[volt-nuts] DAS-46

[Jerry Hancock]

Hello, fellow volt-nuts.

Several weeks ago I was able to pickup a Fluke 332D and a DAS-46, both in nice shape.  Dr. Frank helped me with the 332D which required the replacement of a resistor (two in parallel) in the string to get the 332D working.  

The DAS 46 worked perfectly for a week and then a cap went in the negative supply.  I can only assume this caused the second problem where the output swung negative.  I traced the issue to a Chopper Stabilized Amp on the back of the power supply board.

Unfortunately, the CSA has no schematic supplied.  If it fails, it is to be replaced.  I’ve been trying to reverse engineer it into LTSpice without luck.  There are 32 resistors, many of them high values like 100M, 10M etc., 4 transistors, two op amps, 10 diodes, 10 caps, etc, not an excessive amount of parts, but they are all sort of twisted together on a small double sided board that I find difficult to trace.  I am getting closer every day but not quite there.

I contacted their parent company who offered to repair the unit but having doubts as to the economics of it, I started replacing active components after checking as many diodes and transistors as possible.

There are two FETs, MFE3002 that cross to ECG220 that at first measured differently.  I swapped them and instead of swinging to the negative rail, it swung positive.  Orin, another member of this site, suggested that this could have something to do with the gate voltage variance because after replacing both, they continue to act this way.   Almost all the high value resistors have swung higher.  For instance, the 100M are now between 125 and 130M,; 3M are 3.3M all of these have gold bands so they are out of spec.

After going back and forth with the parent company they have now decided to sell me a CSA.  I can’t wait to hear the price!  I would say the rough BOM would cost about $50 so I expect them to want $500.

Unless someone on this board has either a replacement CSA or the schematic for it by chance, I plan to swap the FETs one more time since they are sensitive to static.  Failing that, Orin and I worked on a CSA using a LTC2057 which I could wire directly to the output transistors to provide the spec’ed current.  I tested this circuit with a ua741 and it worked.  I can also test it with LM308s as used in the CSA and I expect it to work as well.

Any other ideas would be appreciated.

Thank you!

Jerry