Products > Test Equipment
Repair of Return LeCroy/Preamble 1855A differential amplifier
rx8pilot:
Yesterday, I received a LeCroy DA1855A differential pre amplifier from eBay. Advertised as working and has 14 day return option. It looks great in pictures, was packed very well, and physically presents very nice. Not a single scuff mark anywhere on it. But it does not power up. :--
I contacted the seller and he agreed to let me take a look inside for easy and obvious issues. I took the cover off and was greeted with the most convoluted mechanical arrangement I have ever seen. I am nearly positive the mechanical design was done by one of the electrical engineers that has never once had to assemble or service anything. The unit upon application of power - simply has a low level chirp and small pulses of voltage at about 1hz. First thing to check is the power supply, however that requires (apparently) disassembling all of the hand built and very delicate looking pre-amplifier section. I looked at it for an hour trying to guess how it went together in the first place. There is a lot of point to point hand construction on the front end - looks super laborious to manufacture!
Anyway - has anyone every serviced one of these? Trying to decide if I should send it back or dive in. It's a bargain if I can get it to work, but obviously a waste if I damage it in the process of finding the power problem.
TiN:
I'd pry it out of your cold hands, even dead, especially if it's a bargain. Don't mess with front end unless absolutely necessary, it's done that way due to difficult to achieve specs.
Keep it, so we can see moar photos and teardown, please. Let the community wish prevail :-+.
Maybe seller can give you some refund to sweeten the pill.
rx8pilot:
I am highly motivated to make it work. The approximate cost was about $615 US - there were other things like the matching probe in the purchase so I am guessing this part was about that number. Not a bad deal if I can get it working. Most of them are listed at 4x the price on eBay and new ones are $5190 USD! I already have one other of the older Preamble versions - they are nearly the same except this one has a remote option that hooks up to LeCroy scopes. It is a very nice instrument to have for power supply design among other things. I would really like to have two working units.
Challenge number 1:
Disassemble without messing with the clearly delicate front end. So far, I have not figured out how to get the PCB on top removed without needing to de-solder the cables going directly into the hand wired cages that house the first stage of the amplifier. There are also 3 ribbon cables that are soldered direct to the front panel. The other ends are removable but on the bottom of the unit. the screws that hold the whole sandwich assembly inside the chassis are under the bottom PCB - meaning I have to pull all the boards and connections to gain access to the power supply. I have repaired all kinds of very expensive and delicate commercial electronics, but this thing is nothing short of a real challenge.
If a single pot or trimmer gets bumped, I can see LeCroy charging an arm and a leg to get it back to calibration.
Check out the hand work - Nuts. The PCB has no silkscreen either. I have not found a service manual and probably will not. The owners manual states that calibration requires special equipment not commercially available. They are not going to give up any secrets from what I can tell.
rx8pilot:
I dug into it. What an enormous pain in the ass. Stunningly poor mechanical design. I have never seen anything quite like it in all my decades of taking things apart.
Anyway - the power supply, as suspected, is history. The label indicates it should output +5v and +-29V. The main problem is that the +29v is pulsing out +45 to +65v! Crap, even if I fix the the power supply - the rest of the system may be toast after being pummeled with twice the expected voltage. Super crappy news. These types of supplies are hard to work on too - very difficult to get to any of the parts and I don't think this weird thing is going to be easy to source to replace it.
Bummed.
H.O:
Stating the obvious here but can't you try powering it up with an (3) external supply - just to see if the rest of the circuit works? If it does it might be slightly more motivating trying to fix the existing supply or possibly build/buy a supply to fit in the old ones place or even use an external supply to retrofit the unit with.
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