: Addendum:I took the board out. The connector doesn't have any traces leading to or from it on the back. It looks like it's connected directly across the 15V rail. but then other components shoudln't be of any concern, should they?I took the power supply out and connected the fan directly the the 15V rail. There it works fine an shows the full 15.2V across the fan.

author: grumpydoc
author_rank: Super Contributor
timestamp: June 27, 2018, 09:12:02 am
content: Does the 'scope itself work - don't run it without ventilation for any length of time.The fan is fed by a dropper resistor from 15V, is the resistor OK?

author: exit_failure
author_rank: Regular Contributor
timestamp: June 27, 2018, 09:27:09 am
content: If I measured between the power pin of the connector and the +15V pin of the power supply. I got about 800 ohm after letting it settle. The ground pin of the fan connector is a dead short to ground. The orange resistor(?) has the same roughly 800 ohm between its right side and the power pin of the connector and a dead short between its left side and the ground pin of the connector.

author: grumpydoc
author_rank: Super Contributor
timestamp: June 27, 2018, 10:01:26 am
content: Quote from: exit_failure on June 27, 2018, 09:27:09 amI'd have to check the service manual but you can download that without any difficulty, I think both the Lecroy group files section (you are a member I take it, if not do join up) and ko4bb have copies.

author: exit_failure
author_rank: Regular Contributor
timestamp: June 27, 2018, 08:40:16 pm
content: So let me tell you a tale of graciousness, skill, craftsmanship and intelligence:It wasn't enough that work sucked and that university bored me to tears today, not even damaging my scope by putting in the new fan backwards was enough... no... after coming home, I started to repair my 'scope.First I sucked the orange thing, that I assumed to be a resistor out only to notice that it in fact was a capacitor. The resistor in question was the blue device next to it (R52) clearly marked 6R20. I measured it and it showed 330 ohm instead of 6.2. So I tried to suck that out. In the process, I lifted both pads without any usable traces left to use as makeshift pads. Wonderful.So I bodged in 4 parallel 27 ohm half-watt axial resistors (the 1W rating for the original one was a bit low for my taste with a 12V, 200mA fan) between C46 and the 15V rail directly at the main connector. The fan worked now. Yay.I put the scope back together, turned it on and... nothing except the main grid, no status, no controls, no signal... Terrific.I disassembled it again and found, that I broke a bodge wire at on of the ICs. I fixed that with a new wire, checked if it basically worked and put it back together. Finally! Or so I thought...As soon as I turned it on, I let the smoke of of the resistors I put in earlier. Not sure how but I must have shorted something out. Super... Perfect...At least the scope itself seems to be undamaged.I started to take it apart again for a third time and while taking off the front panel I broke off one of the knobs I fixed two days ago. Why... just why?!It's 10:30pm here and I think, I'm going to bed now.

author: grumpydoc
author_rank: Super Contributor
timestamp: June 27, 2018, 09:01:59 pm
content: My heart goes out to you - I've had days like that as well.

author: Bashstreet
author_rank: Frequent Contributor
timestamp: June 27, 2018, 09:34:46 pm
content: Quote from: exit_failure on June 27, 2018, 08:40:16 pmI know the feeling....Strength brother it will be ok.

author: Stray Electron
author_rank: Super Contributor
timestamp: June 27, 2018, 10:30:15 pm
content: You should join the LeCroy mailing list. In that group it's well known that the LeCroy scopes use a very low power fan and that if you use a fan that draws too much power then it will burn out that resister. Also found out the hard way that most fans that claim to use little power draw far more than than they say! Also most of the low power fans don't put out their rated air flow. I don't know of anyone that has found a different fan that works correctly in the LeCroy scopes. All the ones that I've heard of finally gave up on finding a replacement fan and just bit the bullet and bought the correct replacement fan. OTOH I'm wondering about using a 12 or 15 VDC fan and a smaller value resister. Fortunately none of my LeCroys have run into a fan failure yet so I haven't had to resort to testing that idea.

author: grumpydoc
author_rank: Super Contributor
timestamp: June 28, 2018, 12:31:28 pm
content: Quote from: Stray Electron on June 27, 2018, 10:30:15 pmAerocool Sharkhttp://www.aerocool.us/accessory/shark_b12.html- perfectly OK for the 9354, search for my previous posts.Not sure I would put one in a 9374 or 9384 though.

author: exit_failure
author_rank: Regular Contributor
timestamp: June 28, 2018, 03:49:13 pm
content: I am thinking about installing two fans. The Aerocool Shark you mentioned (thanks for the tip) at the back and the Cougar CF-T 12S I bought a couple of days ago blowing out the top where the FDD was supposed to go (my 'scope doesn't have one). I ordered a couple of 6.8ohm 5W resistors that I would use in a 2x2 configuration to run them well below their rating. What do you think?By the way: The resistor LeCroy chose here seems extremely underrated to me considering that the the 'scopes original fan measured 240mA at 12V.

author: grumpydoc
author_rank: Super Contributor
timestamp: June 28, 2018, 04:02:19 pm
content: Quote from: exit_failure on June 28, 2018, 03:49:13 pmDepends on how much of a safety margin you like6R2 at 240mA will dissipate 0.357W, though it will only drop 1.48V so the fan will be taking a bit more juice - I reckon the resistor dissipates half a watt in practice which should be OK.

author: Bashstreet
author_rank: Frequent Contributor
timestamp: June 28, 2018, 07:43:43 pm
content: Quote from: exit_failure on June 28, 2018, 03:49:13 pmMore airflow the better.

author: grumpydoc
author_rank: Super Contributor
timestamp: June 28, 2018, 08:39:00 pm
content: Quote from: Bashstreet on June 28, 2018, 07:43:43 pmKindaTemp vs airflow will be an asymptotic curve so at some point diminishing returns will kick in.Probably before that point the noise will get intolerable - however quiet the fans are just pulling air through the fairly narrow space that the main board occupies will be noisy.Whether it is worth using the floppy cut out for an additional fan I don't honestly know.

author: Bashstreet
author_rank: Frequent Contributor
timestamp: June 29, 2018, 04:20:47 pm
content: Quote from: grumpydoc on June 28, 2018, 08:39:00 pmSomething like this ?

author: GreyWoolfe
author_rank: Unknown
timestamp: June 29, 2018, 05:14:39 pm
content: Quote from: Bashstreet on June 29, 2018, 04:20:47 pmNot enough fans. You left the optical drive uncovered. You should have made a hinged bracket to attach 2 more fans to it.

author: exit_failure
author_rank: Regular Contributor
timestamp: June 29, 2018, 11:23:05 pm
content: I was thinking more about something like this. Blowing my windows out of their frames might be a downside but would be absolutely worth it in my opinion.