Electronics > Repair

HP 3456A repair LM201 replacement

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I once again pulled out my troubled 3456A to try and fix it.  This time I succeeded at finding the issue.  U303, an LM201 configured as a non-inverting amp to produce +30V to power the input FET stage.  It was only outputting about 4V, so I connected an external supply to provided the 30V, and it works perfectly, all error codes gone and minimal current draw.  Checking the inputs and outputs, it's clear that U303 is dead.

So I can easily get hold of LM201A in a plastic DIP, but the originals are metal can TO-99 package.

It seems like a fairly benign part of the circuit, so is there any reason why I wouldn't just drop in a plastic DIP replacement?

Is the unregulated +33V supply ok?
Then use the plastic version. It should be fine.

Check that the pin out is right. On TO-99 the metal tab indicates pin 8.

They have the DIP8 layout on the board, so you can use a LM201, 301 in a DIP package as a replacement.

Looking on my desk I have some well used SFC2101A's date coded 7415 on a board, but unfortunately these are soldered with a TO100 layout ( pins on all 4 sides) to the board. If you can get the CERDIP variant use it, it does look more period.


--- Quote from: SeanB on November 27, 2013, 07:27:49 pm ---They have the DIP8 layout on the board, so you can use a LM201, 301 in a DIP package as a replacement.

--- End quote ---
The LM301 has only 36V absolute maximum supply voltage rating. It's too low for U303. The LM201 has 44V.

I have used 741's on a +-22v rail with no problems. Just use a turned pin socket if you are worried and try 3 or 4 devices and see which ones run the coolest. I had a box full of TTL ( 300 plus devices) which had a supply oops that applied the unregulated 14V rail to them for a few hours. Only device fail was a PM5403 30V open collector NAND gate. Funny that a 30V rated output part failed. That one popped it's top and died, but the rest of the computer still ran perfectly on 14V and still gave correct data out. There i replaced the entire power supply silicon, including the overvoltage crowbar that had quietly failed and did not work.

With that I ordered the pass transistors, and got a query a week later as to if there was an alternative, as the originals were only available as a custom batch, which was a minimum of 100k devices at $10 each. Found a substitute approved for the position that was available in stores instead, I had already ordered power supplies which had a price which was higher than Platinum each. Those would burn up to save the fuses.


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