HP 3456A repair LM201 replacement - Page 1
Forum section: electronics / repair

author: Harvs
author_rank: Super Contributor
timestamp: November 27, 2013, 01:55:26 pm
content: 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?

author: sync
author_rank: Frequent Contributor
timestamp: November 27, 2013, 02:50:12 pm
content: 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.

author: SeanB
author_rank: Super Contributor
timestamp: November 27, 2013, 07:27:49 pm
content: 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.

author: sync
author_rank: Frequent Contributor
timestamp: November 27, 2013, 07:41:13 pm
content: Quote from: SeanB on November 27, 2013, 07:27:49 pmThe LM301 has only 36V absolute maximum supply voltage rating. It's too low for U303. The LM201 has 44V.

author: SeanB
author_rank: Super Contributor
timestamp: November 27, 2013, 07:53:59 pm
content: 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.

author: Harvs
author_rank: Super Contributor
timestamp: November 27, 2013, 10:56:13 pm
content: Quote from: sync on November 27, 2013, 02:50:12 pmYeah the rails are fine. The +33V rail is measuring 44V (which is within spec.) And the negative power is connected to the 5V rail, which is also fine.Thanks SeanB, in this case getting LM201's is easy in PDIP, $1.5ea. It's just getting the can version is expensive, and will require expensive postage on top, making it about a $20+ opamp.I was just concerned about that there was something special about why they've gone an used a mil-spec temp grade opamp in a lab bench meterI've placed an order with RS for a few of the PDIP version, so hopefully that'll have it back up and running.

author: krivx
author_rank: Frequent Contributor
timestamp: November 27, 2013, 11:33:20 pm
content: Quote from: Harvs on November 27, 2013, 10:56:13 pmSeeing as the board seems to be laid out for for a DIP8 package they may have had a shortage of one part or an abundance of another. When you buy in bulk for an entire line of test equipment sometimes it's cheaper to use the more expensive part