My Tek 2430A Released a lot of Smoke - Page 1
Forum section: electronics / repair

author: Ground_Loop
author_rank: Frequent Contributor
timestamp: March 21, 2022, 12:24:31 pm
content: I was running a project test lasting several hours with my 2430A monitoring a couple signals. I left the lab to take care of a couple things and returned to find that haze and smell that says it all. I looked to see my 2430A lifeless. So I cleared the bench and dove in to see what happened. I found two IRF820 FETS in the power supply pre-regulator had spilled their internals along with a gate resistor and the power fuse. Testing other devices in the neighborhood didn't reveal any additional failures. I'm finding that IRF820 FETs are difficult to find from reliable sources. I ordered some from Digikey only to be notified today that they are no longer available and my order is cancelled. I think I found a suitable substitute, but I'm not sure how sensitive the scope is to having an exact replacement. I have selected the following from STM as a likely substitute: STP4N80K5. My question, is the ST part a suitable substitute and for any of you that may have experience with these power supplies do they require exact replacements?EDIT: I found some IRF 820s on Mouser, so I should be ok.I also note in the schematic the resistor values on the gates to be precisely indicated, as in 301.0 ohms. I measured the undamaged one and it's 320 Ohms. Nonetheless, I ordered some 1% 301 ohm resistors as replacements. I ordered 12 of them so I assume I can find a matching pair within the lot. Similar question though, is the circuit really that sensitive to the gate resistor value?

author: m3vuv
author_rank: Super Contributor
timestamp: March 21, 2022, 06:06:49 pm
content: Pass,next question!

author: bdunham7
author_rank: Super Contributor
timestamp: March 21, 2022, 06:26:33 pm
content: Can you tell me the p/n of your PSU board? It should be up near the 50-ish pin blue connector J102.Edit: I have a PSU 670-9908-00 that was working when removed. Resistors R324 and R624 measure 301.2 and 300.1 ohms, respectively. I don't see why a gate resistor would need to be so precise, but there are other 301R resistors in the unit, such as for the optocoupler U265. It is a 301R 1% by the color coding, but I don't know whether that is a BOM management thing or there is some reason that they needed that precision.

author: james_s
author_rank: Super Contributor
timestamp: March 21, 2022, 06:42:33 pm
content: The IRF820 is a fairly old part and IIRC there is nothing particularly special about it. You should be able to sub something similar although it is nice to have an exact replacement when you can get it.

author: Ground_Loop
author_rank: Frequent Contributor
timestamp: March 21, 2022, 07:52:40 pm
content: Quote from: bdunham7 on March 21, 2022, 06:26:33 pmI'm just getting on a plane to San Diego, so won't be able to check the board number for a while. I found the other pair of FETs damaged as well, but I should be able to repair with what I have on the way. Thanks for the response.

author: bdunham7
author_rank: Super Contributor
timestamp: March 21, 2022, 08:45:17 pm
content: Quote from: Ground_Loop on March 21, 2022, 07:52:40 pmNot quite close enough to drive up and collect it, but let me know if you need the whole PSU. My garage would thank you and it's yours for shipping. I have had mixed success repairing things like that once they go xplodey-like. There's often something else lurking beneath.

author: Ground_Loop
author_rank: Frequent Contributor
timestamp: April 02, 2022, 03:22:03 pm
content: And it lives!!!!Replaced 4 MOSFETs, 2 MOSFET gate resistors, 2 Rifa caps, and mains fuse. All those in the power supply. Then verified NVRAM battery, reseated all connectors and did a calibration. It's now running about as good as a 34 year old 2430A can.