Another Opamp replacement question, old LM1436H or MC1436 in T099  - Page 1
Forum section: electronics / beginners

author: RichardM
author_rank: Frequent Contributor
timestamp: November 25, 2021, 11:58:10 am
content: I have a dead version of this in my Fluke 8502a and would love some expert advice on what could replace it.I doubt I will understand the answer, but what foes internally compensated mean ?Any help much appreciated.Richard

author: RichardM
author_rank: Frequent Contributor
timestamp: November 25, 2021, 12:49:21 pm
content: The +-30V supply voltage looks like it will be a problem....

author: Kleinstein
author_rank: Super Contributor
timestamp: November 25, 2021, 01:05:24 pm
content: The +-30 V suplly range can ideed be difficult, but there are a few higher voltage OPs available, e.f. ADA4700 (up to 110 V total). One may need some kind of adapter board. The GBW could still be a problem as too fast or too slow an OP may change stability in more complicated circuit.Internally compensated means that the OP does not need extra capacitors for loop compensation in the normal applications. Most modern OPs are of this type - it is more tricky to find some that are not.

author: Zero999
author_rank: Super Contributor
timestamp: November 25, 2021, 02:19:52 pm
content: Are you sure it's the op-amp which is bad? Have you tested it?Which package style is it? The metal package might be difficult to obtain. If surface mount is acceptable, there's the OPA454.As mentioned above, getting a high voltage part is difficult and expensive. It's not a fast op-amp. If the replacement is too fast, it might oscillate, which could be fixed by adding a capacitor, although it will require knowledge of the circuit.

author: magic
author_rank: Super Contributor
timestamp: November 25, 2021, 04:41:22 pm
content: I see some LM1436H on eBay in Canada.Also in China, but that can be risky. I would definitely perform a high voltage breadboard test before using such an IC from China.What sort of circuit is it in and do you know what the requirements really are?

author: SeanB
author_rank: Super Contributor
timestamp: November 25, 2021, 05:52:40 pm
content: For a test a LM741 would work perfectly here, the performance at least is similar enough, and the offset circuitry is the same as well. At least work well enough so you can check if the opamp itself was dead, or if it was something else doing it. While the 741 is rated to 36V only, it will, I have found, operate fine on up to 44V with no major issues, though admittedly when i was abusing them like that I was using the TO100 parts, and slipped on a TO39 finned heatsink block on each one as safety, seeing as they were driving some rather long cables, and also a 200R load. They did not even get warm, but seeing as the OP22's they replaced had been killed by cooking into shorts, I figured the free update was not a problem.

author: Kleinstein
author_rank: Super Contributor
timestamp: November 25, 2021, 07:19:25 pm
content: Mouser still has a few OPA445 in TO99 case. Not cheap, but at least available. Not sure if they fit (e.g. maybe a bit fast).

author: RichardM
author_rank: Frequent Contributor
timestamp: November 25, 2021, 10:50:28 pm
content: Thanks for the reply. Each rail is currently supplied with 30V, one at +30V and the other at -30V. I dont think the LM741 can go that high? I.e. only 22V

author: Kleinstein
author_rank: Super Contributor
timestamp: November 25, 2021, 10:56:01 pm
content: There are a few higher voltage types available. So check you favorite source. If needed one could use a adapter board. Because of the heat dissipation it may need a custom adapter, but if normal SO8, a standard one for So8 -> dip can work.

author: RichardM
author_rank: Frequent Contributor
timestamp: November 25, 2021, 11:04:58 pm
content: I am pretty sure it is dead and worse it may have been my errant probing!!! It was getting very hot so I had initially suspected it. It’s on the filter board of the Fluke 8502a.

author: RichardM
author_rank: Frequent Contributor
timestamp: November 26, 2021, 12:05:23 am
content: This is the schematic for the filter board

author: Rich S
author_rank: Newbie
timestamp: November 26, 2021, 01:02:42 am
content: In the 1989 NS data book, they recommend the LM343, its maximum supply voltage rating is +/34 volts.(The LM143 is rated for +/-40V). But that IC is obsolete too, though you might find NOS somewhere.A current hi-v single opamp in TO-99 can is the OPA445 - with the "BM" suffixhttps://www.ti.com/lit/gpn/opa445around $15 at Digi-Key

author: Kleinstein
author_rank: Super Contributor
timestamp: November 26, 2021, 08:11:55 am
content: The circuit should be OK with a moderately faster OP, as there is local capacitive feedback to limit the speed. It has a rather high impedance at the inputs, so a FET type is a good idea. Some BJT versions with higher input current noise or offset current drift could be a problem.