Siglent SPD3303X-E mosfet replacement - Page 1
Forum section: electronics / repair

author: R-1125F
author_rank: Regular Contributor
timestamp: February 24, 2023, 06:43:00 pm
content: I am working on repair of a Siglent SPD3303X-E. I found a bad IRFP150N MOSFET. I pulled the old one out and it appears that they used black electrical tape to cover the gate connection on the back. The MOSFET is screwed to a heatsink. I would have expected a thermal pad, but not tape.Is this tape something special or just ordinary electrical tape?

author: R-1125F
author_rank: Regular Contributor
timestamp: February 24, 2023, 07:28:55 pm
content: I think I figured out that it is thermal transfer tape. It is just much thinner than the pads I am used to seeing.

author: tautech
author_rank: Super Contributor
timestamp: February 24, 2023, 10:34:10 pm
content: Quote from: R-1125F on February 24, 2023, 06:43:00 pmWhere exactly, the power or control PCB ?Do you know why it went bad ?

author: R-1125F
author_rank: Regular Contributor
timestamp: February 24, 2023, 11:14:09 pm
content: These are the output MOSFETS for CH1 and CH3, mounted on the left side of the heatsink. I pulled them out and both checked bad. I replaced both MOSFETS them and the CH1 fuse blew. I had already changed the CH1 crowbar triac because that was shorted. I am not sure what to do next.

author: tautech
author_rank: Super Contributor
timestamp: February 25, 2023, 12:00:07 am
content: Quote from: R-1125F on February 24, 2023, 11:14:09 pmOkay, these are on the Power PCB, sorry I don't have one.QuoteWhat happened for these to go bad ?Did the PSU get subjected to back EMF or regenerative charging ?QuoteThou shall test voltages as specified in the Service manual:https://siglentna.com/download/14764/?tmstv=1677277978Pages 41 and 42.

author: JeremyC
author_rank: Regular Contributor
timestamp: February 25, 2023, 01:44:46 am
content: Quote from: R-1125F on February 24, 2023, 11:14:09 pmCheck rectifier bridge(s), all electrolytic caps and transistor(s) driving the MOSFETs.Good luck.

author: R-1125F
author_rank: Regular Contributor
timestamp: February 25, 2023, 04:26:35 pm
content: I will testing it as much as I can today. I need to change the fuse first and see if it blows again.I was told by the seller that he was using it to operate a solenoid for testing. I assume that the inductive kickback from the solenoid killed something in the power supply. He wasn't using CH3 at the time but it took that out too. CH2 still works as it should.

author: tautech
author_rank: Super Contributor
timestamp: February 25, 2023, 06:19:42 pm
content: Quote from: R-1125F on February 25, 2023, 04:26:35 pmYup, back EMF would do that.There is a reverse current protection but it may have been turned OFF or it was large enough to swamp it.Okay work carefully as you have one working channel to follow.My thoughts are the control/output PCB has been damaged causing the power PCB damage too.Repair the power PCB and channels 1 and 3 disconnect from the control PCB and work from there checking voltages as per Service manual.Contact Ohio for the control PCB costs.

author: R-1125F
author_rank: Regular Contributor
timestamp: February 25, 2023, 08:37:17 pm
content: Partial success. I changed the fuse and CH1 now sort of works. The voltage is controllable but something is drawing current without a load. My thermal camera shows that one of the TL072 op amps on the main board is running a bit warmer than the others. That is the only thing that looks unusual on CH1. The 78L05 regulator is working, 15V in and 5V out. I haven't gotten to the CH3 problem yet.

author: R-1125F
author_rank: Regular Contributor
timestamp: February 26, 2023, 02:18:03 pm
content: I did notice that this power supply has been hacked to the X model. Could that cause it to draw current (or at least indicate that it is) without a load?

author: tautech
author_rank: Super Contributor
timestamp: February 26, 2023, 06:53:08 pm
content: Quote from: R-1125F on February 26, 2023, 02:18:03 pmNow you're making shit up, of course not.

author: R-1125F
author_rank: Regular Contributor
timestamp: February 26, 2023, 07:08:50 pm
content: It was on the readback display so it must be true.Later. it occurred to me that since I replaced a bunch of parts that it may just need some calibration.I will work on that after I get the CH3 working and everything put back together.

author: R-1125F
author_rank: Regular Contributor
timestamp: February 27, 2023, 10:44:16 pm
content: I got all the outputs working. The calibration is off, but that is to be expected after replacing a few things. The parts I replaced were CH1 crowbar triac type BTA08, CH1 and CH3 MOSFETS type IRFP150N, CH3 MOSFET driver transistor type 2N3906, and CH1 and CH3 fuses type T6.3A.Next I will learn to calibrate this thing. I see there were some very extensive threads on that subject.Thank all.